The Complete Human Body Dr Alice Roberts

299

NERVOUS SYSTEM

THE ELECTRICAL NATURE OF NERVE SIGNALS Potassium Sodium Sodium–
potassium
Nerve signals are pulses of electricity caused by the mass movement of tiny particles called ions. channel channel pump 1  Resting potential
Electrical charge is a fundamental property of matter. Minerals such as potassium and sodium dissolve Every nerve cell’s sodium–
in bodily fluids and exist as ions, each with a positive charge. The more ions in a certain place, the potassium pump distributes
higher the charge. The fluids inside and outside of cells are electrically neutral, but there is a polarizing sodium and potassium across
shell of charge coating every cell’s membrane, and this creates the resting potential. When ions move the cell membrane, which creates
across the membrane, the associated move of charge creates a pulse of electricity or action potential. differences in concentration and
An action potential measures about 100 mV from peak to trough and is over in 1/250 th of a second. a polarization of electrical charge
at the membrane—the resting
MEMBRANE 30 Action potential Neuron Membrane negative potential—with the inside of the
VOLTAGE Ions move in and out of membrane on the inside cell negatively charged.
(MILLIVOLTS) small patches of the axon’s
0 Resting Peak of membrane to generate 2  Depolarization
potential depolarization an action potential by A stimulus arrives and
changing the cell’s voltage. triggers voltage-gated sodium
–65 3 Sodium ions channels to open. Sodium ions
Repolarization Traveling signals move into flood into the neuron, causing
1 2 The region of reversed neuron Potassium a movement of positive charge.
charge “fizzes” along the channels If this depolarization (reversal
Signal travels TIME length of the axon, much Sodium of the polarity of the membrane)
along axon Hyperpolarization like a lit fuse, before channels close achieves a critical level (called
passing the message on threshold) the membrane
at a synapse (see p.300). open generates an action potential.
Charges across the
Membrane Repolarization membrane are disrupted Membrane now 3  Repolarization
ahead starting ahead of and behind positive on inside The depolarizing change in
to depolarize Electrically the depolarization. voltage causes sodium channels
active patch to snap shut and voltage-gated
Node of Ranvier of membrane potassium channels to open.
Slight gap between Now, potassium ions move out
neighboring sections Potassium ions of the neuron, removing the
of myelin sheath move out positive charge brought in by
of neuron the sodium ions. In fact, a brief
Potassium hyperpolarization occurs (inside
channels Sodium is even more negative) before
open channels returning to its resting potential.
close

Membrane returns
to negative on inside

Oligodendrocyte
Makes the myelin
sheaths in the
CNS; it can extend
“arms” to more
than 30 neurons

Synaptic knob
Conveys nerve signals to
other cells across a tiny gap or
synapse (see pp.300–301)

Myelin sheath A typical neuron Axon terminal
Wrap-around The basic components of a neuron are End of the axon,
similar wherever they occur in the nervous
covering that insulates system: a rounded cell body, containing the which may be
the axon and speeds nucleus and mitochondria, with many dendrites single or branched
signal conduction projecting from it, and a single long axon. The neuron
shown here has been shortened to fit on the page;
Axon in reality, some neurons are up to 39 in (1 m) long.
The neuron’s longest
and thinnest projection;
nerve signals travel from
the cell body along the
axon to the synapse

300

HOW THE BODY WORKS

PASSING ON THE MESSAGE

Nerve messages travel along individual neurons as tiny pulses of electricity.
They change into chemical form, as molecules of neurotransmitters, to cross
the tiny gaps at the junctions, or synapses, between neurons.

At the synapse 1 Neurotransmitter ready
Neurons do not quite touch at their main communication Vesicles travel from the sending
points, the synapses. Their cell membranes are separated by neuron’s cell body to the presynaptic
a synaptic cleft just 20 nanometers wide. As a nerve impulse membrane. An impulse arrives and
in the sending neuron arrives at the synapse, it triggers the makes them fuse with the membrane
release of neurotransmitter molecules. These “jump the gap” and release their contents.
and set off a nerve impulse in the receiving neuron.

2 Crossing the gap
Neurotransmitter molecules
cross the cleft in a few thousandths
of a second and attach to receptor
sites in the postsynaptic membrane
of the receiving neuron.

3 The message continues
Neurotransmitter molecules
bind to receptors on ion channels
in the postsynaptic membrane,
causing them to open. Positive ions
then flow into the receiving neuron.
If enough channels open, a new
wave of depolarization is triggered.

Postsynaptic
membrane
Part of the

receiving neuron

301

NERVOUS SYSTEM

HOW NERVE CELLS COMMUNICATE

Microtubule The basic “language” of the nervous second. These signals are passed onward
Microscopic conveyor system is nerve signals or impulses. to other neurons with which it has synaptic
belt that carries vesicles This language is frequency based—that connections. The pattern of connections
to the synapse is, it “talks” in digital and not analogue between neurons changes over time,
terms. The precise information nerves through natural body development and
carry depends on how many impulses also through learning (see p.307).
there are, how close together, where
they come from, and where they go. In the brain’s cortex, one neuron may
have synapses with more than 200,000
Resting or quiet neurons, for instance, others, so that a piece of cortex the size
might send an impulse every second or of this “o” contains more than 100 billion
two. A highly stimulated neuron—for synapses. The way that each neuron
example, dealing with sudden pressure processes its incoming signals, and
on the skin—might send 50 impulses per what it sends onward, is shown below.

Axon of neuron DEALING WITH Signal summation
Nerve impulses travel MULTIPLE SIGNALS At any instant, a neuron’s activity is affected by
along this to the “summing” the numbers and types of signals it
synapses at its end Some nerve impulses arriving at receives and by their positions on its dendrites and
a synapse are excitatory (causing cell body (and perhaps the axon in certain neurons).
Vesicle depolarization) and thereby contribute
Membrane bag to similar impulses being formed in the Excitatory input (A)
of neurotransmitter receiving neuron and the message being This input comes a
molecules passed on. Other inputs are inhibitory
(causing hyperpolarization), damping short distance from a
Ion down any impulse formation in the neighboring neuron
This electrically receiving neuron. Whether the receiving
charged particle neuron “fires off ” an action potential, Neuron cell body
floats in the fluid or impulse, depends on the sum of its The cell body receives
on either side of the excitatory and its inhibitory inputs. The inputs, as do dendrites
cells’ membranes type of neurotransmitter at the synapse
is also important, as is the structure of Excitatory input (B)
Neurotransmitter the neurotransmitter receptor site. This axon terminal is
molecule from a neuron many
Relatively large inches away
chemical “messenger”
units; there are To send or not to send? Inhibitory input (C)
several main kinds, Each neuron’s inputs (A, B, or C) vary depending on the frequency Information received
including GABA, of arriving signals, their synapse positions, and whether they are excitatory here works against the
acetylcholine, and or inhibitory. As a complex web of electricity ripples around the neuron’s
dopamine membrane, it may send its own signals onward—or not. excitatory inputs

Presynaptic THRESHOLD A+A A+B A+A
membrane A C
The end part of STRENGTH OF STIMULUS (MILLIVOLTS)
the sending neuron 0 Once the threshold
is reached, there is an
Synaptic cleft THRESHOLD all-or-nothing response
Fluid-filled gap
less than 1/5000th the –65 TIME
width of a human hair
Subthreshold Threshold stimulation Hyperstimulation Inhibition
stimulation The greater the excitatory When even greater The inhibitory input (C)
The depolarization of input (A+A), the greater stimulatory impulses cancels out the
this excitatory input (A) the chance of exceeding arrive (A+B), far exceeding stimulatory impulses
is too small to reach the the threshold; here a the threshold level, they (A+A), which would
threshold level, and so series of action potentials result in a higher- normally depolarize
the neuron doesn’t “fire” results for the duration frequency sequence to the threshold, so here
an action potential. of excitation. of outgoing signals. no signal is generated.

302

HOW THE BODY WORKS

THE BRAIN AND
SPINAL CORD

The central nervous system—the brain and spinal
cord—receives information from all body parts
and replies with instructions to all tissues and
organs. These nerve centers are protected
and nourished by an elaborate system of
membranes and fluids, including blood.

INFORMATION PROCESSING

The spinal cord gathers messages from the torso and limbs and
relays them to the brain. But the cord is not just a passive conveyor
of signals; it also carries out basic body “housekeeping,” receiving
and sending messages without involving the brain. In general, the
“higher” the information goes—heading up to the top of the
brain—the nearer it gets to our conscious awareness. As the cord
merges with the brain it leads to the brain stem, where centers
monitor and adjust vital functions, such as heartbeat and breathing,
usually without bothering the upper brain. Higher still is the
thalamus, a “gatekeeper” that selects which information to allow
into the uppermost area, the cerebral cortex. Many of the highest
mental functions occur in the cortex—thoughts, imagination,
learning, and conscious decision-making.

PROTECTING THE BRAIN

Around most of the brain is the rigid, curved case of the upper skull, the

cranium. Bone and brain are separated by a set of three sheetlike membranes

—the meninges—and two layers of fluid. Outermost is the tough dura mater

membrane lining the inside of the skull. Next is the spongier, blood-rich arachnoid.

Spaces called venous sinuses between the dura and the arachnoid contain the Cerebrum
outer cushioning liquid—slow-flowing venous blood leaving the brain to return Large upper dome of
to the heart. Within the arachnoid is an inner cushioning layer of cerebrospinal two hemispheres with
fluid (see opposite). Below this is the innermost and thinnest membrane, the pia highly folded cerebral
cortex covering

mater, which closely follows the brain’s contours directly beneath it. Cerebellum
Small, rear, wrinkled
Between brain and skull
Cerebrospinal fluid circulates in a thin gap, the subarachnoid space (see opposite), part involved in
between the arachnoid and the pia mater. The meninges and fluid work together muscle coordination

to absorb and disperse excessive mechanical forces so they don’t result in injury. Skull bone Thalamus
Central monitoring
Cerebral cortex Dural venous sinus
Outermost layer Venous blood drains area shaped like
away from the brain two hen’s eggs
of the brain
Dura mater Medulla
Blood vessel Outermost and Lower tapering
strongest membrane
Arachnoid part of the
Weblike layer rich in brain stem
blood vessels and fluid
Spinal cord
Pia mater Major brain–body
Thin membrane
around the surface highway, about
as wide as the
of the brain owner’s forefinger

Cervical vertebra

303

NERVOUS SYSTEM

FEEDING THE BRAIN Venous
sinus
The brain has two main sources of nourishment and waste
disposal. One is blood, brought mainly by the carotid and Skull
vertebral arteries in the neck to the Circle of Willis at the
brain’s base. The second system involves a liquid derived from Lateral
blood, cerebrospinal fluid (CSF). This fluid is made at a slow, ventricle

steady rate by the linings of two chambers inside the brain’s Subarachnoid
hemispheres called the lateral ventricles, and it flows within space
and around the brain. About 17 fl oz (half a liter) of
Dura
CSF is produced every day, with up to 5 fl oz (150 mater
milliliters) present at any time. It transports glucose,
proteins, and other materials to brain tissues, and Third ventricle

takes away waste substances; it also carries infection- Fourth ventricle
fighting white blood cells. In addition to metabolic
functions, CSF provides physical comfort for the Spinal cord
Central canal
brain and spinal cord since they “float” in it.
CSF flow
Anterior From the lateral ventricles, CSF flows through two
cerebral central chambers, the third and fourth ventricles,
out into the subarachnoid space around the brain,
artery and also around the spinal cord—which is also
wrapped in meninges. The fluid is absorbed by
Middle tiny mushroomlike projections of the arachnoid.
cerebral
Circle of Willis
artery This system brings together several arteries supplying
the brain and provides links, or communicating
Internal arteries, between them. The communications work
carotid as “bypasses” so that if one artery is narrowed
or damaged, blood can still flow to the brain from
artery another artery in the circle.

INSIDE THE SPINAL CORD space) and within it (along the tiny central canal).
The meninges and CSF ensure that the cord is
The cord mirrors many features of the brain. It is not knocked or kinked as the spinal column twists
protected by bone, in this case the spinal column and flexes. If an infection is suspected, such as
of linked backbones or vertebrae, whose central meningitis (see p.441), a sample of CSF is more
holes align to form a tunnel for the cord. easily withdrawn using a hollow needle from
It is enclosed within the three meningeal layers around the lower cord, by a lumbar puncture
that cushion it within the spinal column. It has or “spinal tap,” than from around the brain.
nourishment-providing CSF circulating
both around it (in the subarachnoid

Epidural Arachnoid
space

Subarachnoid Pia mater
space Central canal

A slice through the brain Dura mater The spinal
This MRI scan through the middle of the brain cord in section
and cord (from front to back) shows their major Cerebrospinal The cord is encased
features. The darker areas of the brain are fluid within the central space
fluid-filled spaces and internal chambers known of the vertebral column;
as ventricles. In blue around the brain are the Vertebral bone FRONT OF BODY its nerve roots (yellow)
protective bones of the skull and, on either side of pass out through gaps
the cord, the bones of the neck (cervical vertebrae). between adjacent
vertebrae.

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HOW THE BODY WORKS

THE CNS IN ACTION LEFT SIDE OF BRAIN RIGHT SIDE OF BRAIN

Our brain and spinal cord are always active—in constant communication Breaks up a whole Intuitively combines
with each other and the rest of our bodies. Messages stream in from the into constituent parts parts into a whole 
peripheral nervous system (PNS), and are channeled to the central nervous
system (CNS), which processes the signals and sends instructions back out. Analytical activity, with Tends to make random
progressive sequencing leaps and links 

Left hemisphere KNOWING LEFT FROM RIGHT Tends to be objective, More subjective and
impartial, detached individualistic 
Longitudinal Anatomically, the nervous system shows left–right
fissure symmetry (see pp.60–63); but in terms of function, it’s More active with words More active with sounds,
not as simple. The brain’s wrinkled cerebrum is almost and numbers sights, and items in space 
Right completely divided by a deep front-to-back groove into
hemisphere two cerebral hemispheres, left and right. Although these Deals more with logic Deals more with ideas
may look outwardly similar, each hemisphere dominates and implication and creativity 
Corpus for certain mental functions (see table, right). The two
callosum hemispheres “talk” constantly via a straplike collection Leads in rational Jumps with insight to
of nerve fibers—the corpus callosum. problem-solving  possible solutions 
Thalamus
Information from the body swaps sides on its way Location of speech and Rarely dominates speech
Cerebellum to the brain. Nerve signals travel within organized bundles language centers  and language
of nerve fibers called tracts, which cross over from the left
Two sides working as one side of the body to the right side and vice versa. So, for Stores literal meanings Gives language context
This vertical “slice” through the brain shows the longitudinal fissure example, sensory information from the body’s left side ends of words, grammar  and accentuation
as a deep furrow between the left and the right cerebral hemispheres. up in the right hemisphere, and motor instructions sent
At its base the corpus callosum, a bridge of more than 200 million from the left hemisphere control muscles on the right More active in recalling More active in facial
nerve fibers, links the hemispheres. side of the body. names  recognition

Controls right side Controls left side
of the body  of the body

Which side takes charge?
Brain scans and studies of brain injury or disease reveal that
the “take-apart” left side is more concerned with logic and
reasoning, while the “put-together” right side is more intuitive
and holistic; although each side assists the other. 

TO THE BRAIN Dorsal root Dorsal column–medial lemniscus tract
Carries sensory Sensory information (other than pain)
AND BACK AGAIN Dorsal root ganglion nerves into the diverges in the spinal cord: one branch
Neuron cell bodies and stays within the cord to synapse with
Information from the world around us reaches synapses relay the signals spinal cord
the brain via the major sense organs (see p.310). An another neuron; the other branch ascends
external stimulus is converted into nerve impulses by into the spinal cord the spinal cord to the medulla

specialized receptor cells. The impulses begin a journey

through the sensory nerves of the peripheral nervous

system and on to the higher centers in the brain; the

route to the cerebral cortex may involve a series of up

to 10 neurons linked by synapses (see p.300). At each

relay station in the sequence, additional messages are

sent out along other pathways, like branches diverging

from a tree trunk. In the cortex, we become aware of

the stimulus and decide to act. The result is a cascade

of outgoing or motor messages that travel in the

reverse direction, out to various muscles and glands. Myelinated axon
The myelin sheath
speeds the nerve
impulse transmission

CROSS SECTION
OF THE SPINAL
CORD

Sensory receptor White and gray matter Motor messages Spinothalamic tract
Responds to activation White matter (axons) Motor nerve Information about pain
synapses with the next
by sending impulses surrounds the central gray matter impulses descend the neuron and crosses over
along its axon (neuron bodies, interconnecting corticospinal tract
within this level of the
dendrites, and synapses) and relay along more spinal cord before
axons to the arm and
ascending to the brain
hand muscles

305

NERVOUS SYSTEM

Initiating output Thalamus Pain and temperature
Instructions for voluntary Major relay station Information concerning these sensations
movements originate in the motor en route to the cortex reaches the somatosensory cortex by
cortex (see p.308) and travel via the a different route from messages about
thalamus before connecting to general touch
motor neurons in the spinal cord
Physical contact and vibration
Two up, one down Touch-related signals reach a particular
Sensory information from any of the patch of somatosensory cortex, and
body’s sensory receptors travels up we become aware of the sensation
one of two pathways to the brain—
the spinothalamic or the dorsal Gray and white matter
column–medial lemniscus routes. In contrast to the spinal cord, gray
Motor instructions travel down only matter (neuron bodies, dendrites,
one pathway—the nerves of the and synapses) sits on the outside
corticospinal tract. of the cortex, and the axon-rich
white matter lies within
Cerebrum
section

Medulla
section

Spinal cord
section

LOCATION OF CROSS SECTION
SECTIONS SHOWN OF THE CEREBRUM

KEY Somatosensory Tracts within the brain
Dorsal column– cortex A computer-colored scan shows nerve fiber tracts from
medial lemniscus tract cerebral cortex to brain stem in blue, from the brain’s
Motor cortex front (on the left) to its rear in green, and in the corpus
Spinothalamic callosum between the hemispheres in red.
tract Connection
or synapse
Corticospinal
tract

FUNCTIONAL MAPS Broca’s area Motor cortex Somatosensory
For speech production and Initiates the process cortex
To the naked eye, the cerebral cortex of conscious or Relates touch, pain,
appears much the same all over. But each articulation; named after voluntary movement and allied sensations,
patch of its surface has a designated code Pierre Broca, 1824–1880 mainly from the skin
CROSS SECTION Medial known as a Brodmann number (from (see p.320)
OF THE MEDULLA 1 to 52), devised by German neurologist Auditory cortex
lemniscus Korbinian Brodmann (1868–1918) Processes sound Visual cortex
Crossing over of tracts Ribbonlike and based on features of microscopic information Analyzes what we
In the upper spinal cord structure anatomy, such as how neurons are layered. (see p.316)
and the lower medulla, where Distinct from these numbers, but partly see (see p.315)
most nerve tracts cross the main overlapping with them, are cortical areas
over (decussate) to the sensory tract dealing with certain functions, such as the
other side of the body crosses over visual cortex for input from the eyes or
Broca’s and Wernicke’s areas for language.
“Live” brain scans using methods such
as PET (positron emission tomography)
and fMRI (functional magnetic resonance
imaging) are revealing ever more details
about how the cortex works.

Ventral root Cortical brain map Wernicke’s area Geschwind’s territory
Motor axons leave Major mental functions are localized in certain For understanding Connects Wernicke’s
the cord here to areas of the cerebral cortex. These areas do not and Broca’s areas;
take instructions work alone, they communicate constantly with spoken words; named after Norman
to the muscles each other and with inner brain parts. Some are named after
named for their function, while others reference Geschwind, 1926–1984
the scientists who discovered their function. Carl Wernicke,
1848–1905

306

HOW THE BODY WORKS

MEMORY AND EMOTION Brain areas involved in memory
There is no single “memory center.” Information is
Memory is not just the storage and recall of facts. It encompasses all processed, selected for memorizing, and stored in
kinds of information, events, experiences, and contexts—from names various brain parts. For the memory of a roller-coaster
to faces and places—and references our emotional state at the time. ride, for example, what we saw resides in the visual
areas, sounds in the auditory areas, and so on. These
Caudate nucleus Frontal lobe are pulled together to recall the whole experience.
Involved in learning and
Temporal Fornix
especially feedback to lobe Important in forming
modify procedural memories and recognition
of scenes and words
memories for actions
Putamen
Cingulate gyrus Involved in procedural
Deals with learning and memories and well-learned
physical skills
memory processing;
suppresses overly Thalamus
powerful reactions
and behaviors Parietal
lobe
Central executive
Coordinating area

that calls up information
from other parts and

formulates action plans

Hypothalamus
Links brain to hormonal
system; center for major

drives, instincts,
emotional reactions,

and feelings

Olfactory bulb
Preprocesses smells
(which are closely tied
to emotions) ahead of

olfactory areas

Pituitary gland
Chief hormonal gland;
responds to instructions
from the hypothalamus,

just above

TYPES OF MEMORY Mammillary bodies Hippocampus
Process and help to Screens experiences,
Current thinking describes five main recall memories, selects those to
kinds of memory. Working memory is the remember, and
short-term retention of information, such especially smells; also carries out
as a telephone number or the position recognition of long-term storage
of doors in a room, just long enough sensations
to be useful, before rapidly fading away.
Semantic memory is for detached facts, Amygdala Pons Cerebellum
independent of our personal existence, Central to the Serves as a
such as the date of a famous historical processing and recall switchboard
event. Episodic memory recalls episodes of the emotional connecting the
and events from our personal perspective, components of cortex and the
including our sensations and emotions, cerebellum
such as a happy birthday party. Procedural memories MEMORY TYPE
memory is for learned, well-practiced THALAMUS
physical skills, such as walking, bicycling, Memory-processing areas PARIETALWORKING
and tying shoelaces. Implicit memory For the four best-understood LOBESEMANTIC
affects us without our awareness, for types of memory, several brain CAUDATEEPISODIC
example being more likely to believe areas work in a coordinated NUCLEUSPROCEDURAL
something is true if we’ve heard it before. fashion. The thalamus is a MAMMILLARY
general gatekeeper and the BODY
frontal lobe, in particular, has an FRONTAL
overall executive capacity in LOBE
both learning and recalling most PUTAMEN
kinds of memories. AMYGDALA
TEMPORAL
LOBE
HIPPOCAMPUS
CEREBELLUM
CINGULATE
GYRUS
OLFACTORY
BULB
FORNIX
CENTRAL
EXECUTIVE

307

NERVOUS SYSTEM

HOW EMOTIONS AFFECT MEMORY

The “emotional brain” is a term often nerve signals to various brain parts Average working
applied to the limbic system, a group that then convey their own nerve signals memory holds five
of parts nestling on top of the brainstem, to various muscles, often through the words, six separate letters,
under and within the overarching autonomic nervous system (see p.297). or seven single numbers.
dome of the cerebrum. They include the For example, in response to a sudden Training memory, such
amygdala, thalamus, hypothalamus, fornix, scare, the hypothalamus takes control as reordering to assign
and mammillary bodies (see opposite), and tells the heart to beat faster, the a meaning, can usually
plus inward-facing (medial) areas of the skeletal muscles to tense, and the adrenal double this.
cerebral cortex and the cingulate gyrus glands to release epinephrine, ready for
that form a collar-shape around them. sudden action—the “fight or flight” Lasting memories
response. The hypothalamus also links Events such as our first day at school, first time
The limbic system takes the lead via a thin stalk to the pituitary gland (see riding a bicycle, and getting married involve
in deep-seated feelings and instinctive p.386) below it. This gland secretes various strong emotional components, such as anxiety
reactions that seem to well up inside us hormones and other substances that affect mixed with achievement, so the memories
during times of great emotion, and which other hormonal glands, to complement persist and stay “real.”
the rational-thinking parts of the brain and reinforce the nervous system’s actions.
may have trouble controlling. In particular,
the fingertip-sized hypothalamus—almost Several limbic parts are also intimately
at the anatomical center of the brain involved in memory formation, especially
—plays vital roles in powerful basic drives episodic memory (see opposite). This
for survival such as hunger, thirst, and fact explains why being in a state of high
sex, and the strong emotions that may emotion helps form strong memories
accompany them, for instance rage or at the time, and why we feel emotional
ecstatic joy. The hypothalamus sends out again when we recall such memories.

FORMING MEMORIES Sleep and memories EXTREME HUMAN
Electrical traces and scans show the
Each memory is formed by a unique pattern of brain is very active during sleep. With PEOPLE WHO CANNOT
connections between the billions of neurons in no distraction from conscious thoughts, FORGET
various parts of the brain, especially the cerebral the memory circuits may sift through
cortex. The event to be memorized—from reading a recent events, move some to longer-term Total recall, or hyperthymestic syndrome,
number to meeting a celebrity—occurs as a particular storage, and consolidate established is a rare condition in which people can
set of neurons sending impulses to each other during memories while we sleep. remember vast amounts of information,
the initial experience. Activating this set of signals again, from incredibly significant to numbingly
by remembering the experience, strengthens its pattern KEY trivial, for many decades. Even if they try to
of links so they are more likely to occur together—a forget, they cannot. But the memories tend
process known as potentiation. After several activations Brain activity levels, based not to be “total” in that, when questioned
the links become semipermanent. Triggering a few on the uptake of glucose about a past event, they may recall the
of them, by a new thought or experience, activates date, place, and what people said, but not
the pattern’s whole network and recalls the memory. HIGHEST LOWEST what they were wearing. Similarly, most
of their memories are centered on their
Neuron personal life and experiences, and less on
what was happening in the wider world.
Initial Hyperthymestic people show tendencies
input to obsessive–compulsive traits, such as
collecting memorabilia and keeping diaries.

Repeated Regular
input input

Existing link New link New link Connection not
reinforced enough,
1 Initial experience 2 Further modification
A stimulus causes one neuron to “fire” and Repeating the stimulus strengthens the initial so it is lost
send a particular string of nerve signals to the next link, or synaptic communication, and also recruits
one. This is part of the process of thinking and other neurons into the network. In reality, this 3 Consolidation, or not Hyperthymestic syndrome
being aware of a fact, experience, or learned skill. occurs with thousands of neurons. Regular use of connections both maintains One of the first people with hyperthymestic
them structurally and increases the strength of syndrome studied by scientists in the US, Jill
Price can recall every day since she was 14.
synaptic signaling between the neurons. Links that

are not refreshed regularly tend to fade and are lost.

308

HOW THE BODY WORKS

HOW WE MOVE Making a move Posterior
These views show with arrows which parts of our parietal
Every split second, the brain coordinates the precise brains are “talking to each other” during the execution cortex
tensing and accurate contraction of more than 600 of a simple sequence—Ready, Get Set, Go!
muscles all around the body, from full-speed running
to the blink of quick an eye. Such a huge task would
be impossible with every muscle under conscious
control, so the brain has a hierarchy of delegation.

VOLUNTARY MOVEMENT

A voluntary action is one we plan with Dorsolateral
awareness and carry out with purpose. frontal cortex
We may hardly be aware of turning a
book’s pages, or we might concentrate Auditory
on its every detail, but both are cortex
intentional. Central to these voluntary
movements is the motor cortex—a strip Putamen
of gray matter arching “ear to ear” on Thalamus
the brain’s outer surface (see also p.305).
Moving—part of everyday life It sends and receives millions of nerve Visual
The motor cortex works intimately with other areas impulses every second—even when we cortex
of the brain involved in movement, such as the do not move, because muscles are still
cerebellum (see opposite), so that we can move needed to hold the stationary body in READY ...
around almost without thinking. position or it would simply flop in a heap.
The visual and auditory brain centers relay sensory
Different patches of motor cortex deal information to the dorsolateral frontal cortex,
with instructions to certain parts of the which continually assesses the start time. The
body—it’s a similar “map” of size-related putamen feeds its memories and preparations for
specialization to that in the somatosensory well-rehearsed movement patterns to the posterior
cortex (see p.321). Parts that need intricate parietal cortex, whose activity is largely subconscious.
muscle control, such as the lips and
fingers, have a correspondingly larger
patch of motor cortex dedicated to them,
compared with those needing less refined
control, such as the thigh.

INVOLUNTARY MOVEMENTS— Duck and dive Sense danger Eyes blink
REFLEXES Protective reflexes, such as ducking to avoid a Long-term training and real-time Reflex 1: eyelids blink and
fast-approaching object, are rooted deep in our vision warn that a blow to the screw up to shield the eyes.
Most involuntary actions begin not at the conscious level, evolutionary past. Ducking is a cascade of four head is on the way.
but unintentionally. They happen automatically, although reflexes (see right) that are “learned” as one; the order Face turns
even as they start, we become aware of them and can reflects the journey the motor signals take from the Subconscious processing Reflex 2: neck muscles twist
start to modify them. Many involuntary actions are lower brain down the spinal cord to the body. Sensory information alerts lower the head to the side.
reflexes—set patterns of movements in response to levels of conscious, especially
a specific situation or stimulus. Reflexes such as lifting the thalamus. Head jerks back
the foot up after having stepped on a sharp object have Reflex 3: upper body muscles
survival value. They protect the body by carrying out a Motor output begins draw the head back.
fast reaction to danger, even if we are not paying attention. Motor areas organize all aspects
Reflexes receive sensory nerve messages about a stimulus, of the action a split second Hands throw up
“short-circuit” these through the spinal cord or the before awareness clicks in. Reflex 4: arm muscles raise
subconscious parts of the brain, and then send out motor hands for extra protection.
signals to initiate muscle action, without “permission” of
the conscious mind. As these nerve circuits quick-fire their
impulses, they also send signals up to the brain’s higher
centres where, a fraction of a second later, they register in
our awareness. We can then take over voluntary control.

Premotor Supplementary Motor 309
cortex motor cortex cortex
NERVOUS SYSTEM
Posterior
parietal cortex Motor
cortex

Dorsolateral Basal
frontal cortex ganglia

Thalamus Basal
ganglia
Cerebellum
Pontine
nucleus To muscles

... GET SET ... ... GO!

The dorsolateral frontal cortex formulates a The motor cortex gives the order. It has two-way
conscious impending intention to move; the command–feedback links with the cerebellum
posterior parietal cortex signals the same (via (itself linked to the pontine nucleus) and the basal
the basal ganglia). Both alert the thalamus to relay ganglia. The cerebellum fills in details of muscle
signals to the supplementary and premotor areas, coordination and relays back to the motor cortex,
which make “action plans” with the motor cortex. for output to the muscles.

THE “LITTLE BRAIN” to the spinal cord and then the body. The cerebellum
also has intimate relationships with other movement-
In some ways, the rounded, grooved cerebellum (“little controlling brain zones, such as the basal ganglia. Its chief
brain”) at the brain’s lower rear mirrors the dominating role is to fill in fine details of the broad instructions for
domed cerebrum above. Like the cerebrum, it has gray movements coming from the motor cortex, send these
matter formed of neuronal cell bodies, dendrites, and back to the motor cortex for detailed output to muscles,
synapses in its outer layer, or cortex, with an inner medulla and monitor feedback to ensure that all movements are
of mainly nerve axons (fibers), arranged in tracts or smooth, skilled, and coordinated.
bundles linking it to many other brain parts. The cerebellar
cortex is even more highly folded than the cerebral cortex. Recent research shows that the cerebellum is also
active in focusing attention onto a situation, and in
Its anatomical location allows the cerebellum to “see” speaking and understanding language.
all the sensory information on its way to the brain as well
as all the motor instructions on their way from the brain

The cerebellum is only 10 percent of the brain’s Cerebellum in cross section
volume, yet it contains more than twice the number The cerebellar cortex (palest yellow) is intricately folded around a
of neurons than the other 90 percent put together. multiple treelike branching system of nerve fiber tracts (red). At the
thickest “trunks” of the trees are clusters of neurons, or gray matter,
known as cerebellar nuclei, which are coordinating centers for the
massive inputs and outputs of motor nerve messages.

310

HOW THE BODY WORKS

HOW WE SENSE 1 2
THE WORLD 5 6

The brain itself is surprisingly insensitive. With hardly
any sensory nerve receptors of its own, it is incapable
of feeling that it is being touched or injured. However,
it is highly attuned to what happens in the rest of the
body—and in the world outside—through the work of
sense organs as they respond to many kinds of stimuli.

OUR MAIN SENSES of pain is handled differently by the
nervous system compared with other
The idea of five senses is oversimplified. sensations (see opposite).
Four of them and their stimuli are well
defined: vision using light rays (see p.312), The body also has internal sensory
hearing and sound waves (see p.316), receptors in muscles, joints, and other
smell involving airborne odor molecules parts (see Inner Sense opposite). But at
(see p.318), and taste from waterborne the simplest level, all sensory parts do
flavor molecules (see p.318). the same thing. Scientifically, they are
transducers, changing energy from their
Other modes of sensation are more specific stimuli into the nervous system’s
complex. Balance (see p.316) is less of common “language” of nerve impulses.
a discrete sense and more of an ongoing
process involving several senses A sensational world
simultaneously as well as the muscular We can imagine the main sensory inputs in
system. Touch is based in the skin, but these situations (clockwise from top left: ears,
not exclusively, and is a multifactored balance, tongue, nose, skin, and eyes), yet the
sense that responds not just to physical only actual stimulus here is light for vision.
contact but also to vibration and to
temperature (see p.320). The sensation

SYNESTHESIA also bring on a taste of cheese, while sardines
are tasted while listening to certain instruments
In normal sensory nerve pathways, messages play. This condition is known as synesthesia
travel from a sense organ to specific regions and affects about 1 person in 25, although
of the brain, especially to the cerebral cortex, to varying degrees. Synesthesia can also be
where they enter conscious perception. brought on by certain chemicals, especially
Signals from the eyes, for instance, end up perception-altering or psychedelic drugs.
in the visual cortex, and so on. Rarely, these
pathways diverge and connect to other Painting by music
sensory brain regions. In such cases a person British artist and synesthete David Hockney said,
may experience more than one kind of of designing the sets for the LA Opera, that the
sensation from a single type of stimulus. colors and shapes “just painted themselves”
For example, seeing the color blue may when he listened to the music.

311

NERVOUS SYSTEM

3 HOW WE FEEL PAIN Initiation of pain
Injury causes the release of chemicals such as
7 Pain is a sensation that is very difficult prostaglandins and bradykinin, which prompt
5 to measure objectively. We have a set nociceptors to initiate pain signals.
3 of terms to describe it, such as aching,
1 stabbing, burning, and crushing. Pain Spinal cord
begins in specialized nerve endings— Nerve signals travel in pain-related axons
7 nociceptors—in the skin and in many (fibers) into the dorsal horn of the spinal cord
other body parts. When nociceptors for onward transmission.
or tissues are damaged they release
substances such as prostaglandins, Brain stem
adenosine triphosphate (ATP), and The signals pass via the medulla and activate
bradykinin. These stimulate the the sympathetic division of the autonomic
system (see p.297).
4 nociceptors to transmit pain signals.
The signals follow a different pathway
from touch or other sensations from
that body part (see p.304), especially
in the spinal cord. Most end up in the
cortex of the cerebral hemispheres,
where we perceive them as pain
related to a particular body part.

6 Midbrain

4 Whole-brain pain Pain-registering regions monitor the signals
2 Left: These fMRI scans show sequential horizontal and trigger the release of the body’s own
“slices” up through the brain of a healthy person analgesics in the brain stem and spinal cord.
being subjected to a painful stimulus. The yellow

areas show brain activity, reflecting how widely

pain is dealt with by different parts of the brain.

Pain pathways Cerebral cortex
Right: In all sensations, nerve signals take time to Signals reach several areas of the cerebral
travel from their receptors to the brain and enter cortex. The pain is felt consciously and
our conscious awareness. In the time gap of a regionalized to a body part.
second or so, damage could already be advanced.

INNER SENSE BLOCKING PAIN and spread in the blood and nervous system. They affect
AND SENSATIONS transmission of nerve signals carrying pain information by
Without looking or touching, we know where interfering, for example, at the level of synapses (see p.300)
our arms and legs are, if we are upright or lying Despite its unwanted nature, pain has survival value as by preventing the production of certain neurotransmitter
down, what our posture is like, and how we are it warns us that a part of our body is in trouble, that any chemicals or blocking receptor sites, so that impulses do
moving through space. This body sense is known potential cause of the pain should be spotted and removed, not continue in the receiving neuron.
as proprioception; it makes us aware of our and that the part should be protected and rested so it can
position and movements. heal. The body has its own pain-reducing or analgesic Levels of relief
substances, principally the endorphins group, which are Pain messages travel to the higher brain centers along a series
Proprioception relies on internal sensory parts, released by the brain’s hypothalamus and pituitary gland of neurons and their synapses. So, there are several opportunities
mostly microscopic, known as proprioceptors. to block these pathways and lessen the perceived pain.
There are many thousands spread throughout
the body, being especially numerous in muscles PAINKILLERS HOW THEY WORK ANESTHETICS HOW THEY WORK
and tendons, and in the ligaments and capsules GENERAL
of joints. They respond to changes in tension, OPIOIDS Like endorphins, these work mostly within the ANESTHETICS Act primarily on the brain but also affect the
length, and pressure in their particular area, (for example, central nervous system and inhibit the brain’s spinal cord, causing muscle relaxation and
such as when a relaxed muscle is stretched. Such morphine) conscious ability to perceive pain.  producing loss of consciousness; precise
information is integrated with signals concerning mechanisms are unclear.
changes of orientation and position in space, for
example, via hair cells in the vestibule and the ACETAMINOPHEN This analgesic is similar to a weak opioid. LOCAL Impede peripheral nerve impulses in a specific
semicircular canals in the inner ear (see p.316). It inhibits prostaglandin formation and also ANESTHETICS part, for example, by blocking sodium channels
affects formation of the neurotransmitter in neuron membranes (see p.299) to reduce all
As the proprioceptors are stimulated, they AEA (anandamide), mainly within the sensory information.
send streams of nerve signals through the central nervous system. 
peripheral nervous system to the brain. For
example, messages coming from proprioceptors NSAIDS Ibuprofen and other NSAIDs suppress the EPIDURAL Injected into the cerebrospinal fluid around
in the biceps muscle of the upper arm inform formation of certain prostaglandins that would ANESTHETICS the dura mater (the outermost of the meninges
the brain that they are being compressed and (nonsteroidal otherwise produce pain sensations. They work surrounding the spinal cord) to quash all
shortened, meaning that the elbow is bending. anti-inflammatory mainly in the peripheral nervous system. sensations felt from below the site of injection.
drugs)

312 Lens refracts and Light rays cross Inverted image
fine-focuses over inside eye is smaller than
HOW THE BODY WORKS light rays object in view

HOW WE SEE Light rays Cornea refracts
reflected light rays
For most people, vision is the most important sense. from object
Using information in the form of light rays, gathered A
by our eyes, the brain creates clear images of the world
allowing us to experience our surroundings. B

THE VISUAL SYSTEM Optic
nerve
Cushioned within sockets in the skull, their surfaces
washed by tears and wiped by blinking of the eyelids, the eyeball. As in a modern camera, the process of focusing Image production
eyes relentlessly scan the surroundings to collect light rays is automatic, as is the adjustment of the size of the iris, Refracted by the cornea and lens, light rays
reflected or generated by objects in view. Those rays enter which controls the amount of light entering the eye. When cross over and create on the retina a sharply
the eye through a clear, bulging window, the cornea. light hits the retina’s photoreceptors, they generate billions focused, upside-down, and back-to-front
Aided by the adjustable lens behind it, the cornea focuses of nerve impulses that stream along the optic nerve to image of the object in view.
light rays onto the retina, the thin layer of light-sensitive the visual areas at the back of the brain. Here signals are
receptors that lines the inside of the rear part of the analyzed to give a mental impression of what we are
looking at, where it is, and whether or not it is moving.

BENDING LIGHT projected onto the retina. The cornea does most of the Cornea
light bending, but its shape and, therefore, refractive Domed transparent
Light rays usually travel between objects in a straight line. powers, cannot be altered. It is the elastic lens that
When they pass through both the cornea and the changes shape to fine-focus light (see opposite). membrane that
transparent lens they are bent, or refracted. As a result of covers front of eye
refraction, a clear, inverted view of the outside world is
and refracts light

LIGHT REFRACTION Convex lens Point of greatest refraction
Light rays refracted
When light rays pass from one transparent by a convex lens are Light rays
medium to another they bend, or refract. focused on a single converge
This is the case when light enters and leaves focal point. The thicker
the eye’s lens, which is convex—curving the lens, the more the Rays intersect
outward on both surfaces. The greater the light rays are refracted. at focal point
angle at which light hits the surface of the Light rays enter lens
convex lens, the more it is refracted inward.

BRIGHT LIGHT NORMAL LIGHT DIM LIGHT LIGHT CONTROL
Pupil is
constricted Pupil is The eyes can operate in most light conditions
dilated because of a control system that automatically
and unconsciously regulates the amount of light
Circular Radial muscle entering through the hole at the center of the iris, the Inner iris
muscle fibers fibers contract pupil. The iris, the colored part of the eye, has two layers This colored electron
contract of muscle fibers: concentric circular fibers, and radial micrograph shows the inner
fibers arranged like the spokes of a wheel. These muscles surface of the iris (pink). To the
contract on signals from the autonomic nervous system right (dark blue) is the edge of
(see p.297). The system’s opposing parasympathetic and the pupil, and the folded
sympathetic branches ensure that the pupil shrinks in structures in the center (red) are
bright light to avoid dazzling, and expands in dim light to the ciliary processes.
allow enough light into the eye to make vision possible.

Narrow pupil Normal pupil Wide pupil Under normal conditions, the pupils of both
Stimulated by parasympathetic In normal light conditions both Stimulated by sympathetic eyes respond identically to a light stimulus,
nerves, circular muscle fibers in circular and radial muscle fibers nerves, radial muscle fibers in regardless of which eye is being stimulated.
the iris contract to make the pupil partially contract. The pupil is the iris contract to make the pupil
narrow—less light enters the eye. neither too wide nor too narrow. wider—more light enters the eye.

Ciliary muscle
Ring of muscle
that alters lens

shape

ACCOMMODATION of the cornea is not adjustable. For close
vision, the ring of ciliary muscle surrounding
However close or distant objects in view the lens contracts and shrinks, the ligaments
might be, the eyes employ an automatic, suspending the lens go slack, and the elastic
fine-focusing mechanism to project an lens bulges. For distant vision, the ring of
image that is sharp, not blurry, onto the ciliary muscle relaxes and widens, pulling
retina. This adjustment process, called the suspensory ligaments taut so that they
accommodation, involves changing the stretch the lens and make it thinner.
shape and, therefore, the light-bending
capability, of the lens; the refractive power

Pupil
Hole in iris that becomes
narrower in bright light

Lens Highly divergent Lens rounded
Transparent, bulging light rays
disc of tissue that
changes shape for
near or far vision

NEAR VISION Image focused
on retina
Suspensory ligaments
Hold lens within the ring Close objects
of ciliary muscle Light rays from close objects diverge sharply as
they approach the eye. A thicker lens is required
Iris to refract those rays sufficiently to focus a sharp
Ring of muscle that image on the retina.
changes size of pupil to
regulate amount of light Almost parallel
entering the eye light rays

DISTANT VISION Distant objects
Light rays entering the eye from
distant objects are relatively Lens
parallel. A flatter, less curved lens flattened
is needed to refract and focus
these parallel rays precisely
on the retina.

Vitreous humor Rod cells RETINA AND FOVEA
Jellylike fluid that gives The cylinder-shaped rods,
shown in the image above, The retina has an area only twice that of a thumbnail, yet
bulk, shape, and it provides our amazingly detailed, colorful view of the
transparency to the cannot discriminate colors; world. It lies against another layer of the eyeball, the
they respond to most choroid, and is itself multilayered. The retina’s outermost
eyeball’s interior wavelengths of light in the layer contains photoreceptive cells called rods and cones,
which generate nerve signals when light energy falls on
Retina same way as brightness them. The 120 million rods are found mainly toward the
Innermost layer with detectors. When light front of the retina, and the five million cones largely at the
photoreceptive and above a certain intensity rear. Cones are concentrated at the fovea or yellow spot,
a small patch where the part of an image that we want to
other cells strikes a rod, it produces scrutinize in most detail falls. There are three types of
nerve signals. cone cell—red, green, and blue—that allow us to see in
Macula color. Each responds to a certain wavelength or color of
Area of dense Sclera light ray, and their combined nerve signals are analyzed
rod and cone cells Tough outermost by the brain to produce the millions of colors we
sheath of the eyeball perceive. Cones need more light to respond than rods. As
Fovea light dims, our cones work less well and rods provide most
Small pit-shaped area of the visual information, so the scene tends to “gray out.”

of most densely 3 21
packed cone cells for
greatest visual acuity Ganglion Bipolar
cell cell
Choroid
Blood-rich layer that Light ray Horizontal
nourishes the retina cell
Inner surface
and sclera of retina Back
of retina
BLIND SPOT Amacrine
cell Rod cell
Where the axons of ganglion cells come
together to form the origin of the optic Bundled Cone
nerve, there are no rods and cones. So this axons cell
patch of retina, the optic disc, cannot
respond to light and causes a “blind spot.” Optic
The brain gets used to this dark zone and nerve
uses information from areas immediately
around it to fill in what is probably there. SEEING CELLS
Also, axons and blood vessels on the
retina’s inner layer shade many rods and 1 Rods and cones react to light
cones beneath from incoming light. Again, Light must pass through the first layers of the
the brain is adept at filling in these gaps. retina to reach the light-sensitive cells. Substances
called visual pigments in these cells change shape
Optic disc Receiving light Blood vessels as they are energized by photons or packets of
The disc appears as the pale patch (left) in the The paper-thin retina lies tightly on the Form a branching light, producing a change in membrane polarization
image above. The fovea is in the center of the adjacent choroid layer. Light rays pass network on the inner that starts a nerve signal (see p.298–301).
dark red macula, with blood vessels also in easily through the vitreous humor—an surface of the retina
red. To avoid blank areas the eyes dart around ultra-transparent, gel-like fluid filling the 2 Bipolar–horizontal layer
a scene and the brain guesses what’s there. bulk of the eyeball—to be focused Optic nerve Inside the rod-and-cone layer is a layer
exactly on the retina, with the central Bundle of about one of long, slim bipolar cells, cross-connected by
part of the scene on the fovea. million nerve axons horizontal cells. This is the part of the retina’s
(fibers) carrying neural network that provides initial processing
messages to the brain of the impulses generated by the rods and cones,
adding up or summing them into fewer signals.

3 Ganglion–amacrine layer
Within the bipolar layer are ganglion cells,
with cross-connections of amacrine cells. These
continue the simplifying of impulses from the rod
and cone cells, and send them out along their
nerve fibers or axons, which are bundled together
on the retina’s innermost surface and gather
to form the optic nerve.

Optic radiation 315
Fanlike nerve axon
tracts from thalamus NERVOUS SYSTEM

direct to primary Visual cortex
visual cortex Analyzes nerve
signals for visual
VISUAL PATHWAY Thalamus information
Lateral geniculate
Although the eyes are in front of the brain, the cerebral nuclei relay visual Optic chiasma
areas that process their information are located at the signals and also link Half the axons
rear. Nerve impulses from the eyes pass along the million to other sensory from each optic
nerve cross over
inputs here

or so axons (nerve fibers) of each optic nerve. These Optic nerve
About one
two nerves converge in the underside of the brain at Right visual field million axons
the optic chiasma, where about half of the fibers from carry nerve
impulses from
each cross to the other side. Next, each set of fibers the retina into
the brain
passes to a dedicated area known as the lateral
Right visual field
geniculate nucleus in the thalamus (see
Binocular visual field
p.302). This screens the information
Image formed
for relevance to what is going on in the on right retina

conscious mind and for links to other Right optic nerve
Optic chiasma
senses. Axons from each nucleus then Retinal cells Right optic tract
Convert light energy Thalamus
fan out through the brain tissue, to nerve signal energy Right cerebral
and preprocess initial hemisphere
as the optic radiation, to the primary information Right visual cortex

visual cortex at the lower rear

of the brain. Here the information Left
is initially processed, sorted, and then visual field

partitioned to other areas of the brain.

These include zones of secondary visual

cortex around the primary cortex, which Look, then see

discriminate features such as lines, angles, Both eyes angle toward an object in
colors, shapes, and movements, and the center of the visual field. The nerve
the temporal lobe on the side of the brain signals produced travel along a three-
for recognition of familiar objects. stage pathway before being analyzed
and consciously perceived.

DEPTH AND DIMENSION Left visual field

We experience the visual field in three dimensions, with Combined image
depth, and can determine whether one object in a scene composed by brain
is closer than another. The brain achieves this by
combining information from many varied sources. Image formed
on left retina
Memory is important. We recall that mice are small
and elephants are big. Linking this to relative size in the Retina
visual field, we expect a mouse we see as large to be
closer than an elephant that appears smaller. Movements Left optic
in and around the eye when viewing objects also supply nerve
information on their distance. The more the two eyes
angle inward as detected by sensors in the eyeball- Seeing 3-D Left optic
moving muscles, and the more the lens bulges, due An object in the binocular tract
to ciliary muscle contraction, the closer the object. visual field is seen by each
eye at a slightly different Left cerebral
The fact that we have two eyes and the visual pathways angle. This means that the hemisphere
swap information left to right also plays a part. Each eye view of the image received
has its own visual field, which overlap in the middle to by each side of the visual Optic
form the binocular visual field. Nerve fibers cross at the cortex from each eye, radiation
optic chiasma, so the left part of the visual field of each is different. By combining
eye ends up in the left visual cortex, and the right half in and comparing the views Left visual
the right visual cortex. The brain then compares the differing the brain can judge depth. cortex
views from each eye, known as spatial binocular disparity.

17,000

The average number of times the
human eye blinks each day—
that is once every five seconds.

316

HOW THE BODY WORKS

HEARING AND BALANCE

Our ears greatly complement our eyes in providing vast amounts of The cochlea
information about the world around us—indeed, we can often hear what Three fluid-filled ducts spiral within the cochlea and
we cannot see. Balance is anatomically adjacent to hearing, and employs carry sound vibrations. The outer scala vestibuli and
similar physiological principles, but has no direct connection. scala tympani connect at the apex, or point,ß of
the spiral. Between them is the cochlear duct,
divided from the scala tympani by the basilar
membrane bearing the organ of Corti.

HOW WE HEAR Sound waves arrive Vibration
Air pressure waves are
Sounds consist of areas of alternating high and low funneled by the outer ear Tympanic
pressure, called sound waves, propagating through air. flap, or pinna, into the slightly membrane
The auditory sense allows us to perceive sounds in the S-shaped external acoustic (eardrum)
mind through a series of conversions. The first occurs meatus (canal). They bounce
when sound waves hit a skinlike sheet, the tympanic off the tympanum, which is Suspensory ligament
membrane (eardrum). These pressure waves then pass about the size of the little Incus (anvil)
from the eardrum through the middle ear, causing fingernail, causing it to vibrate.
vibrations along a chain of the three smallest bones in Vibration
the body, called the ossicles. The last ossicle butts against Sound External Stapes
another flexible membrane, the oval window, set into waves acoustic meatus (stirrup)
a fluid-filled chamber in the inner ear. The vibrations (outer ear canal) Oval
change into waves of fluid pressure rippling through the window
snail-shaped cochlea. Within the cochlea lies the organ Middle-ear vibrations Malleus
of Corti, containing a fine membrane in which hair cells The tympanum is connected (hammer)
are embedded. The vibrations distort these hairs, causing to the first ossicle, the malleus.
them to produce nerve signals. These signals pass along Vibrations proceed from here Tympanic
the cochlear nerve, which becomes part of the auditory through the air-filled middle ear membrane
nerve, to the brain’s auditory cortex—just under the skull, cavity, along the incus, and then (eardrum)
almost alongside the ear itself. Here the nerve impulses to the stapes. The base of the
are analyzed to gauge the frequency (pitch) and intensity stapes presses against the Sound
(loudness) of the original air pressure waves—and we hear. membrane of the oval window, wave
and as it vibrates, it pushes and
pulls against the window.

BALANCE the ground, the skin registers pressure as we Ampulla Cupula Fluid Cupula
lean, and muscles and joints detect levels of Hairs swirls bends
Balance is an ongoing process, coordinating many strain (see proprioception, p.311). Balance Macula
sensory inputs. It does this largely at subconscious levels, information comes from the fluid-filled organs Hairs rotated
with outputs to muscles all over the body, enabling us to in the inner ear, via the vestibular nerve. deflected
retain our poise and adjust our posture. For example, Gravity pulls
vision monitors the head’s angle to horizontals such as Fluid membrane
Otoliths (mineral crystals)
Lateral canal Macula
of utricle cover membrane
Posterior Membrane
canal Macula Hairs
of sacule Hair cell
Ampullae
Vestibular
nerve

Superior Responding to
canal movement
The utricle and saccule have a
Utricle patch of hair cells, the macula,
the hair tips of which are set
Vestibule Saccule into a membrane bearing
mineral crystals. The pull of
Organs of balance gravity on the membrane
depends on the position of
Three semicircular canals, each at right angles the head. At one end of each Utricule and saccule Semicircular canals
semicircular canal is a wide With the head level, gravity pulls evenly on the A head movement makes the fluid in at least one
to the others, detect head movements. Two area, the ampulla, with hair membrane. As the head nods, gravity tugs it and canal swirl around. This disturbs the cupula and
cells set into the cupula. distorts the hairs, whose cells produce nerve signals. bends the hair cells, generating nerve impulses.
neighboring chambers, the utricle and saccule, are

more specialized for the head’s static position.

317

NERVOUS SYSTEM

Facial nerve SCIENCE Primary
auditory cortex
Vestibular nerve RANGE OF HEARING

Cochlear Our ears detect a range of sound Corresponds
(auditory) nerve frequencies (pitches), from a very to base of
deep 20 Hz (vibrations per second) to cochlea
Scala a shrill 16,000 Hz. Frequencies above
tympani (ultrasound) and below (infrasound) 16000 HZ
(tympanic cannot be heard by people. However, 8000 HZ
canal) hearing range varies among 4000 HZ
Scala individuals and reduces with age, 2000 HZ
vestibuli especially for higher frequencies. 1000 HZ
(vestibular
canal) 500 HZ

Perceiving sound Corresponds to
frequencies apex of cochlea
The cochlea responds best to
lower frequencies at its tip and
higher ones toward the base.
This is mirrored from front
to back along the primary
auditory cortex, which is
the brain’s hearing center.

THRESHOLD OF HEARING (dB) 80

70

Incoming 60 “Middle C” Top of hearing
vibrations 50 is at 262Hz range; above
Travel from
oval window 40 this is ultrasound
along scala
vestibuli 30 Bottom of
hearing range;
Residual 20
vibrations
Vibrations spiral 10 below this is
back along scala
tympani to round 0 infrasound
window
-10
Helicotrema
Apex of cochlear -20
spiral 7.8 15.6 31.2 62.5 125 250 500 1000 2000 4000 8000 16,000

FREQUENCY (HZ)
Audiogram
An audiogram is a graph that shows the hearing threshold level of the softest
sounds a person can hear at different frequencies. It reveals that the ear is
most sensitive to sounds in the middle frequency range, such as speech.

Cochlear duct Cochlear nerve Scala Organ of Corti
The basilar membrane
Stereocilia fibers tympani bears inner and outer rows
Protrude from tip of of thousands of hair cells. The
Eustachian tube Vestibular ganglion hair cells and bend in Inner hair Tunnel of Corti tips of their hairs, or stereocilia,
response to vibrations cells Outer hair cells are embedded in the tectorial
Inside the cochlea Vestibular nerve Tectorial membrane membrane. Pressure waves
A cutaway of the cochlea Cochlear make the membranes vibrate,
shows how the ducts curve nerve Hair cell tips are bending the hairs so their cells
around its central cone embedded here generate nerve signals.
of bone, the modiolus,
and how nerve fibers Modiolus Basilar
from hair cells are Vibrations membrane
bundled within Membrane along
this as the spiral which organ of
ganglion. Corti is located

Stapes in Modiolus Cochlear
oval Spiral ganglion duct
Scala tympani
window Scala vestibuli Reissner’s membrane
Divides cochlear duct
Round and scala vestibuli
window
allows for Scala vestibuli
exapansion Conveys vibration to
of fluid in basiliar membrane
cochlea
Cochlear duct

318

HOW THE BODY WORKS

TASTE AND SMELL Dura mater Mucus-secreting
Glomerulus gland
The senses of taste and smell both detect chemical substances, are
adjacent, work in similar ways, are fine-tuned for survival value, and Olfactory bulb
seem inextricably linked as we enjoy a meal. Yet until their sensations
reach the brain, there is no direct connection between them.

HOW WE SMELL Epithelial cells Ethmoid bone
Separated by smooth supporting cell ends, tufts of Nerve fiber (axon)
Smell particles, or odorant molecules, are detected by the cilia, each from an olfactory receptor cell, dangle
olfactory epithelia—two patches, each thumbprint-sized, from the surface of the olfactory epithelium. Basal cell
one in the roof of each nasal cavity, left and right. These Receptor cell
epithelia contain several million specialized olfactory Supporting cell Air flow Mucus
receptor cells, whose lower ends project into the mucus
lining the nasal cavity and bear hairlike processes, called Cilia Odor molecule
cilia, on which are located receptor sites. When suitable
odorants dissolve in the mucus and stimulate receptor Cilia Olfactory epithelium
sites, the cells fire nerve impulses. This may happen when Receptor cells send signals along their axons, through holes in
an odorant fits onto a site like a key in a lock. But there is the skull’s ethmoid bone, to the olfactory bulb. This outgrowth
also a “fuzzy coding” component that is less understood, of the brain processes signals at ball-like groups of nerve
where each odor produces a variable pattern or signature endings (glomeruli) and sends them along the olfactory tract.
of impulses. Smell information is analyzed by the brain’s
olfactory cortex, which has close links with limbic areas,
including emotional responses. This is why smells can
provoke powerful recollections and feelings (see p.307).

HOW WE TASTE combinations of five main tastes—these being sweet, salty, Up to three-quarters of what we
savory (umami), sour, and bitter. Most of these are think of as taste is a combination
Like smell, taste or gustation is a chemosense. Its stimuli detected equally in all the parts of the tongue furnished of taste and smell perceived
are chemical substances, in this case taste molecules with taste buds. A similar “lock and key” system to smell simultaneously—blocking off the
dissolved in food juices and the saliva that coats the (see above) probably works for gustation, with receptor nose makes foods taste very bland.
tongue and the inside of the mouth. The main organ for sites for different taste molecules located on the hairlike
taste is the tongue, which has several thousand tiny cell processes of gustatory receptor cells in each taste bud.
clusters called taste buds distributed mainly on its tip and
along its upper sides and rear. The buds detect different

Lingual tonsil Vallate Taste pore
papilla Taste hair
Vallate
papilla Filiform
papilla

Filiform Tongue Supporting
papilla epithelium cell

Foliate Fungiform Nerve fiber
papilla papilla

Fungiform
papilla

Taste bud

The tongue Mucus- Nerve fiber Papillae Epithelium Gustatory
The upper surface has projections secreting Vallate papillae are large of tongue receptor
called papillae, most bearing taste domes; filiform ones are cell
buds on and around them. Vallate gland slimmer with branched tips;
papillae form a V shape across the foliates are like folded leaves; Taste buds
rear. Papillae help to grip and abrade and fungiform papillae are Each bud has 20–30
food and move it when chewing. mushroom-shaped. receptor cells with hairs
projecting into a surface
gap, the taste pore.

Amygdala Retronasal
Distributes warning flow
messages if odors or Odors carried
tastes are associated with into rear of
fear, such as a burning smell nasal cavity on
normal expired
Olfactory tract airflow
Carries smell signals
from olfactory bulb Odor
molecules
to olfactory cortex in food

Olfactory bulb

Olfactory epithelium
Patch rich in olfactory
receptor cells

Nasal cavity

Odor in
expired air

Orthonasal smell
For this type of smelling, air
comes directly from outside,
in through the nostrils. Sniffing
sucks in more odor molecules
and makes the air swirl upward,
nearer the olfactory epithelia.
A quick sniff is an automatic
or reflex action when a smell
catches our attention, to take
in more odor molecules.

Orthonasal flow
Molecules enter

through each nostril

Airborne odor
molecule

Retronasal smell Facial nerve
Air enters the nasal cavity from the rear, carried Conveys nerve
up from the mouth below on the regular exhaled signals from taste
flow from the lungs. It carries odor molecules buds on front
released by chewing foods. The sensory inputs
to the brain from it coincide with taste, creating of tongue
a full range of olfactory–gustatory flavors.
Glossopharyngeal
WHY DO WE EXPERIENCE nerve

DISGUST? Conveys nerve signals
from taste buds on
Both smell and taste are situated at rear of tongue
the entrance to the digestive tract
and monitor food being chewed and
drinks before swallowing. Worrying
odors and flavors such as rottenness,
contaminating fecal matter, or
intense bitterness warn that food
may be bad, infected, or unpalatable.
The resulting reactions of grimace,
nostril-wrinkling, and gagging in
disgust make it very difficult
to eat.

320 Somatosensory cortex Foot to brain
Left side receives touch A touch on the foot sends nerve
HOW THE BODY WORKS signals from right signals along peripheral fibers
side of body in the leg to the spinal cord,
then up to the brain stem. Here
TOUCH Medial lemniscus the fibers cross over, right to left,
Fibers cross over to in the medial lemniscus and
continue up to the thalamus
other side here and the brain’s somatosensory
cortex (see opposite).
Touch does far more than detect physical contact. It tells us Spinal cord
about temperature, pressure, texture, movement, and bodily Carries signals up Ganglion
location. Pain seems to be part of touch, but it has its own Concentration of
dedicated receptors and sensory pathways. ascending tracts neuronal (nerve
into brain stem cell) bodies

TOUCH PATHWAYS Under pressure Sacral plexus
The largest skin receptors are Pacinian corpuscles, Nerve junction
The skin contains millions of touch receptors of different about 1/32 in (1 mm) long. They register changes where
kinds, including Merkel’s discs, Meissner’s and Pacinian in pressure and fast vibrations in particular. information is
corpuscles, and free nerve endings (see p.279). Although shared and
most receptors show at least some reaction to most kinds coordinated
of touch, each kind is specialized to respond to certain
aspects of touch. Meissner’s corpuscles, for example, Lateral branch
react strongly to light contact. The more a receptor is of tibial nerve
stimulated, the faster it produces nerve impulses. These Carries nerve
travel along peripheral nerves into the central nervous impulses up leg
system at the spinal cord, then along the dorsal column–
medial lemniscus tract (see p.304) to the brain, which Stimulus
figures out the type of contact from the pattern of impulses. Light touch on
skin of outer heel
SPINAL NERVES Dermatomes
Each spinal nerve carries sensory information
Snaking out from the spinal cord, through the narrow via its dorsal root into the spinal cord from a
gaps between adjacent vertebrae, are 31 pairs of spinal specific skin area or dermatome. Facial skin
nerves (see pp.148–49 and 178–79). They divide into (V1–3) is served by cranial nerves (see p.114).
smaller peripheral nerves that extend to all organs and
tissues, including skin. Most of these nerves carry both FRONT VIEW REAR VIEW
sensory nerve signals about touch on the skin to the
cord, and motor signals from the cord to muscles. V1 C2
V2
Cervical region C3 V3 C3
Eight pairs of cervical C4 C2 C4
nerves serve skin C5
covering the rear C6
head, neck, shoulders,
arms, and hands T1-12

Thoracic region T2–12 C7
Twelve pairs of C5
thoracic nerves C8
connect to skin on C6 L1
chest, back, and T1 L2
underarms L3
L4
Lumbar region C7 L5
Five pairs of lumbar C8
nerves serve skin on S1
the lower abdomen, L1 S2
thighs, and fronts of S3
the legs S4
S5
Sacral region L2
Six pairs of sacral S2 L1
nerves connect to S3
skin on the rear of L3 L2
the legs, feet, and L4
anal and genital L5 L3
areas L4
S1 S1
Spinal regions S2
Each pair of spinal nerves, from L5
the upper neck to the lower
back, links to one of four
specific regions of the body.

321

NERVOUS SYSTEM

THE FEELING BRAIN Hand Arm Head Trunk

The main “touch center” of the brain is the primary LOCATOR Leg
somatosensory cortex. It arches over the outer surface
of the parietal lobe, just behind the motor cortex. It has Fingers Foot
two parts, left and right. Because of the way nerve fibers and thumb Toes
cross to the other side in the brain stem (see opposite),
the left somatosensory cortex receives touch information Eye Genitals
from the skin and eyes on the body’s right side, and vice Face
versa. Touch information starting as nerve signals from a Sensory homunculus
particular body region, such as the fingers, always ends up Lips If body parts are modeled according to their
at a corresponding dedicated region of the somatosensory sensitivity to touch—in other words, the relative
cortex. Skin areas with more densely packed touch Tongue area they have in the somatosensory cortex—the
receptors, giving more sensitive feeling—as in the result is the figure known as a sensory homunculus.
fingers—have proportionately larger regions of cortex.

Touch map
The surface of the somatosensory cortex has been mapped to skin
areas. The order of these, from the lower outer side, up and over to
its medial or inner surface, reflects body parts from head to toes.

EXPERIENCING PAIN suffers physical injury or microbial infection (see p.311). Mast cell with
The nociceptors send their nerve signals into the spinal histamine
Pain information comes from a class of receptors, called cord along specialized nerve fibers (axons) of two main Mast cells are scattered
nociceptors, present not just in skin but throughout the kinds, A-delta and C. Instead of crossing to the opposite throughout tissues and
body. However, the skin has the highest numbers, so we side up in the brain stem, as for touch (see opposite), pain play roles in inflammation
can localize a pain here more easily—in a fingertip, for information moves to the opposite side at its entry level following injury, and in the
example—whereas pain within organs and tissues is vague in the cord (see pp.304–305). The signals then pass allergic response. When
and difficult to pinpoint. Nociceptors respond up the spinal cord to the medulla and thalamus, where damaged or involved in
to many kinds of stimuli, such as temperature extremes, automatic reactions such as reflexes are triggered. fighting microbes, they
pressure, tension, and certain chemical substances, release granules (dark
especially those released from cells when the body purple in this micrograph)
containing heparin and
C-fiber Inflammatory “soup” Granule histamine. Heparin
Lacks myelin An “insult” to the body breaks tissues and damages cells, which prevents blood clotting
insulation, impulses release various substances into the general extracellular fluid to and histamine increases
travel slower cause inflammation and begin repair. Several of these substances, blood flow and swelling.
such as bradykinin, prostaglandins, and ATP, stimulate nociceptors.
A-delta fibers ATP
Myelin sheath Tissue injury Dermis Damaged Epidermis
increases speed membrane K+
of nerve signals releases
chemicals Mast cell releases
histamine
Nociceptor (pain Histamine
receptor) at site
of injury Bradykinin

ATP and K+ break Bradykinin and ATP
down to form bind to nerve
bradykinin receptors

Prostaglandin released Nerve endings
by damaged cells release substance
P, stimulating other
Histamine causes nerves to do the
capillary to swell same, causing redness
at site of injury
Pain fibers Blood vessel
Dedicated sensory nerve fibers convey pain information toward the Red blood cell
brain. A-delta fibers have myelin sheath insulation, carry impulses fast
and serve a small area, usually a 1 mm² patch of skin. C-fibers are
more widespread and diffuse and their impulses are slower.

NOSE TRACHEA

Air usually enters the body via This main airway, also known as
the nostrils, which open into the the windpipe, channels air from
nasal cavity. The linings of both the nose and throat to deep
help filter out dust particles. within the lungs.

LUNG

The highly branched “tree” of
tubes in each lung end at millions
of balloonlike alveoli where
gas exchange takes place.

Every living cell in our bodies requires a RESPIRATORY
constant supply of oxygen and the removal of SYSTEM
waste carbon dioxide. The respiratory system
brings air from the atmosphere into the body
so that this vital exchange of gases can occur.

324 Frontal sinus Respiratory passage
The deliverance of oxygen into
HOW THE BODY WORKS the lungs, and the reciprocal
expulsion of carbon dioxide is
Sphenoidal a process known as respiration.
sinus
Pharynx
JOURNEY OF AIR Conchae Epiglottis
Larynx
The respiratory tract is responsible for transporting air Vocal cords
into and out of the lungs, and for the essential exchange of
oxygen and carbon dioxide between the blood and the air in
the lungs. It also protects the entire body by providing key lines
of defense against potentially harmful particles that are inhaled.

AIR FLOW 20.9% 0.06% 0.4%

With every breath, air is drawn into the Oxygen Other gases Water vapor
alveoli of the lungs via the respiratory
tract. It travels from the nose or mouth, 0.04% Esophagus
past the pharynx, through the larynx, and Trachea
enters the trachea. This splits into two Carbon dioxide
smaller tubes, one entering each lung,
called the primary bronchi, which in turn 78.6%
branch into increasingly smaller bronchi
and then into bronchioles attaching to the Nitrogen
alveoli (tiny air sacs). During this long
journey, the air is warmed to body Breathable air Right lung
temperature and has any particles filtered Nitrogen is the gas that occupies the largest part
out. Used air makes the same journey in of atmospheric air, yet at the pressure at sea level,
reverse, but as it passes though the larynx very little dissolves in human blood, so it is able to
it can be employed to produce sound. pass harmlessly into and out of the body.

NASAL CONCHAE Protection Blood vessels Air warms and
Cold, inhaled air is lie close to moistens as it
the surface passes conchae
Three shelflike projections in the nasal gradually warmed
cavity provide an obstruction to inhaled and humidified by
air, forcing it to spread out as it passes over the conchae as it
their surfaces. This fulfills several roles. The passes over their
moist surfaces.

moist, mucus-lined conchae humidify Mucus-
passing air and entrap inhaled particles, lined
while their many capillary networks warm
concha

the air to body temperature before it Nasal hairs Primary
reaches the lungs. Nerves within the obstruct bronchi
conchae sense the condition of the air particles

and, if needed, cause them to enlarge—if

the air is cold, for example, a larger surface
area helps warm it more effectively. This is Inhaled air

what gives a feeling of nasal congestion.

Frontal sinus PARANASAL SINUSES

Ethmoid sinus Four pairs of air-filled cavities called paranasal sinuses

sit within the facial bones of the skull. They are lined

Maxillary sinus with cells that produce mucus, which flows into the nasal Bronchi
passageways through very small openings. The roles of Bronchioles

Sphenoidal the sinuses are to lighten the heavy skull bones and to Alveoli
sinus improve the resonance of the voice by acting as an echo

Continuous space chamber. Their effectiveness becomes KEY
The paranasal sinuses are filled with obvious during a cold, when the small Inhaled air
air that moves into and out of them openings into the nose become blocked,
Exhaled air
from the nasal passageways. giving a nasal quality to the voice.

325

RESPIRATORY SYSTEM

TRACHEA SNORING

The trachea (or windpipe) acts as Inhaled Swallowed Over one third of people snore. The
a conduit for air from the larynx to air food mass incidence is higher in older people
the lungs. It is kept open by rings of and those who are overweight. The
C-shaped cartilage, which encircle Epiglottis folds noise is produced by the vibration Sleepless nights
it at intervals along its length. The over trachea of soft tissues in the airways as air is Severe snoring can cause “obstructive
ends of these rings are connected breathed in and out. When a person sleep apnea”, a condition where the
by muscles that contract to increase Air flows Trachea is is awake, the soft tissues at the back snorer stops breathing during sleep.
the speed of air expelled during in through drawn upward of the mouth are kept out of the
coughing. In order to swallow, the open trachea way of the airflow by the tone of the
trachea closes against the epiglottis, surrounding muscles. During sleep
a cartilage flap, and the vocal cords Breathing these muscles relax and the soft tissues
close tightly shut. Cells that line The trachea remains open, flop into the air stream and cause it to
the trachea either produce mucus allowing air to flow freely vibrate, producing the snoring noise.
or display cilia (see below), which into and out of the lungs.
transport mucus up to the mouth. Food enters Collapsed soft
palate
Swallowing esophagus Inhaled air Airflow
Tonsils The main soft tissues that
The trachea is pulled upward so can disturb air flow to
produce snoring are the
that it is closed off by the epiglottis. nasal passageways,
the soft palate, and the
Food passes down the esophagus. tongue. Swollen tonsils
can also contribute.
Pulmonary venule
carrying oxygenated Capillary bed Tongue
Inhaled air
blood Exhaled air Constricted and
vibrating air
Pulmonary arteriole Alveolar sac
carrying deoxygenated CILIA

blood The air passages from the nose through to the bronchi are lined
with two types of cells: epithelial cells and goblet cells. The more
Left lung numerous epithelial cells have tiny, hairlike projections called cilia
on their surface. Cilia continually beat toward the upper airways.
Alveoli The goblet cells produce mucus, which they secrete into the lining
Tiny air sacs, encased by a network of capillaries, are the final of the airways where it can trap inhaled particles, such as dust. The
destination of inhaled air. In each alveolar sac, oxygen is traded cilia then act as a conveyor belt, transporting the mucus, along with
for carbon dioxide in a process called gas exchange (see p.326). any trapped particles, away from the lungs into the upper airways,
where it can be coughed or blown out, or swallowed.
DUST INHALATION KEY TO PARTICLE SIZE
Large – 6μm or over Cilia beat
Many particles of varying size are Small – 1–5μm The speed at
inhaled along with air and can lodge Tiny – under 1μm which cilia beat
along the airways. To prevent these depends on
particles from damaging the airways’ temperature. They
lining, or causing infection, defenses slow down below
such as mucus and cilia (see right) 90°F (32°C) and
are in place. For microscopic over 104°F (40°C).
particles, white blood cells called
macrophages patrol the alveoli Rhythmic cilia
and destroy invaders. transport mucus

Mucus

Final defense Dust filter Cilia Goblet
A macrophage (green) Large particles, such as dust, lodge in the cell
checks a lung cell for nasal cavity; smaller ones, such as fine coal Epithelial cell
foreign particles. Once dust, in the trachea; and the tiniest, such as
a threat is destroyed, cigarette smoke particles, reach the alveoli. Mucus transport
the macrophage will Mucus is a viscous
migrate into the secretion produced
bronchioles to be in the airways. Its
expelled from the sticky surface protects
airways via mucus. the lungs by adhering
to invading particles.

326

HOW THE BODY WORKS

GAS EXCHANGE Hundreds of millions
of alveoli provide a total
Cells need a continual supply of oxygen that they combine with glucose to surface area of 750 sq ft
produce energy. Carbon dioxide is continually generated as a waste product (70 sq m), over which gas
of this process and is exchanged for useful oxygen in the lungs. exchange can take place.

PROCESS OF GAS EXCHANGE Deoxygenated Trachea Oxygen-rich Oxygenated
blood enters Aorta blood leaves blood is pumped
The respiratory tract acts as a transport system, taking air to millions right lung heart via aorta
of tiny air sacs (alveoli) in the lungs where oxygen is traded for carbon via right to body cells
dioxide in the bloodstream. This exchange of gases can take place only pulmonary
in the alveoli. However, during normal breathing, air is only drawn into artery
and out of the respiratory tract as far down as the bronchioles. This
means that the alveoli are not regularly flushed with fresh air and Deoxygenated
stale, carbon dioxide-rich air remains in them. Carbon dioxide and blood enters
oxygen in the alveoli therefore have to change places by moving left lung via left
down a concentration gradient—the oxygen molecules pulmonary artery
migrate to the area where oxygen is scarce, while the carbon
dioxide molecules migrate to the area where carbon dioxide Oxygenated
blood returns
is scarce. Using this process, to heart via
known as “diffusion,” pulmonary
oxygen enters the alveoli, veins
and from there diffuses
into the blood (see Deoxygenated
below), while carbon blood from
dioxide moves out of body returns
the alveoli and into the to heart via
bronchioles, and is superior
exhaled normally. vena cava

Lung tissue Deoxygenated Heart
A color-enhanced micrograph of blood returns
a section of a human lung clearly
displays the numerous alveoli, to heart via
which form the site of gas exchange. inferior vena cava

DIFFUSION FROM ALVEOLI Oxygen Carbon dioxide Deoxygenated blood
enters leaves alveolar sac arrives from heart
In human lungs there are nearly 500 million
alveoli, each of which is around 1/128 in alveolar
(0.2 mm) in diameter. Taken together, the sac
alveoli represent a large surface area over
which gas exchange can take place. To Capillary
move between the air and the blood,
oxygen and carbon dioxide have to cross Capillary bed Carbon dioxide
the “respiratory membrane,” which surrounds diffuses into air
comprises the walls of the alveoli and their alveolus
surrounding capillaries. Both of these are Oxygen diffuses
just one cell thick, so the distance that into blood
molecules of oxygen and carbon dioxide
must travel to get into and out of the blood Respiratory membrane Oxygenated Exchange of gas
is tiny. The exchange of gas through the The vast number of capillaries that blood returns Capillaries alongside
respiratory membrane occurs passively, surround the alveoli mean that up to alveoli give up their waste
by diffusion, where gases transfer from 32 fl oz (900 ml) of blood can take to heart carbon dioxide and pick
areas of a high concentration to a low part in gas exchange at a given time. up vital oxygen across the
concentration. Oxygen dissolves into the respiratory membrane.
surfactant (see p.329) and water layers of
the alveoli before entering the blood, while
carbon dioxide diffuses the opposite way,
from the blood into the alveolar air.

327

RESPIRATORY SYSTEM

HEMOGLOBIN No oxygen Oxygen DIFFUSION INTO
molecules molecules CELL TISSUES
Hemoglobin is found in red blood cells and is a
specialized molecule for transporting oxygen. It is made Deoxyhemoglobin Oxyhemoglobin Body cells constantly take in oxygen from
up of four ribbon-like protein units, each containing a Deoxyhemoglobin is hemoglobin Oxygen binds to deoxyhemoglobin hemoglobin (see left) and excrete their waste
heme molecule. Heme contains iron, which binds oxygen without oxygen. Once it has lost one in the lungs to form oxyhemoglobin. into the bloodstream. As a result, the
to the hemoglobin and therefore holds it within the red oxygen molecule, the hemoglobin Once one oxygen molecule has been concentration of oxygen in the capillaries is
blood cell (oxygenating the blood). When oxygen levels changes its shape to make it easier picked up, the structure changes so low, and the concentration of waste products
are high, for example in the lungs, oxygen readily binds to release its remaining oxygen. more oxygen will quickly attach. is high; a situation that prompts hemoglobin to
to hemoglobin; when oxygen levels are low, for example give up its oxygen. The free oxygen then diffuses
in working muscle, oxygen molecules detach from into the cells, where it is used to create energy,
hemoglobin and move freely into the body cells. while carbon dioxide diffuses out of the cells
and into the blood. Hemoglobin picks up
around 20 percent of this carbon dioxide, yet
most returns to the lungs dissolved in plasma.

Oxygenated red blood
cells enter capillary

Carbon dioxide diffuses
out of tissue cells, through
the capillary wall, and into

the blood plasma

Oxygenated
red blood cell

Essential supply
Oxygen absorbed in the lungs is taken
in the blood to the left side of the heart,
which pumps it through the body. When
it reaches the capillaries, oxygen is
exchanged for carbon dioxide. Carbon
dioxide is then transported in the blood
to the right side of the heart, which
pumps it to the lungs to be exhaled.

Body cells
Capillary bed

Oxygen is Capillary gas exchange
released by Blood flows through the capillaries, where
hemoglobin hemoglobin releases oxygen, and carbon dioxide
within the red dissolves in plasma to be taken back to the lungs.
blood cells

Smoke inhalation THE BENDS
Inhaled smoke particles travel deep into the
lungs. They damage the alveolar walls and Divers breathe pressurized air, which forces
cause them to thin and stretch. This results more nitrogen than usual to dissolve into the
in the individual air sacs fusing, which reduces blood (see p.324). If they ascend too fast,
available surface area for gas exchange. nitrogen forms gas bubbles in their blood,
Breathing difficulties can then arise in later life. blocking the vessels and causing widespread
damage, known as “the bends.” Treatment is
Deoxygenated to redissolve the bubbles in a decompression
red blood cell chamber until nitrogen levels return to normal.

Deoxygenated
blood is carried
back to the heart

328

HOW THE BODY WORKS

MECHANICS OF BREATHING IN
BREATHING
For forced inhalation, contraction of
The movement of air into and out of the lungs, known as the diaphragm is combined with the
respiration, is brought about by the action of muscles in the neck, contraction of three key accessory
chest, and abdomen, which work together to alter the volume of muscles: the external intercostals,
the chest cavity. During inhalation fresh air is drawn into the lungs, scalenes, and sternocleidomastoids.
and during exhalation stale air is expelled into the atmosphere. This dramatically increases the
volume of the chest cavity.

MUSCLES OF RESPIRATION depth of the chest cavity and drawing air into the lungs. Lungs
Normal, quiet exhalation is passive and brought about Air is drawn into
The diaphragm is the main muscle of respiration. It is by the relaxation of the diaphragm as well as the elastic
a dome-shaped sheet of muscle that divides the chest recoil of the lungs. If extra respiratory effort is required, lungs as chest
cavity from the abdominal cavity, attaching to the for example during exercise, when the body’s cells need cavity expands
sternum at the front of the chest, the vertebrae at the a greater supply of oxygen to function efficiently, then
back of the chest, and to the lower six ribs. Various contraction of the accessory muscles bolsters the action
accessory muscles are located within the rib cage, neck, of the diaphragm to allow deeper breathing. Different
and abdomen, but these muscles are used only during accessory muscles are used for inhalation and exhalation.
forced respiration. For normal, quiet respiration, the
diaphragm contracts and flattens to inhale, increasing the

PLEURAL CAVITY Lung held in Collapsed lung Diaphragm
place by negative Contracts and
The pleural cavity is a narrow space flattens to draw
pressure
chest cavity
between the lining of the lungs and the downward,
which increases
lining of the chest wall. It contains a small lung volume

amount of lubricating fluid (pleural fluid)

that prevents friction as the lungs expand

and contract within the chest cavity. Pleural

fluid is held under slight negative pressure.

This creates a suction between

the lungs and the chest wall Lung is sucked Circular breathing enables a single
that holds the lungs open against the continuous exhalation by
and prevents the alveoli from chest wall inhaling while exhaling air stored in
closing at the end of exhalation. the cheeks—the longest exhalation
Lung tissue on record has exceeded 1 hour.
If the alveoli were to close
Collapsed lung
completely, an excessive Pleural If air enters the pleural cavity it
amount of energy would cavity cancels the suction effect, causing
the lung to collapse (pneumothorax).
be needed to reinflate

them during inspiration.

NEGATIVE AND Chest Chest
POSITIVE PRESSURE cavity cavity
expands contracts
The generation of “pressure gradients” is what causes air
to move into and out of the lungs. When the muscles of Inhalation Exhalation
inhalation contract to increase the volume of the chest Enlarging the chest cavity creates Reducing the chest cavity volume
cavity, the lungs, which are sucked onto the chest wall a negative pressure in the lungs, exerts a positive pressure on the
by the effect of pleural fluid, expand. This reduces the causing air to be drawn into them. lung tissue and forces the air out.
pressure in the lungs relative to that of the atmosphere
and air flows down the pressure gradient into the lungs.
For exhalation, the elastic recoil of the lungs compresses
the air within them, forcing it out into the atmosphere.

329

RESPIRATORY SYSTEM

Sternocleidomastoid BREATHING OUT
Increases volume
of chest cavity by For forced exhalation, the passive
pulling up rib cage recoil of the diaphragm and lungs
is not sufficient. Accessory muscles,
Scalenes including the internal intercostals,
Contract to raise external obliques, and rectus
the upper ribs abdominis, all contract to forcibly
reduce the chest cavity volume.

External Lungs Internal intercostal
Deflate as muscles
intercostal chest cavity Contract to tilt ribs
contracts downward and inward
muscles
Contract to tilt Ribs
ribs upward Tilt downward and
and outward inward in response
to muscle contraction
Ribs
Tilt upward External obliques
and outward Contract and
in response shorten, working
to muscle with rectus
contraction abdominis to
pull lower ribs
Diaphragm downward
Relaxes to
Rectus abdominis
reduce lung Pulls rib cage
volume downward, reducing
volume of chest cavity

Contraction Relaxation
Top of diaphragm Diaphragm rises
can move down
by up to 4 in (10 cm) back up to its
normal position

SURFACTANT Type II Water molecules Type I cells form
alveolar cell alveolar wall
Cells lining the alveoli are coated with a layer of water molecules. These produces new
Surfactant
have a high affinity for each other, meaning that the water layer tries to surfactant molecules
molecules
contract and pull the alveolar cells together, like a purse string. To prevent Dust particle
Water molecules
the alveoli from closing under this pressure, a layer of surfactant spreads pull toward Alveolar
each other macrophage
over the water surface. Oil-based surfactant engulfs tiny
Low-affinity dust particles
molecules have a very low affinity for each other Oily layer surfactant that enter
and can therefore counteract the pull of the water A surfactant molecule’s molecules alveolar sac
molecules, ensuring the alveoli remain open. water-loving end (see p.325)
Alveoli are made of two types of cell: Type I form dissolves in water; its resist the pull
the alveolar walls and Type II secrete surfactant. fat-loving end forms a of the water
boundary with the air.

330 TRIGGER

HOW THE BODY WORKS Clusters of specialized cells, known
as chemoreceptors, located in the
INSTINCTIVE aortic and carotid bodies (peripheral Medulla oblongata
BREATHING chemoreceptors) and the brain stem Contains the
(central chemoreceptors), monitor respiratory center
The aim of respiration is to maintain the necessary levels of carbon dioxide and oxygen
blood levels of oxygen and carbon dioxide for the in the blood. They then send signals Glossopharyngeal
corresponding level of activity. The trigger to breathe, to the brain to trigger a response. nerves
as well as breathing itself, is subconscious, but the rate Convey signals from
and force of breathing can be consciously modified. Central chemoreceptors the carotid bodies
Chemoreceptors in the
Carotid bodies
medulla oblongata of the
brain stem are sensitive Vagus nerves
to chemical changes in Convey signals from
the cerebrospinal fluid, the aortic bodies
which alters its acidity in
response to increased Aortic bodies
carbon dioxide levels
RESPIRATORY DRIVE in the blood

Oxygen is vital for cells to function, yet cells’ ability to function properly. Therefore, Peripheral
the drive to breathe is mainly determined breathing is triggered by rising levels of chemoreceptors
by levels of carbon dioxide in the blood. carbon dioxide or acid, and only very Chemoreceptors located in
Hemoglobin, the oxygen-carrying low oxygen levels stimulate breathing. the aortic bodies (on the
molecule (see p.327), has a built-in reserve, aortic arch) and the carotid
and can continue to donate oxygen to Specialized cells called chemoreceptors bodies (on the carotid artery)
cells even when blood levels of oxygen measure blood levels and send nerve detect rising levels of carbon
are low. However, carbon dioxide readily impulses to the respiratory center of the dioxide, or low levels of
dissolves in plasma and is converted to brain stem within the medulla oblongata. oxygen, in the blood. Signals
carbonic acid, which quickly damages the Corresponding messages from the brain to the respiratory center in
then activate the respiratory muscles. the medulla oblongata are
sent via the vagus and the
glossopharyngeal nerves

Heart

PATTERNS OF 90% Overbuilt Aortic bodies
BREATHING Quiet breathing uses less than Contain
Excess space 10 percent of the total lung
During normal breathing, only 18 fl oz chemoreceptors
(500 ml) of air flows into and out of the 10% capacity. These huge reserve
lungs. This is known as the tidal volume. volumes enable a person
The lungs have extra, reserve capacity Used space with one lung to survive.
(the vital capacity) for both inhalation and
exhalation so that they can increase the 6000 MAXIMUM POSSIBLE INSPIRATION
amount of air they take in during exercise. 5000
4000 LUNG VOLUME (ML) Vital Aortic arch
The maximum amount of air that the capacity
lungs are able to hold is around 204 fl oz 3000 Tidal Spirometer reading Blood sampling
(5,800 ml), but about 35 fl oz (1,000 ml) 2000 Residual volume Total lung volume The volume of air held within The aortic bodies are located along
of this remains within the respiratory 1000 capacity the lungs is determined by the aortic arch. Like the carotid
passages after each out breath. This blowing into a machine called bodies, they have their own blood
is called the residual volume and 0 a spirometer. The results are supply, from which they sample
cannot be displaced voluntarily. recorded as a graph (left). levels of gas and acid.

Divers often exceed EXTREME HUMAN to dive for longer without feeling they need to
depths of 328 ft (100 m), breathe. However, this is highly dangerous
which involves them not FREE DIVING because their cells may run out of oxygen before
breathing for several their brain realizes they need to take a breath.
minutes at a time. Some forms of free diving involve divers They risk blacking out under water and drowning.
competing to go as deep as possible without
using breathing apparatus. They train by Into the deep
exercising on land while holding their breath Free diving with fins, or flippers (as shown here),
to get their muscles used to working without provides extra propulsion and allows divers to
oxygen. Prior to the dive, some divers reach depths beyond their usual capabilities.
hyperventilate in an effort to rid their blood of
as much carbon dioxide as possible—high
levels would normally tell their brain of the
need to stimulate inhalation. This allows them

RESPONSE 331

If carbon dioxide levels rise or RESPIRATORY SYSTEM
oxygen levels fall, the respiratory
center signals to the muscles of Respiratory center
respiration, via the nerves, to trigger
breathing, increasing both its rate Cervical vertebrae REFLEXES
and depth. These signals are sent
continually so that respiration always Intercostal nerves Inhaled air often contains particles of dust
matches the demands of the body. The intercostal nerves take or corrosive chemicals that could damage
impulses from the respiratory the surfaces of the lungs and reduce their
Phrenic nerves center to the intercostal muscles ability to function. Cough and sneeze
Messages from the respiratory and cause them to contract. reflexes exist to detect and expel such
center pass down the phrenic Each nerve leaves the spinal irritants before they reach the alveoli.
cord at the same level of the Nerve endings in the respiratory tract
nerves, which originate from muscle that it supplies are very sensitive to touch and chemical
the spinal cord in the neck, irritation and, if stimulated, send impulses
to the brain to initiate a sequence of
and stimulate the diaphragm events that causes the offending object or
to contract and expand chemical to be coughed or sneezed out.
the thoracic cavity

Forcible explusion
Schlieren photography, which
registers density changes, reveals
the air turbulence from a cough.

Intercostal Deep
inhalation
muscles of air
Contract to
expand the Open throat
rib cage

Inhaled irritant

Diaphragm Diaphragm Diaphragm
Contracts via is relaxed contracts
innervation by
the phrenic 1. Irritation 2. Inhalation
nerves Inhaled particles or chemicals irritate The brain signals to the respiratory
sensitive nerve endings, which send muscles to contract, causing a sudden
signals to alert the brain to the intrusion. intake of breath (88 fl oz/2,500 ml).

Throat closes Expelled air
dislodges
irritant

KEY Chest cavity Throat opens
contracts as
Glossopharyngeal Intercostal nerves Direction of closed system Chest cavity
contracts
nerves nerve impulse Air pressure sharply
rises in lungs
as abdominal Diaphragm
and accessory quickly relaxes
muscles start
to contract

Vagus nerves Phrenic nerves 3. Compression 4. Expulsion
The vocal cords and the epiglottis shut The epiglottis and vocal cords open
tightly and the abdominal muscles suddenly, expelling the air at high
contract, raising air pressure in the lungs. velocity and taking the irritant with it.

VOCALIZATION Back of tongue Vocal cords
press together
Speech involves a complex interaction between the brain, vocal cords, soft Epiglottis
palate, tongue, and lips. When air passes against the vocal cords they vibrate Restricted air
to produce noise. Muscles attaching them to the larynx can move the cords Open vocal flow causes
apart for normal breathing, together to create sound, or stretch them to cords
increase pitch. Vibrations are articulated into words by the soft palate, lips, vocal cords to
and tongue. Higher air pressure beneath the vocal cords will increase Air passes vibrate
volume. The voice itself finds resonance in the paranasal sinuses (see p.324). through
trachea Speaking
Vocal cords vibrate at a variety of speeds depending on how tightly they During normal speech, the muscles of the larynx move
are stretched: faster vibrations create high-pitched sound. For example, the Back of throat the vocal cords close together so that air passing
vocal cords of a bass singer vibrate at around 60 times per second, whereas through them causes them to vibrate.
those of a soprano can vibrate at up to 2,000 times per second. Breathing
The vocal cords are held fully open during breathing.
Air passes easily between them without causing any
vibration and no sound is made.

HEART

Sitting at the center of the
circulation, the muscular heart
pumps all of the blood around
the body once every minute.

ARTERIES VEINS

Blood vessels that carry blood away Blood vessels that bring blood
from the heart have thick, muscular, back to the heart have thinner,
elastic walls that cope with the high expandable walls and one-way
pressures generated by a heartbeat. valves that prevent backflow.

CAPILLARIES CARDIOVASCULAR
SYSTEM
Oxygen diffuses out of these
minute, thin-walled vessels
to supply body cells, while waste
carbon dioxide diffuses in.

The heart is a pumping engine, powering the
transport of life-giving blood around the body.
Blood carries oxygen, nutrients, and immune
cells to every part of the body via arterial
vessels, and carries away waste via the veins.

334

HOW THE BODY WORKS

BLOOD Constant supply
Blood flows to every
Adults have approximately 11 pints (5 liters) of blood, which consists of specialized cell in the human body.
cells suspended in plasma. It supplies cells with nutrients and oxygen and removes Throughout the body,
their waste. Blood also transports hormones, antibodies, and cells that fight infection. the cells continually release
chemicals to ensure that
BLOOD AS TRANSPORT they get enough blood to
supply them with nutrients
Blood is the main transport system of the body. The lactic acid, and transports them to the liver and kidneys, and remove any waste.
which expel or excrete them from the body. Carbon
heart pumps all 11 pints (5 liters) of a resting adult’s blood dioxide is taken from the cells and excreted by the lungs. Blood vessel

around the body every minute. Components of the blood Blood also transports hormones (see p.384) from the
glands in which they are produced to the cells they affect.
pick up nutrients absorbed from the gut as well as oxygen Cells and other substances involved in healing and
fighting infection circulate in the blood stream, only
In the stream from the lungs and deliver these becoming active when they are needed.
This magnified image to the body’s cells. The blood
reveals the cells and also removes the cells’ waste
platelets in blood. chemicals, such as urea and

COMPONENTS OF BLOOD 54% Plasma
1% Plasma is a straw-colored
The liquid component of blood (plasma) is 92 percent 45% liquid that forms the largest
portion of blood.
water, but also contains glucose, minerals, enzymes,
White blood cells
hormones, and waste products, including carbon dioxide, and platelets
These cells play a vital role
urea, and lactic acid. Some of these substances, such in immunity and clotting.

as carbon dioxide, are just dissolved within the plasma. Red blood cells
Each milliliter of blood
Others, such as the minerals Mainly water contains around 5 billion Capillary
iron and copper, are attached Blood is made up of red blood cells. network
to specialized plasma transport around 46 percent solids
proteins. Plasma also contains (cells), suspended in
antibodies that fight infection. 54 percent liquid plasma.

BLOOD CLOTTING PRODUCTION OF CELLS Blood vessel
wall
When a blood vessel is damaged, platelets rush to the Red and white blood cells, as well as platelets, are
site to plug the gap. As they adhere to the damaged produced in the bone marrow, and pass from here into Protein
area, they release chemicals. These trigger what is the circulation. White blood cells, involved in immunity,
called the clotting, or coagulation, cascade. This results can also pass into the lymphatic system (see p.344–49).
in the formation of strands of a protein called fibrin, Red blood cells, which lack a nucleus, remain in the
which cross-link to form a robust plug, or clot, with blood circulation, where they can live for up to 120 days.
platelets and red blood cells trapped within.

Blood flow Platelets rush Red blood cell Waste product Useful products Cells form in bone
to fill gap of blood cell returned marrow
Platelet plug
Platelets are Waste is Life of a red blood cell New red
attracted to the excreted After about 120 days of life, blood cell
exposed collagen from red blood cells are broken
fibers in the body down by white blood cells Enters
damaged vessel wall called macrophages. Waste circulation
and form a plug. Macrophage products are excreted while
in liver or useful ones return to the
Released Fibrin bone marrow.
chemicals strands Blood clot spleen engulfs
red blood cell
Blood clot
Chemicals trigger Tired red
the formation of blood cell
strands of fibrin,
which mesh the
platelets and red
blood cells together.

335

CARDIOVASCULAR SYSTEM

BLOOD TYPES GROUP A GROUP B GROUP AB GROUP O

Blood type is hereditary. It is determined by proteins, called antigens, A antigen
Anti-B
on the surface of red blood cells. The main antigens are called BLOOD
GROUP
A and B, and cells can display A antigens (blood group A), B antigens
ANTIGENS
(group B), both together (AB), or none (O). Antigens are triggers for
ANTIBODIES
the immune system. An individual’s immune system ignores antigens

on their own red blood cells, but produces antibodies to recognize

and help destroy foreign cells that display new antigens. So, in blood None
Anti-A and Anti-B
group A, cells display the A antigen, Antigens B antigen A and B antigens
which the immune system ignores, There are 30 different antigens Anti-A None
but it produces antibodies to the B that red blood cells can display,
antigen, and destroys foreign cells but the ABO antigens, illustrated
displaying this antigen. here, are the most well known.

RED BLOOD CELL WHITE BLOOD CELL PLATELET

Vital for oxygen transportation, red blood Many types of white blood cells (leukocytes) Important in blood clotting, platelets are cell
cells (or erythrocytes) contain hemoglobin, exist in the body (see p.345). They are key fragments produced in the bone marrow
a protein that binds to oxygen molecules to immunity, fight infections, trigger allergic from large cells called megakaryocytes.
(see p.327). It also creates the cells’ red reactions, and remove foreign bodies. Platelets lack a nucleus and last 8–12 days.
pigmentation. Their biconcave disk shape
increases their surface area for oxygen
absorption, and also increases flexibility.

CHOLESTEROL MICELLE

Balls of fatty molecules, grouped with
water-loving (hydrophilic) parts on the
outside, and water-repellent (hydrophobic)
parts inside. Hydrophobic fatty substances
such as cholesterol are carried in the core.

336

HOW THE BODY WORKS

CARDIAC CYCLE Pulmonary
veins carry
The heart is a two-sided muscular pump. The right side of the heart receives blood from
deoxygenated (oxygen-poor) blood from the body and pumps it to the
lungs, where it is topped up with oxygen. The left side receives oxygenated the lungs
(oxygen-rich) blood from the lungs and pumps this around the body.
Cardiac cycle
PUMPING HEART Cardiac echo Contraction of the heart muscle
Echocardiography occurs in response to electrical
The heart combines two separate pumps (or echo) produces an activity within the cardiac
within a single organ—one for oxygenated ultrasound of the heart, conducting system (see p.338).
blood (left), and one for deoxygenated visually recording the Under normal circumstances this
(right). When at rest, it beats on average real-time movement of electrical activity follows a strict
100,000 times per day. Every heartbeat blood through its four pattern, with contractions of the
involves the coordinated contraction chambers. Echo reveals heart chambers following suit.
(systole) and relaxation (diastole) of the any abnormalities of the Despite this regulation, the heart
heart’s four chambers. These regulated valves or of the pumping can easily respond to the demands
muscular pulses transfer blood from the ability of the heart. of the body by altering the rate, as
upper two chambers (atria) into the lower well as the force, of its contractions.
two (ventricles) via a system of valves, and Pressure builds
from there eject it from the heart through as right atrium Aortic valve Pressure builds as
the aorta and the pulmonary artery. Known fills with blood closes left atrium fills
as the cardiac cycle, this process divides with blood
into five key stages (see opposite).
Left ventricle Blood is forced Left atrium
contracts through aortic valve

CARDIAC MUSCLE

Cardiac muscle (myocardium) can be The divisions between cardiac muscle cells

distinguished from the other types of are highly permeable, allowing electrical

muscle (skeletal and smooth) by its impulses (action potentials) to flow quickly

appearance and behavior. Apart from and easily between cardiac muscle cells

being branched, cardiac muscle fibers so that all of the cells in an area of muscle Pulmonary
can contract as one. Cardiac muscle also valve closes
Striated muscle look similar to
A colored micrograph skeletal muscle, contains large numbers of energy- Tricuspid valve
shows pink muscle fibers yet they behave producing mitochondria, meaning that remains
and oval mitochondria. very differently. it doesn’t fatigue, unlike skeletal muscle. closed

HEART VALVES OPEN VALVE CLOSED VALVE Ventricle walls
relax
Mitral valve remains closed

Four heart valves, two at the Heart 5 ISOVOLUMIC
RELAXATION
exit of the atria and two at Blood flows Blood is Isovolumic relaxation is the earliest
the exit of the ventricles, freely through unable to phase of diastole. The ventricles start
prevent blood from open valve flow backward to relax and the pressure of blood
into atrium within them falls to below that of the
flowing backward into the blood in the aorta and pulmonary
Chordae artery; therefore the aortic and
heart chambers. They tendineae is pulmonary valves both close.
pulled taut However, the pressure in the
open or close passively ventricles is still too high to allow the
Papillary mitral and tricuspid valves to open.
depending on the pressure muscles
contract
of the blood surrounding

them. If the blood pressure behind

the valves is greater than that in front of

them they will open; if the pressure in front atria and the ventricles have specialized Held tight Valves and pressure
is greater, they will close—the closing of attachments called papillary muscles Papillary muscles contract along with Ventricular pressure decreases so
the valves is what creates the familiar and chordae tendineae. These prevent the ventricle, pulling taut the chordae the pulmonary and aortic valves
“lub-dub” sound of a heartbeat. The mitral the valves from opening backward into the tendineae (attached to the valve) close, yet it is not low enough for the
and tricuspid valves located between the atria when ventricular pressure rises. in order to keep the valve tight shut. mitral and tricuspid valves to open.

Superior vena cava Left atrium Pulmonary veins 337
returns blood from fills with carry blood from
the body oxygenated the lungs CARDIOVASCULAR SYSTEM
blood
Right atrium Left atrium
contracts contracts

1 DIASTOLE 2 ATRIAL SYSTOLE
At this stage the ventricles are The right and left atria contract
relaxed. In early diastole the mitral simultaneously, forcing any remaining
and tricuspid valves open and blood blood into the ventricles, which are
that has been building up in the atria still relaxed, through the mitral and
during systole rapidly flows into the tricuspid valves. After atrial systole the
ventricles. Following this, blood ventricles are full, yet the contraction
returning to the heart flows passively of the atria has only contributed to
from the atria into the ventricles. 25 percent of this volume.
At the end of this process the
ventricles are about 75 percent full. Valves and pressure
Even higher pressure in the now
Mitral valve opens and contracting atria keeps the mitral
blood flows passively and tricuspid valves open. The aortic
into left ventricle and pulmonary valves remain closed.

Valves and pressure Remaining blood
High pressure in the atria opens in atrium is forced
the mitral and tricuspid valves. Low into left ventricle
ventricular pressure means the aortic
Inferior vena cava Right atrium fills with and pulmonary valves remain closed. Remaining blood in Pulmonary Left atrium
returns blood deoxygenated blood atrium is forced valve remains continues to
from the body Tricuspid valve opens closed fill with blood
and blood flows passively into right ventricle
into right ventricle Right atrium
continues to
fill with blood

An adult heart pumps an average 3 ISOVOLUMIC Tricuspid Mitral
of 15,200 pints (7,200 liters) of CONTRACTION valve closes valve
blood around the body every day. closes
This is the first stage of systole, when
Left ventricle begins
the muscle of the ventricles starts to to contract

contract and increase the pressure

on the blood within the ventricles.

This increased pressure is enough to

close the mitral and tricuspid valves,

Blood is forced into Aorta branches but not enough to open the aortic
pulmonary arteries into smaller
from right ventricle arteries to supply and pulmonary valves. Therefore
blood to the body
Blood is forced into aorta during this stage the ventricles
from left ventricle Descending aorta
contract as a closed system.

Pulmonary arteries carry Valves and pressure
Increased ventricular pressure means
4 EJECTION blood to lungs the mitral and tricuspid valves close,
Eventually the ventricular yet it is not high enough to open the
pulmonary and aortic valves.

contraction causes the pressure of the

blood within the ventricles to exceed

the pressure of the blood in the aorta Pulmonary Aortic valve
arteries carry remains
and pulmonary arteries. At this point blood to lungs closed

the aortic and pulmonary valves are

forced open and blood is powerfully

ejected from the ventricles. The

papillary muscles prevent the mitral Left atrium
continues to
and tricuspid valves from opening. fill with blood Right ventricle begins
to contract

Valves and pressure Left SCIENCE
The aortic and pulmonary valves are ventricle
forced open by high pressure in the contracts ARTIFICIAL HEART
contracting ventricles. The mitral fully
and tricuspid valves remain closed. Many people die while waiting for
heart transplants because there are not
Right atrium continues enough donors to satisfy demand.
to fill with blood Artificial hearts were therefore developed
to help these people survive until a heart
Pulmonary became available. They may eventually
valve opens replace transplanted hearts altogether, and
allow more patients to live a normal life.
Aortic valve Right ventricle
opens contracts fully

338 Sinoatrial node Right atrium Currents
Also called the pacemaker Electrical impulses
HOW THE BODY WORKS of the heart, the SA node rush through the
emits an electrical impulse atrial walls
CONTROLLING that runs through the atrial
THE HEART walls and stimulates atrial

systole. This is what
instigates a heartbeat

The heart beats around 70 times per minute, although
this varies dramatically throughout the day. Heart rate
is finely tuned by nerves and circulating hormones
that work to ensure the speed is just right to provide
all the cells in the body with the blood that they need.

CARDIAC CONDUCTING SYSTEM

The cardiac conducting system consists of specialized cells that transport electrical
impulses through the cardiac muscle in order to trigger its contraction. The impulse
for each heartbeat starts in the sinoatrial (SA) node, which is located in the right atrium.
It flows rapidly through the atria and causes them to contract (atrial systole). Electricity
cannot pass directly between the atria and ventricles; instead it is channeled into the
atrioventricular (AV) node, where it is delayed slightly to ensure that the atrial contraction
is over before the ventricles start to contract. After leaving the AV node, the electrical
impulse rushes through the bundle of His and Purkinje fibers, which are conducting
fibers that run through the ventricle walls, to stimulate contraction of the ventricles.

ELECTRICAL ACTIVITY Atrioventricular node
The electrical current
The heart’s electrical activity can be recorded using an
cannot breach the fibrous
electrocardiogram (ECG). Electrodes are positioned on tissue dividing the atria and Tricuspid
valve
the chest and limbs in such a way that electrical currents ventricles. It enters the AV
node and is delayed there Right
in all areas of the heart can be monitored. The recording ventricle
for 0.13 seconds, before
displays the voltage between pairs of electrodes. In a being quickly propelled
through the ventricle walls

typical ECG, each heartbeat produces three distinctive

waves (P, QRS, and T), showing a regular Electrical activity
beat. In addition to recording the heart’s in the SA node
rhythm, an ECG can pinpoint the site of any instigates atrial
damage that disturbs the flow of electricity, systole

as the waves will form an unusual pattern.

Electrical rhythm 1. The P wave Purkinje
Each heartbeat is triggered by the flow fibers
of electricity through the muscle in an Electrical impulses
exact sequence that can be detected
using an ECG. Deviations from the spread from the SA
horizontal line on the ECG
tracing are caused by electrical node, through
activity resulting in specific
actions within the heart. the atria, to the

SA node Electrical AV node.
prepares for
next heartbeat impulse

AV node forward Papillary
electrical impulse to muscle

contract ventricles Conductors of the heart
Both the SA and the AV nodes are capable
3. The T wave Electrical 2. The QRS complex of self-excitation, meaning that the heart will
Represents the electrical impulse Electrical activity continues beat without input from the nervous system—nerves
recovery (repolarization) of recedes from the AV node through regulate, rather than instigate, the heartbeats (see
the ventricles. Both atria and as heart the ventricles to produce opposite). The SA node sets the heart’s rhythm, but
ventricles relax completely. resets itself ventricular contraction. if the impulse from the atria is blocked, the AV node
can stimulate the ventricles to contract.

339

CARDIOVASCULAR SYSTEM

Left NERVE AND Nerve supply Sympathetic
atrium BRAIN CONTROL Parasympathetic nerve nerves
supply to the heart,
Bundle Nerves from both the sympathetic and from the vagus nerves,
of His parasympathetic nervous systems (see p.297) begins in the medulla
fibers directly supply the cardiac conducting system, oblongata (brain stem).
as well as being widely distributed throughout Sympathetic supply is
Mitral the cardiac muscle. Sympathetic nerves release from the spinal cord.
valve norepinephrine, which can increase both the
heart rate and the force of muscle contraction. Medulla oblongata
These actions considerably increase the
volume of blood that the heart ejects (the Vagus nerves
cardiac output). The vagus nerves, which form (parasympathetic)
part of the parasympathetic nervous system,
release acetylcholine, a chemical that conversely Spinal cord
slows the heart rate, thus reducing the cardiac
output. These opposing systems complement
each other to regulate the heart muscle and
ensure that sufficient blood is pumped to
meet the demands of the body.

The heart is self-excitable Muscular heart
and continues to beat
even if its nerve supply
is severed completely.

BLOOD SUPPLY

The heart is the most active muscle in the

body and needs a constant supply of blood

to deliver oxygen and nutrients to its cells

and remove their waste. Although the heart

chambers are always full of blood, this cannot

reach all the cells of its thick walls, so the

heart has its own blood vessels: the coronary

circulation. The coronary arteries that supply

the heart are forced

shut under the pressure Vital supply

Purkinje of the contracting A colored angiogram
fibers muscle. They therefore shows large coronary
can only fill when the arteries branching into
heart is relaxed, a network of smaller
during diastole. blood vessels that
supply the heart.

Papillary SCIENCE
muscle
DEFIBRILLATOR
Left ventricle Bundle of His and
Defibrillators can deliver electric shocks to
Purkinje fibers kick-start a heart that has stopped beating
These specialized conducting properly. They are also used to treat abnormal
fibers transport electrical impulses heart rhythms, where the heart cells contract
extremely rapidly throughout the in a haphazard way. The external dose of
ventricle walls to ensure that all electricity causes all the heart cells to contract
the muscle cells in the ventricles at once, which resets them and allows them
contract almost simultaneously to resume working in a coordinated manner.
These machines can be external, as shown, but
they can also be implanted into patients who
are susceptible to abnormal heart rhythms.

340

HOW THE BODY WORKS

Arteriole wall Blood Arteriole wall Arteriole narrows
is relaxed flow contracts to locally limit
blood flow
BLOOD VESSELS

Blood vessels are a network of branching tubes that join Arteriolar
together to form part of the circulatory system. They can dilate diameter
or contract to adjust blood flow and in this way finely tune the Muscle in the walls of
blood supply to organs, as well as assist with thermoregulation. arterioles allows them
to alter their diameter
and adjust blood flow
in response to the
needs of nearby cells.

BLOOD VESSELS Outer layer Muscular layer Elastic fiber Inner layer
(Adventitia) layer (Endothelium)
Great variation in the size and structure of blood vessels allows each to
perform a specific task. Arteries (the largest) carry oxygenated blood Arteries
away from the heart. They expand to fill with blood and then propel Carry blood
it forward as they return to their normal diameter. Less muscular, veins
return deoxygenated blood to the heart, via a series of valves. Capillaries, away from
the smallest vessels, are the site of gas exchange (see pp.326–27). Their the heart
walls are just one cell thick to allow easy gas diffusion. The smallest is just
7μm in diameter, whereas the diameter of the aorta (the largest artery)
is 1 in (2.5 cm), with walls so thick they require their own blood supply.

DOUBLE CIRCULATION Blood vessel network
Arteries, carrying blood from
The circulation has two main divisions: pulmonary (lungs) and systemic the heart, branch into smaller
(body). The pulmonary circulation takes blood from the right side of the arteries and arterioles that
heart to the lungs, where it is oxygenated and releases carbon dioxide. supply the body’s organs.
Blood is then returned to the left side of the heart. The systemic Arterioles feed capillary beds,
circulation takes the oxygen-rich blood to the body’s cells, picks up which then join to leave the
carbon dioxide and waste products, and returns to the right side. organ as venules. These form
progressively larger veins that
Cerebral veins Cerebral arteries return blood to the heart.
Deoxygenated Oxygenated blood
blood returns travels to brain Arterioles
from brain Sprout from small
Aorta arteries and feed
Superior capillary bed
vena cava

Pulmonary Pulmonary veins Capillary bed
artery Oxygenated blood A network of
returns to the heart microvessels
Deoxygenated from the lungs. The joins arterioles
blood travels to pulmonary veins are and venules
the lungs in the the only veins to
carry oxygenated
only artery blood
that carries
deoxygenated Systemic arteries
Oxygenated blood
blood flows to the limbs and
organs in the chest
Systemic veins and abdomen
Deoxygenated

blood returns to
the heart

Vessels of lower body Vessels of internal organs

Multiple blood supplies
The pulmonary and systemic circulatory systems ensure a constant supply
of blood to the lungs and to the body. A third system—the coronary
circulation—supplies blood directly to the heart itself (see p.339).

341

CARDIOVASCULAR SYSTEM

THERMOREGULATION Thermal imaging 37˚C COLD HAND HOT HAND
On the far right, a thermal 35˚C
When ambient temperature increases, circulating chemicals signal to scan shows a hot hand
blood vessels in the skin to dilate (widen). In this way, warm blood is that radiates red heat, as 30˚C
diverted to the skin, where it can lose its heat to the surrounding air, thus warm blood flows through
cooling the body. When the temperature falls, blood vessels constrict so its vessels. On the near 25˚C
the skin loses less heat, and therefore essential warmth is retained in the right, the hand is cold, 21.5˚C
core of the body, where the vital organs are. This mechanism helps to blood flow through the
keep the body temperature at a constant level of around 98.6°F (37°C). vessels is reduced, and less
heat is radiated (blue).

Open Blood flows Closed Blood cannot Inner layer Elastic fiber layer SKELETAL MUSCLE PUMP
valve flow back (Endothelium)
upward valve Muscular layer Pressure in the veins is too low to actively
pump blood back to the heart against gravity.
Valve Outer layer Therefore, veins have to rely on pressure from
(Adventitia) their surrounding tissues to squeeze blood
Vein valves back toward the heart. In the chest and
Vein pressure only reaches 5–8mmHg Veins abdomen, organs such as the liver perform
(millimeters of mercury); therefore a one-way Carry blood back this task. In the limbs, the contraction and
valve system is in place to keep blood from to heart relaxation of muscles during movement
flowing backward under the force of gravity. effectively “pumps” blood toward the heart.

Capillaries Compressed
The smallest capillaries are vein
so narrow that red blood
cells must travel in single Surrounding
file to squeeze through. muscle
This brings them into close
proximity with the body Venous Contraction
cells that they supply blood flows of the muscle
with oxygen via upward forces blood
gas exchange. upward

RELAXED MUSCLE CONTRACTED MUSCLE

Pumping muscles
When the muscle contracts, blood in the vein is
squeezed upward. When it relaxes, the one-way
valves prevent blood from flowing back down.

BLOOD PRESSURE

Blood pressure, measured in millimeters

of mercury (mmHg), refers to the pressure

within the arteries. It peaks (systolic pressure)

Venules as blood pumps into the arteries. As the heart
Channel blood
from capillary relaxes, pressure in the vessels falls, but the
bed into veins
tone of the artery walls never allow it to reach
Cell wall
A single layer Peaks and troughs zero, so blood always
of endothelial A heartbeat has a systolic flows. This lower
cells forms (peak) and a diastolic pressure is called the
capillary wall (minimum) pressure. diastolic pressure.

PRESSURE (mmHG) 120 Systolic
pressure
100
Diastolic
80 pressure
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4
TIME (SECONDS)

LYMPH NODE

Lymph flows slowly through
nodes, where it is filtered.
Antibodies are made in nodes,
which enlarge during infection.

WHITE BLOOD CELLS

White blood cells are produced in
bone marrow. The chief immune
cells, lymphocytes, are stored in
the spleen and lymph nodes.

VESSEL LYMPHATIC AND IMMUNE
SYSTEM
Thin-walled lymph vessels are
valved and work as a similar
way as veins, transporting clear
lymph fluid around the body.

Running in parallel with the blood’s circulation,
the lymphatic system collects excess tissue fluid
from the body (via a network of lymph nodes
and lymph vessels) and returns it to the blood.
This system has vital immune functions.

344

HOW THE BODY WORKS

LYMPHATIC SYSTEM Vessels of head Blood and lymph
and upper body This schematic diagram
of the body shows the
The lymphatic system is a network of vessels and ducts, with close links between
associated lymph nodes, that collects and drains fluid from body the blood vessels and
tissues. It has important roles in maintaining tissue fluid balance, their associated lymphatic
dietary fat absorption, and the functioning of the immune system. vessels that enable
drainage of body tissues.
LYMPHATIC CIRCULATION Right Right internal jugular vein
subclavian vein Left internal jugular vein Right lymphatic
duct
The lymphatic circulation, closely linked to the blood Thoracic (left
lymphatic) duct
circulation, plays a key role in draining fluid from body Left lung

tissues. Delivery of nutrients to body cells and the Heart
Vessels of
elimination of waste products via the blood is not a abdominal
cavity
direct process, but occurs by means of the interstitial
Vessels in gut permit
fluid, which is derived from blood plasma (see below) absorption of fat and
fat-soluble vitamins
and bathes the cells of the tissues. The lymphatic system from small intestine

prevents a buildup of this fluid by collecting and Plasma
filters out
returning it to the blood, via a series of vessels found of capillary
Interstitial fluid enters initial
throughout the body. Once it has entered the lymphatic lymphatic, carrying white blood cells

circulation it is referred to as Left
subclavian
lymph. Lymph re-enters the Right lymphatic duct vein Right
blood via ducts that drain Lymph drains into blood at lung
into the left and right Drainage of right
subclavian veins (see right). junction of right internal lymphatic duct
jugular and subclavian veins
The lymphatic system also Drainage of
forms the basis of an effective Thoracic duct thoracic duct
surveillance network for the Lymph drains into blood at
junction of left internal jugular

and subclavian veins

body’s immune cells (white

blood cells) that monitor Drainage of the body

tissues for signs of The right lymphatic duct drains Valve
infection. These cells move, fluid from the right side of the Allows fluid to enter
via lymph, through lymph head and neck, the right arm, and
nodes located throughout part of the thorax. The remainder initial lymphatic
the body (see opposite). of the body is drained by the
thoracic, or left lymphatic, duct. Body cell

MOVEMENT OF LYMPH

Fluid components of blood plasma, containing nutrients, Vessels of Initial lymphatic Lymph moves
hormones, and amino acids, filter out of the blood lower body Entry point of lymph into into circulation
through the capillary walls, and enter the interstitial spaces
of body tissues. This interstitial fluid is secreted faster than Body cells lymphatic system
it can be reabsorbed. Blind-ended channels, called initial
lymphatics, allow the excess fluid to drain into the Interstitial
lymphatic system, via one-way valves, forming lymph. space
White blood cells also migrate into the system in this way.
Initial
The initial lymphatics drain into the main lymphatic lymphatic
vessels, which carry the lymph around the body.
These vessels have contractile walls that aid the forward Fluid pressure
movement of lymph, and bicuspid valves that prevent When the pressure of
reversal of flow as lymph circulates around the body. fluid outside the initial
lymphatic is greater than
Vessel valves the pressure of fluid within
A bicuspid valve it, the valve in the vessel
(left) permits wall opens, allowing
one-way fluid interstitial fluid to drain
flow. Reverse through, forming lymph.
lymph flow
causes it to shut.

345

LYMPHATIC AND IMMUNE SYSTEM

LYMPHOID TISSUES AND ORGANS GENERATION OF BONE MARROW
IMMUNE CELLS T cells
The primary lymphoid tissues are the thymus and bone marrow, both Thymus
White blood cells, or immune cells Generated within T cells migrate to the thymus
associated with immune cell generation and maturation. Secondary (see below), are all produced in the bone the bone marrow
marrow. Cells involved in innate immunity to mature
lymphoid tissues—lymph nodes, spleen, adenoids, tonsils, and (see pp.346–47) migrate to the blood and B cells
tissues after maturation. Adaptive immune Generated and Lymphoid organs and tissues
gut-associated lymphoid tissue (GALT)—are cells are T and B lymphocytes: T cells mature in the Mature T and B cells
mature in the thymus, while B cells mature bone marrow migrate here
where adaptive immune Adenoids in the bone marrow. Maturation results
responses originate (see in their collective ability to recognize a Innate immune Blood and body tissues
huge range of specific pathogens (see cells Innate immune cells migrate into
pp.348–49). Lymph nodes are Tonsils pp.348–49). Mature lymphocytes migrate
to secondary lymphoid tissues, and Generated and the blood and body tissues
integrated with the lymphatic Thymus circulate and scan for infection. mature within the

system, while the spleen acts Sites of production bone marrow
as a lymph node for the blood. Lymph nodes Blood cell generation initially takes place in most
bones, but by the time of puberty it is centered
Adenoids, tonsils, and GALT Bone marrow on the sternum, vertebrae, pelvis, and ribs.

are key for generating immune Nodes in lungs
responses at mucosal surfaces.

Spleen

KEY Gut-associated
lymphoid tissue
Primary lymphoid tissues
Lymph nodes and spleen Guarding the body
Mucosa-associated The main locations of lymphoid structures show
lymphoid tissue their close links with entry points for infection.

LYMPH FILTERING via efferent vessels. As lymph travels through the node IMMUNE CELLS
it is screened for signs of infection by immune cells.
Lymph nodes are small, encapsulated structures that filter A pathogen may simply flow into the node via the lymph, White blood cells carry out immune responses. The many
passing lymph. They are home to cells of the immune or it may be actively carried in by another immune cell different types reflect their varied roles in combating infection.
system, primarily T and B lymphocytes but others, such as and presented to resident lymphocytes. Recognition Immune cells broadly divide into two groups: innate cells
macrophages, are also present. B cells are concentrated of infection will result in an adaptive immune response respond similarly to all infections; adaptive cells respond to
in the outer cortex, while T cells are found more centrally (see p.348–49). Numerous lymph nodes are positioned specific pathogens toward which they generate immunity.
at intervals along draining lymphatic vessels, enabling
in the inner (paracortical) region. Lymph enters them to monitor particular regions of the body. Monocyte (innate)
through afferent lymphatic vessels, and exits Precursor immune cell, found in the blood.
Migrates to the tissues where it differentiates
Outer cortex Efferent vessel into both macrophages and dendritic cells.
Area where B cells Carries lymph
are concentrated away from node Neutrophil (innate)
Phagocytic cell. Often the first immune cell to
Inner Blood supply reach an infection site, these are short-lived and
(paracortical) Allows engulf microbes via phagocytosis (see p.347).
cortex lymphocytes
Area where T cells to cross from Macrophage (innate)
are concentrated blood stream Phagocytic cell. Long-lived cells, often resident
into node in tissues. Able to promote adaptive immune
Valve responses via interactions with lymphocytes.
Ensures one-way
Natural killer cell (innate)
movement of Cytotoxic cell. Specialized for targeting intracellular
lymph pathogens (those living inside body cells) as well
as malignant tumor cells.
Afferent vessel Hilum
Carries lymph Area where efferent Mast cell / Basophil (innate)
toward node vessel connects to node Inflammatory cells. When activated they release
inflammatory factors that promote an immune
Capsule Recticular fibers response. Also responsible for allergic reactions.
Fibrous casing Fibrous meshwork forms
for lymph node supportive structure of node Eosinophil (innate)
Inflammatory cell. Specialized for targeting larger
Centers of recognition pathogens such as parasitic worms. Associated
The structure of a lymph node with allergic reactions.
maximizes the chances of both
the capture of infective material Dendritic cell (innate)
carried in the lymph, and also Primary antigen-presenting cell (see p.348). They
of its exposure to immune cells present material linked to infection to lymphocytes
—in particular T and B cells. to promote adaptive immune responses.

T and B lymphocytes (adaptive)
Key cells of the adaptive immune system. T cells
target body cells infected with specific pathogens.
B cells secrete antibodies to target infected fluids.

346

HOW THE BODY WORKS

INNATE IMMUNITY

The specialized cells and molecules of the innate immune system, supported by barrier
immunity, respond rapidly to the typical signs of infection produced when pathogens
gain entry to the body. Although highly effective, innate immunity relies upon the recognition
of generalized pathogen characteristics and may not be effective against all infections.

BARRIER IMMUNITY ACTIVE IMMUNITY Micrograph of a blood clot
Blood clots (see p.334), seal
A key strategy in keeping the body free from infection If barrier immunity is breached, for example by a skin wound, and pathogens enter broken tissues and prevent the
is to prevent the entry of harmful organisms in the first the body, the innate immune system then becomes actively involved. Key to this is the entrance of harmful microbes.
place. Barrier, or passive, immunity acts as a first line activation of an inflammatory response and the deployment of immune cells (see p.345).
of defense against pathogens, providing protection via
the physical and chemical barriers presented by the Tissue damage results in inflammation, which helps to prevent microbes from
various surfaces of the body. These include both external spreading. The capillary walls in the affected area become more permeable, enabling
surfaces, for example, the skin, as well as mucus-lined immune cells to easily enter the intersitital fluid and access the infected tissue. Damaged
internal surfaces, for example the airways and the gut. cells release chemicals that attract the immune cells once they have migrated from the
blood stream. The first cells to arrive are usually phagocytes (predominantly neutrophils),
Each body surface forms an initial physical barrier but other elements, including Natural killer cells (see below) and the complement
to infection, and this is then supplemented by a variety system (see opposite) may also be engaged. If innate immunity cannot resolve the
of substances secreted at these barriers that exhibit infection, the adaptive immune system may be set in motion (see pp.348–49).
antimicrobial properties, such as enzymes, which break
down bacteria. Additional mechanisms function to expel Swollen, Phagocytes
or flush out microbes from the body, for example, red tissue attack microbes
coughing, sweating, and urination.
Broken skin Invading Chemicals released Phagocytes
Tears microbes by damaged cells exit from
Flush the eyes and associated capillary wall
membranes and contain the
Breaching the barrier Inflammatory response
enzyme lysozyme, which Injury to a body surface results in bacteria gaining access to internal Local blood vessels dilate, allowing more blood to pass through the
disrupts bacterial cell walls. tissues. To minimize damage, a defensive inflammatory response is area. Tissue permeability to blood plasma increases, and the now
immediately activated as the injured cells release chemicals that attract more-porous capillary enables phagocytes to access the interstitial
Saliva phagocytes to the scene. Inflammation of body tissue is characterized fluid. The “chemical trail” produced by the damaged tissue then leads
Flushes the oral cavity, by four key features: swelling, heat, pain, and redness. them to the site of infection where they attack invading microbes.
trapping microbes. Contains
lysozyme and lactoferrin INTRACELLULAR INFECTIONS
(antimicrobial agents).
Natural killer (NK) cells target body cells infected with
Mucous membranes pathogens. Body cells display surface receptors, called the
Secrete mucus to trap major histocompatibility complex (MHC), that provide
microbes. Cilia (see p.325) information about the cell’s internal environment and
line the airways and transport indicate when it is infected. NK cells closely monitor these
microbes up to the mouth. receptors, as infected body cells may avoid displaying them
to evade detection. However, NK cells become activated
Skin when they detect reduced numbers of MHC on a cell
Physically blocks pathogens. surface and will target such cells for destruction.
Sebaceous secretions contain
Malignant targets
fatty acids that disrupt NK cells are also able to identify and attack malignant cancer
microbial membranes. cells, as shown in this electron micrograph. The NK cell (white)
extends long projections to wrap around the cancer cell (pink).
Stomach acid
Produces very low pH in the

stomach that helps to kill
many (but not all) microbes

present in ingested food.

Urine
Flushes the vessels of the
genitourinary system, helping
to keep them free of infection.

First line of defense
The body’s physical, chemical, and mechanical
barriers are maintained constantly and, as such,
are a passive means of defense. If they are
unable to keep pathogens out of the body,
an active immune response takes over.

347

LYMPHATIC AND IMMUNE SYSTEM

EXTRACELLULAR INFECTIONS Phagocytosis 0 sec 10 sec 20 sec 30 sec
This series of time-lapse, 40 sec 50 sec 60 sec 70 sec
Fundamental to the innate immune response are cells known as phagocytes microscopic images illustrates
(macrophages and neutrophils) that “eat,” or engulf, microbes that have infected the process of phagocytosis. The Digested
tissue fluids. This process is known as “phagocytosis.” The cell surfaces of bacteria are bacterium (green) is identified by cellular
composed of materials that are different from those of human tissues, and this fact the phagocyte (red) via surface fragments
has allowed a system of contact recognition to evolve. Once identified, an invading contact and has been completely
bacterium is enveloped, absorbed, and then digested by the phagocyte. ingested within 70 seconds.

Phagocyte Phagolysosome Phacogyte expels
extends encases bacterium waste products

pseudopods

Bacterium Bacterium is Expulsion
gradually digested Aggressive chemical reactions ensure that the
Recognition bacterium is killed quickly. Digested cellular fragments
Recognition of a target bacterium by the phagocyte Digestion that cannot be broken down further by the phagocyte
is achieved on contact of the two cells’ surfaces. The The bacterium is contained within a specialized vesicle are then expelled.
phagocyte then extends projections (pseudopods) called the phagolysosome, in which it is neutralized
that engulf and absorb the bacterium. and broken down by the internal molecular killing
mechanisms of the phagocyte.

COMPLEMENT SYSTEM Approach Membrane attack Perforation Rupture
Bacterial surface proteins The proteins combine to form The resultant hole allows The combined fluid influx
Specialized proteins, together known as the complement activate the complement system, the “membrane attack complex” extracellular fluid to enter the causes the bacterium to swell
system, circulate freely in blood plasma where they causing the individual proteins —a structure that punches a hole bacterium. This process occurs and eventually rupture.
target microbes. They are ordinarily present as separate to assemble at the cell surface. in the bacterium’s surface. repeatedly over the cell surface.
molecules, yet once activated the proteins act together
as a “cascade,” initiating a complementary chain reaction
that attacks and destroys microbes. Like phagocytes,
complement proteins can be activated by bacterial
surface features, allowing them to easily respond
to infections throughout the body, accessing tissues via
inflammation (see opposite). They also react to pathogens
that have been bound by antibodies (see p.349).

INFECTIOUS AGENTS VIRUS BACTERIUM FUNGUS PROTOZOAN PARASITIC FRIENDLY BACTERIA
WORM
Causes of infection and disease are often microscopic, and The human gut represents a huge surface
broadly divide into five categories. Bacteria and viruses, area that is vulnerable to infection. A large
the smallest and most prevalent, cause many well-known population of harmless bacteria that colonize
illnesses. Fungi infect the skin and internal mucosa, the gut wall form another key barrier to
causing systemic disease in the immunocompromized. infection. These “friendly” bacteria prevent
Protozoa (single-celled animals with nuclei) cause serious harmful bacteria from gaining a foothold,
diseases, such as malaria. Parasitic worms infect areas such and subsequently infecting the body.
as the gut, causing debilitating, or even fatal, diseases.

348

HOW THE BODY WORKS

ADAPTIVE IMMUNITY CELL-MEDIATED RESPONSE

The adaptive immune system provides the body with the means This immune response targets pathogens that infect body cells, for
to develop highly specific immune responses to particular example viruses. It occurs when an APC bearing a microbial antigen
pathogens encountered during its life span. Crucially, such derived from the infected tissue migrates to a lymph node and presents
responses may be quickly redeployed if a pathogen reinfects. the antigen to a T cell that is able to recognize it. Recognition results in
activation of the T cell and triggers a series of reactions that create a swift,
coordinated attack. Killer T cells target the infected body cell, while
helper T cells produce key signaling molecules that shape the
immune response. Only a few T cells of each specificity exist
within the body, yet their rapid circulation maximizes their
chances of encountering target antigen.

AGENTS OF SPECIFIC RESPONSE T CELL RECOGNITION APC
Presentation by the APC in the lymph Presents antigen
T and B lymphocytes are the key agents of the adaptive node results in recognition of the
immune response. Unlike innate immune cells, they can antigen by the killer T cell. If that to killer T cell
recognize and target specific pathogens that enter the recognition is confirmed, via signals, by
body, and are capable of remembering a specific pathogen an activated helper T cell nearby, the Antigen fragment
and acting quickly to eliminate it if it should ever reinfect. killer T cell then becomes activated.
T and B cells can attack particular pathogens through their Killer T cell
ability to recognize specific molecular targets, called Recognizes antigen
antigens, as foreign. Antigens are recognized via cell-surface
receptors displayed by lymphocytes. These receptors are Multiple attack CLONAL SELECTION Activated killer
individually programed to recognize a specific antigen. T cells are able to target body cells that have Once activated, the killer T cell T cell
become malignant, as seen in this micrograph, undergoes a process of division called
Two types of T cell—killer, or cytotoxic (attack cells) and where four T cells (red) attack a cancer cell (gray). “clonal selection.” This involves the Undergoes
helper (coordinating cells)—respond to cellular infections; B production of multiple effector cells and clonal selection
cells respond to fluid infections (see opposite). These cells memory cells. Effector cells exit the
circulate through the body, via the secondary lymphoid lymph node to locate and attack the to produce
tissues, in search of their target antigen. pathogen—the APC will have imprinted hundreds of
the original killer T cell with information clone T cells
Surface Maturation of T and B cells about the site of infection, and this is
receptors As they mature T and B cells gain receptors that enable them transferred to effector cells. Memory cells Memory cells
collectively to recognize a huge range of specific antigens. stay in the lymph node, but may be Remain in lymph
During maturation, any cells that recognize, and may activated subsequently to provide a rapid node to recognize
therefore attack, body tissues are eliminated. This usually response if the same pathogen reinfects. future infections
ensures that antigens that are recognized are foreign in origin.
HELPER T KILLER T B CELL

ANTIGEN PRESENTATION Uptake of antigen Identification
A virally infected body cell Killer T cells monitor body cells for
T cells are only able to recognize an antigen if it is bursts, releasing microbial target antigen displayed via their MHC
“presented” to them by other immune cells—most antigen. APCs absorb this receptors—these denote the condition
commonly dendritic cells, but also macrophages. These antigen for presentation to of the cell’s internal environment.
are known as antigen-presenting cells (APCs) and are T cells in the lymph node. Recognition of target antigen
widespread in body tissues. During infection, APCs absorb indicates that the cell is infected
antigen fragments and migrate, via lymphatic vessels,to Ruptured body cell
local lymph nodes. Here they present the fragment
to resident T cells, enabling any with a corresponding Released microbial
receptor to recognize the antigen and launch an attack antigen
(see opposite). B cells can interact directly with antigens
carried in the lymph, independently of APCs. For adaptive Body cell Infected MHC
immune cells, the lymphatic system therefore forms a body cell
comprehensive surveillance network for the entire body. APCs
(dendritic cells) Granzymes
Interaction Pierce cell
An electron APC T-receptor Antigen MHC receptor
micrograph presents interacts with T-receptor Denotes internal membrane to
captures the antigen condition of induce chemical
remarkable fragment antigen body cell breakdown of cell
interaction
between a MHC T cell DEATH BY T CELL Virus
T cell (pink) and Once the infected body cell has been
a dendritic cell Presentation of antigen positively identified, the killer T cell Microbial antigen
(green) that occurs An APC presents an antigen to a T cell via a attacks. It releases cytotoxic molecules Displayed on cell surface via MHC,
during antigen receptor called the major histocompatibility (granzymes), which penetrate the cell and indicates that cell is infected
presentation. complex (MHC). If the antigen is recognized, membrane and induce directed cell
the T cell will become activated (see opposite). death, known as “apoptosis.” This involves
the degradation of the cell’s contents but
without the release of the components,
limiting the possible spread of virus
particles to neighboring cells.