How body works

98 99

Other sensations

There may be more than just the
five basic tastes. Fat receptors
have been found, and some sour
receptors bind to carbon dioxide,
affecting the taste of sodas. We
may also be able to detect the
chalky taste of calcium. Metallic
tastes and the astringent sensation
from tea are unexplained by our
five-taste understanding. Some
familiar food and drink sensations
are not tastes at all, but responses
of hot, cold, pain, and touch senses.

Cool receptors Heat and pain Touch receptors
Nerve endings on our Heat receptors report the The tongue contains
tongue respond to cold temperature of our food. touch receptors that
temperatures. These Capsaicin in chili activates detect the texture of
nerve endings are made these nerves, misreporting our food, and these may
more sensitive by the contribute to the sensations
menthol in mint, which to our brains that the caused by the bubbles of
food is burning us. carbonated drinks and
is why mint feels so
refreshing. other sparkling
beverages.

Pain receptors
Pain receptors signal
various types of pain.
Some receptors respond
to dangerous heat, while
horseradish and wasabi
activate a receptor type
on the tongue that is
sensitive to itch and

inflammation.

MIRROR BOX THERAPY Visual information
from eye
Many amputees suffer from “phantom limb”
pain. The brain interprets the lack of sensory Mirrored Complete
input from the missing limb as a sensation image of limb
that the muscles are clenched and cramping. complete
By tricking the brain into “seeing” the
phantom limb with a mirror box, limb
movements in the retained limb can often
relieve the pain.

Body Balance
position sense information

How do you know where your hand is if from ear
you’re not looking at it? Sometimes called
our sixth sense, we have receptors dedicated Tension receptor
to telling our brains where each part of our Organs within your tendons detect
body is in space. We also get a sense that how much force your muscles are
our body parts belong to us. exerting by monitoring muscular
tension (see pp.56–57).

Muscle
Golgi tendon organ senses
changes in muscle tension

Bone

Tendon

Position sensors Stretch receptor
Tiny spindle-shaped sense
There is a range of different receptors that organs buried in your
help the brain calculate the position of our muscles detect changes in
body. For a limb to move, the joint must the length of the muscle,
change position. Muscles either side of the telling the brain how
joint contract or relax, changing in length or contracted the muscle is.
tension. Tendons that attach muscle to bone Muscle spindle organ detects
are stretched, as is the skin on one side of the changes in length of muscle
joint, while the skin on the other side relaxes.
By combining information about each of these Nerve sends
components, the brain can construct a fairly signal to brain
accurate picture of the body’s movements.
Muscle

Cerebral Cerebellum 100 101SENSITIVE TYPES
cortex Conscious Body position sense
Subconscious pathway
pathway Integrator

The brain combines information from the sensors located
in and around the muscles as well as your other senses to
interpret how your body is positioned. The conscious element
of this is controlled by the cerebral cortex and allows you to
run, dance, or catch. The cerebellum, at the base of the brain,
is in charge of the unconscious elements that keep you upright
without your thinking about it.

BODY OWNERSHIP SENSE

Bone Your sense that your body is your own is more complicated and flexible
Touch-sensitive than it seems. The rubber hand illusion shown here creates
nerves the feeling that a fake hand belongs to you. A similar technique can
Joint receptors invoke out-of-body experiences, using a virtual reality headset. This
Receptors within your joints flexibility allows us to cope if we lose a limb, or to include tools and
prosthetics in what we think of as part of our body.
detect its position. They are
most active when your joints 1 2
are at their extremes to help
prevent damage through Visual attention
overextension. However, they on rubber hand
may also play some role in
detecting the position of joints Rubber Brain accepts rubber
in normal motion. hand hand as part of the
body
Ligament receptors

Ligament

Covering Rubber hand and
real hand stimulated
in the same way

ESTABLISHING THE CONNECTION RUBBER HAND SEEN AS PART OF BODY

Skin stretch BODY POSITION SENSORS
Special receptors in the skin IN THE JAW MUSCLES
(see p.75) can detect stretch. AND TONGUE HELP
This helps us determine the YOU FORM THE
RIGHT SOUNDS
movements of our limbs, WHEN YOU SPEAK
particularly changes in the
angle of a joint, which causes
the skin on one side to stretch
while the skin on the opposite

side is slackened.

Integrated senses

Your brain makes sense of the world around you by BRAIN
combining information from all your senses. But,
surprisingly, sometimes one sense can actually
change how you experience another.

How senses can interact Sound and vision
When things happen simultaneously,
Everything you experience is interpreted you often assume they are linked, even
by your senses. When you see and pick though your senses are sending you different
up an item, you feel its shape and texture. messages. If you hear an alarm close to your
You look for where sounds or smells are car, you will disregard the location of the
coming from and “eat with your eyes” sound (unless it is very widely different), and
before tasting your food. Your brain believe the alarm is coming from your car.

performs complex processing to If sound of alarm is distant from
integrate this information correctly. car, it can be distinguished

Sometimes, this combination of

information can cause multisensory CAR ALARM CAR
illusions. If information from different SOUND You move toward
senses seems to conflict, the brain Sound car assuming it is
favors one sense over another, and of alarm is the alarm’s source
close to car

depending on the situation, this can

be helpful or misleading.

STALE TASTY Sound of crunching is
played to one person
while eating

STALE POTATO CHIPS IN NOISY
Taste and sound ENVIRONMENTS
If someone listens to the sound of crunching while eating stale potato YOU LIP-READ,
chips, they will claim they taste fresh. Tactically, manufacturers make USING WHAT YOU
chip bags crackly so the chips seem crunchier. SEE TO INTERPRET
MUFFLED SPEECH

102 103SENSITIVE TYPES
Integrated senses

Smell and taste SOUNDS AND SHAPES
Taste is a simple sense, made up of crude
sensations such as “sweet” or “salty”. Most When shown these shapes and
of what you think of as flavor is actually what asked to name one Bouba and
you are smelling. Smell can also influence the the other Kiki, most people call
crude sense of taste itself. Smelling vanilla can the spiky shape Kiki because of
make food or drink taste sweeter, but only in its spiky sound, while deciding
parts of the world where vanilla is a common the softer Bouba fits the rounded
flavor for sweet foods. shape. This pairing holds across
a wide variety of cultures and
languages, indicating a link
between the senses of sound
and sight.

Non-sweetened Vanilla pod
ice cream tastes emits its
sweet distinctive
scent

Image of ball and spring Pressure of ball
bouncing on virtual and spring felt
version of hand
on real hand

VIRTUAL REALITY REAL LIFE

Touch and vision
When gamers pick up objects in virtual reality, visual
cues give them physical sensations, even though their
touch sense gives them no such information. What your
eyes can see can actually influence what you feel.

Using your voice

Talking is achieved by a complex yet flexible 3 Producing sound
network of nerve pathways in the brain and As you exhale, your vocal cords
physical coordination of the body. Tone and vibrate as the air stream passes them,
inflection influences how words are spoken, making sound. Vibration speed dictates
which can add numerous meanings to your voice’s pitch, and this is controlled by
even the simplest of sentences. muscles in the larynx. If you want to shout,
you need a stronger airstream.

1 Thought process Broca’s area on Vibrating cords
Firstly, you must decide the left side of brain cause sound
what words you want to say. This
activates a network of regions in formulates speech
the left hemisphere of your brain,
including Broca’s area, drawing
on your memory store of words.

Vocal cords Larynx 4 Articulation
open to allow air Your nose, throat, and mouth
into lungs

act as resonators, while lip and tongue
movements introduce specific sounds,
altering the buzzing produced by the
vocal cords into recognizable speech.

2 Breathing in
Your lungs provide the
constant stream of air that you
need in order to speak. When
inhaling, vocal cords open to
allow air to pass through, and
then air pressure begins to
build in the lungs.
MAKING AN
Air pressure in “EE” SOUND
lungs builds
MAKING AN MAKING AN
How do you talk? “AA” SOUND “OO” SOUND

The brain, lungs, mouth, and nose all play vital roles when Making different sounds
producing speech, but the voicebox, or the larynx, is the most Your tongue moves to mould sounds created by your
important. Located in your throat above your windpipe, it vocal cords, aided by the teeth and lips. Changing
contains two sheets of membrane that stretch across the inside. the shape of your tongue and mouth produces
These are the vocal cords, and they are the structures that vowels like aa, or ee, and the lips interrupt air flow to
produce the sound you craft into speech. produce consonants, such as p and b.

104 105SENSITIVE TYPES
Using your voice

Pathway of speech Motor cortex sends MOTOR CORTEX
Each area of the brain is connected instructions to muscles
via nerves. The bundle of nerves
linking Wernicke’s and Broca’s to articulate reply
areas, the arcuate fasciculus,
is comprised of nerve cells
that fire at high speeds.

BROCA’S Bundle of nerves
AREA links Wernicke’s
and Broca’s areas
Broca’s area allows
listener to plan WERNICKE’S
reply based on AREA
speech heard Wernicke’s area
AUDITORY processes word
AREA meanings
Auditory area
analyzes speech

Speech reaching listener’s ear HOW DO YOU SING?

Processing speech When you sing, you use the same
physical and cognitive networks as
Air vibrations caused by speech reach the ear when you speak, but it requires much
and trigger nerve cells deep inside, which then more control. Air pressure is greater, and
send signals to the brain for processing. Wernicke’s several chambers, such as the sinuses,
area is vital for understanding the basic meaning of mouth, nose, and throat are used as
the words, while Broca’s area interprets grammar resonators, producing a richer sound.
and tone. These regions are part of a larger
network that understands and produces speech. Frontal sinus
Damage to either area can lead to speech problems.

Nose Nasopharynx
Mouth
Upper
RESONATING throat
CHAMBERS
Lower
throat

Reading faces

We are a social species, so recognizing and understanding Facial expression cues
faces is vital for our survival. This means we have evolved When recognizing a face, you look at
to be very good at noticing them—even sometimes the ratio between the eyes, nose, and
seeing them where they don’t really exist, mouth. Movements of these can help
like on a piece of burned toast!
you detect emotions; for example,
Importance of understanding faces raised eyebrows and an open mouth
would signal surprise. These signals are
From birth, babies are fascinated by faces, and show a preference for
looking at them above everything else. As you age, you not only quickly interpreted by your eyes and nerve
become an expert in recognizing faces, but also reading expressions. signals are sent to the fusiform face
This allows you to identify those who would help or harm you. area in your brain to be processed.
Individual faces can stay in your memory for a remarkable
length of time, even if you haven’t
seen the person in years.

Fusiform face area UNDERSIDE OF BRAIN
This area of the brain, named the
fusiform face area, is activated when
you look at faces. It is thought that
this area of the brain is specialized
in facial recognition. However, it
also becomes active when you
are looking at objects with
which you are familiar—if you
were a pianist, it may become
active when you see a keyboard.
Whether it is face-specific is
still being studied.

Location of fusiform face area
on both sides of brain

RECOGNIZING FACES

Humans tend to spot faces
in random patterns and
places—from cars to cheese,
clothes washers and pieces
of wood. This is because
it was important for our
ancestors to interpret the
faces of others in order
to thrive in a complex
social hierarchy.

106 107SENSITIVE TYPES
Reading faces

Expression muscles when to leave a person alone, or when to offer
comfort. Picking up even the subtlest cues, such as
Your face contains muscles that pull your skin and the furrowing of the brow or the curling of the lip,
change the shape of your eyes and the position of can mean the difference between interpreting a
your lips, making your face highly expressive. The frown or a smirk correctly.
ability to read these expressions on other faces
allows you to judge other people’s moods, intentions,
and meanings. Faces tell us when to ask for a favor,

Corrugator Frontalis muscle inactive How you smile
supercilii furrows when smiling Smiles are caused by
the brow the zygomaticus major
How you frown Genuine pulling the corners of
When you frown, the smiles may use your mouth up and out.
corrugator supercilii orbicularis oculi True smiles may also
draws your eyebrows engage the orbicularis
down, wrinkling the Levator muscle oculi, causing a crinkling of
skin between them. lifts upper lip the eyes, whereas insincere
Meanwhile, the smiles do not. Each person
orbicularis oculi uses different muscles
narrows the eyes and when they smile; one
a network of muscles, person’s grin may be
including the another’s smirk.
depressor anguli oris,
angles the mouth
down at the corners.

Orbicularis
oculari
muscle

narrows
the eyes

Zygomaticus major
muscle pulls mouth
and corner of lip up

and sideways

Depressor anguli oris
muscle pulls mouth
and corner of lip down

GAZE AND EYE CONTACT PEOPLE
BORN
TYPICAL GAZE People with autism (see p.246) usually BLIND
don’t focus on the eyes and mouth when PRODUCE
looking at faces. They find socializing THE SAME
confusing and difficult, and may miss EXPRESSIONS AS
vital social cues when communicating. SIGHTED PEOPLE
Babies may even exhibit this averted WHEN EMOTIONS
gaze, and they may go on to develop ARE PROVOKED
the condition, so it could be used as
an early warning sign for autism.

People with autism show different
THOSE WITH AUTISM patterns of looking behavior

What you don’t say

You communicate using more than just your words. INVADING
Facial expressions, tone of voice, and hand gestures SOMEONE’S
can speak volumes, and noticing these signals is vital PERSONAL
for understanding what someone really means. SPACE CAN
INSPIRE FEAR,
Nonverbal communication AROUSAL, OR
DISCOMFORT
When you are talking to someone, you are subconsciously picking up on
subtle signals from the other person’s voice, face, and body. Interpreting
these signals correctly is most important when what is said could be

ambiguous. Most of these signals allow you to gauge the mood of a
person or group so you act appropriately in social situations. For
example, in a meeting at work, assessing the body language
and moods of your colleagues can be advantageous to you
if you are waiting for the right time to pitch a big idea.

AL EXPRES TYPE OF CLOTHIN G

FACI SIONS

Types of signals HA ND GESTURES BO DY POSTU RE
Facial expressions, hand D SPEED ICAL CON
gestures, body posture, TONE AN OF VOICE PHYS TACT
and the tone and speed
of somebody’s voice are Head tilted Body language
all signals you process Physical The way your body moves as you speak
when communicating. contact can often be just as telling as what you
What someone is wearing say. Holding eye contact, mirroring the
is also important because Mirrored facial expressions and posture of others,
it can provide clues about legs and physical contact, are generally
their personality, religion, interpreted as positive signals.
or culture. Physical Folded arms, hunched shoulders,
contact can add and positioning yourself away
emotional weight from others can produce
to what is said. negative vibes.

Arms folded,
forming barrier
Body turned away

from others

NEGATIVE POSITIVE

108 109SENSITIVE TYPES
What you don’t say

Pausing Caught in a lie Microexpression
You may pause more when you lie
because thinking of a fabricated It is sometimes an 1 SECOND
response takes longer than providing advantage to deceive
a natural one. Even if you are telling those around you, but Microexpressions
a story that actually happened, if also useful to be able Lightning-quick expressions appear
your emotions toward the event to tell when someone unconsciously on the face of a liar and
are untrue, pausing is still a is deceiving you. usually show an emotion he or she is
suggestive sign of lying. However, there are attempting to conceal. These expressions
signals that can give last less than half a second and are usually
Visible hand twitches you away when you lie.
can be a giveaway The best liars convince missed by the average person, but can be
themselves they are detected by a trained viewer.
Hand movements telling the truth—if you
Movements of the body are unedited truly believe your lie, SUPERMAN POSE
by consciousness so are often a more your body language
reliable indicator of lying. When you can’t give you away. Body language is so
lie, you often wring your hands, make powerful that it can even
gestures, or have nervous twitches. Twitches of a change the way you feel
person’s toes about yourself. Adopting a
CAN WE may be an powerful stance for just a minute
DETECT ALL LIES? indicator raises your levels of testosterone,
No—everyone has of lies in both men and women, and
their own ways of lying. reduces levels of the stress
One person may pause and hormone cortisol. This increases
another may twitch their toes, feelings of control, the likelihood
while both these signs could that you will take risks,
point to a host of hidden and your performance
meanings other than in job interviews
improves too.
dishonesty. This shows that
movements of
your body
can influence
emotions, and
proves the old
saying “fake it till
you make it” really
is good advice!



THE HEART
OF THE
MATTER

Filling your lungs 1 Breathing in
Air is warmed and moistened
Your lungs act like a giant pair of bellows, drawing air as it passes through the nose or mouth.
in and letting it out to extract oxygen and expel waste Air breathed in Nasal hairs filter out dust particles that
carbon dioxide. You breathe around 12 times per minute
at rest and 20 times per minute or more during exercise; could irritate the trachea or lungs
which all adds up to roughly 8.5 million breaths per year. and cause a coughing fit.

NASAL CAVITY

Controlling breaths BRAIN Drawing breath TONGUE

Your breathing rate speeds up SIGBNRAALINTO Air drawn in through Air traveling through
or slows down due to signals the nose or mouth passes the throat
from chemical receptors in the down the trachea, or
blood vessels. These receptors windpipe, which channels Air traveling down
provide a feedback loop air into the left or right the trachea
between the blood vessels, bronchus, and then into
brain, and diaphragm. smaller and smaller air
passages called bronchioles.
Receptor monitors levels of Between the trachea and the
oxygen in blood vessels ends of the bronchioles, your
airway divides 23 times.
Direction of
nerve signals Blood vessel

NERVE Cluster of receptors monitors TRACHEA
levels of oxygen in blood
Signals sent to diaphragm HEART from the heart
to control breathing rate DIAPHRAGM
Bronchiole LUNG
Feedback system Lining of right lung
Chemical receptors detect
changes in oxygen, carbon
dioxide, and acidity levels in
the blood. This information is
sent to the brain, which controls
the diaphragm’s movements,
increasing or decreasing rate
and depth of breathing to
keep blood levels constant.

SIZE MATTERS Pleural cavity T BRONCHUSRIGHT BRONCHUSTHE HEART OF THE MATTER
Bronchioles branching
The surface area of all the tiny 2 Into the lungs into microscopically Filling your 112 113lungs
air sacs (alveoli) in the lungs Air travels down each bronchus small airways
measures an incredible 753 sq ft into ever-smaller passages, eventually LEF
(70 sq m)—this is 40 times greater ending in tiny air sacs called alveoli.
than the surface area of your skin! The lungs are separated from the
This maximizes the amount of chest by a pleural cavity filled with
oxygen you can absorb. pleural fluid. This thin layer of fluid
acts as a sticky lubricant, letting your
SKIN lungs slide over your chest wall and
preventing them from pulling away
LUNGS as you breathe out.

Mechanics of breathing ALL YOUR
AIRWAYS
Chest muscles and the rib cage LAID END
influence breathing, but the main TO END WOULD
powerhouse is the diaphragm. MEASURE 1,490 MILES
It is a large domed muscle that (2,400 KM)
separates the chest from the
lower organs. To breathe in, the Air breathed in Air breathed out
diaphragm contracts and pulls
down like a piston. At the same Air enters lungs Air exits lungs
time, muscles between your ribs Chest expands Chest tightens
contract, lifting your ribs so your
lungs expand and air rushes in. Lungs expand Lungs reduce in size
When your diaphragm and chest Diaphragm Diaphragm relaxes and
muscles relax, air is forced out. contracts
domes upward
Direction of Direction of
movement movement

INHALING EXHALING

From air to blood Vein carries
oxygen-rich
Every cell in your body needs oxygen and your lungs blood to heart
are highly adapted to extract this life-sustaining gas
from the atmosphere. This extraction occurs from 300 Ring of stiff cartilage
million tiny air sacs called alveoli, which give your that stops bronchiole
lungs a spongelike texture. from collapsing
CLUSTER ARTERY
Deeper into the lungs Alveolar sacs
VEINLUNGS The bronchioles lead
Inhaled air passes from the throat into the trachea to reach into grapelike clusters
tiny branches called bronchioles. Mucus covers each BRONCHIOLE of alveoli, each of which
bronchiole, which keeps them moist and traps inhaled is wrapped in capillaries,
particles. Each bronchiole is lined with thin strips of muscle. the smallest type of
In people with asthma, a sudden constriction of these blood vessel. In contrast
muscles narrows the airways, causing shortness of breath. to blood vessels in the
rest of the body, it is
THE AIR YOU BREATHE OUT the arteries that carry
CONTAINS 16 PERCENT oxygen-poor blood
OXYGEN, ENOUGH TO to the capillaries.
RESUSCITATE
SOMEONE! Artery carries
oxygen-poor
blood from the
heart to the lungs

OF ALVEOLI

WHY CAN WE Capillaries wrap
SEE OUR BREATH around every alveolus

IN COLD AIR?

The air you breathe is warmed
in your lungs, so when you
exhale, water vapor in your

breath condenses into clouds
of water droplets.

Types of gas Exhaled air contains 100 Blood going back to heart to
Oxygen times more carbon be pumped around the body
Carbon dioxide
dioxide than inhaled air HIGH ALTITUDE
One-cell-thick
wall of capillary Inhaled air contains At high altitudes, air is thinner and
21 percent oxygen less oxygen is present. You may find
LUS One-cell-thick yourself automatically taking deep
wall of alveolus breaths, since your body will detect
lower amounts of oxygen in your
Blood ALVEO bloodstream than it normally expects.
plasma rich ARY

in carbon
dioxide

Oxygen-poor m x1000 ft x1000
red blood cell 10 30

9 20

8 10
PERMANENT 0
7 THE HEART OF THE MATTER

Carbon 6 From air to 114 115blood
dioxide
entering air 5

4

3

1 Carbon 2
dioxide
Carbon dioxide diffuses 1 TEMPORARY
0
from the blood plasma
through the one-cell-thick
walls of the capillary and Acclimatizing Adapting
alveolus. Blood can absorb CAPILL People who travel Those who live
oxygen and get rid of carbon Oxygenated to high altitudes can their entire lives at
red blood cell adapt by producing high altitudes may
more red blood inherit larger lungs,
dioxide simultaneously. cells to carry more wider chests, and
oxygen in their more efficient
Gas exchange Oxygen entering circulation. Full oxygen-processing
red blood cell adaptation takes genes in order to
Capillaries are in such close contact with alveoli that around 40 days, but cope permanently
gases are able to cross over rapidly. Carbon dioxide 2 Oxygen is not permanent. with the hardships.
leaves the blood in exchange for oxygen, and the newly The oxygen we breathe diffuses
oxygenated blood is distributed around the body by the from alveolar air into the blood. Here, it
heart. Since you do not exhale all your inhaled air in one is captured by red blood cells, turning
breath, oxygen-poor and oxygen-rich air mixes in your them, and the blood, bright red.
lungs, which is why exhaled air contains oxygen.

Why do we breathe?

The oxygen we breathe is vital for staying alive because we use it
to create energy. Tiny capillaries, the smallest type of blood vessel,
transport oxygen to the 50 trillion cells that make up your body. One

person uses about 1,160 pints (550 liters) of oxygen per day.

Oxygen-depleted blood

HEMOGLOBIN Turning blood red Oxygen for energy
Red blood cells are packed
with a pigment (a colored Blood transports oxygen to every single
protein) called hemoglobin. cell in the body. Each cell uses oxygen
When oxygen is absorbed in a chemical reaction to break down the
into the blood, it binds to iron sugars ingested from food to produce
atoms in the hemoglobin, and energy. This process is called cellular
in doing so, it enriches the respiration, and is happening constantly
pigment’s color, turning the throughout your body. A byproduct of
blood vivid red. this reaction is carbon dioxide, which is
carried back to the lungs by veins and
Oxygen molecule breathed out as we exhale.
binding to iron atom
within haemoglobin Blood cell loses
Oxygen molecule oxygen, therefore
loses its red colour

Red blood cell
rich in oxygen

Cells of the
body hungry

for oxygen

116 117THE HEART OF THE MATTER
Why do we breathe?

Gas exchange One-cell-thick
Oxygen diffuses, or drifts, from capillary wall
where it is in high concentrations
(in red blood cells) to where THIN CAPILLARIES
there is a low concentration
(in body cells). Likewise, Capillaries connect tiny arteries
carbon dioxide diffuses (arterioles) with tiny veins (venules).
from the body cells The thin walls of capillaries allow
into the blood. the exchange of oxygen and carbon
dioxide. They are thin enough to
Red blood cell access all body tissues from bones to
No oxygen molecules attached skin, yet only just wide enough for
red blood cells. Red blood cells even
to the iron atoms have to change their shape to
in the deoxyhaemoglobin squeeze through some capillaries.

MOGLOBIN Body cell HUMAN HAIR
0.08MM
Red blood cell BLOOD CAPILLARY
without oxygen 0.008MM

DEOXYHE Blue blood?
When hemoglobin is carrying oxygen,
it is called oxyhemoglobin. When it
releases oxygen into your body tissues,
it becomes deoxyhemoglobin, and
turns a dark red color—the color of
oxygen-depleted blood. The blood is
not really blue, even though veins look
blue beneath your skin.

IF YOU HOLD YOUR
BREATH, THERE IS
ENOUGH OXYGEN
IN YOUR BLOOD
TO STAY CONSCIOUS
FOR SEVERAL
MINUTES

Particles irritate Sound waves
nerve endings radiate

Air forced THROAT Air drawn in
from throat Vibrating soft palate Air flowing over
back of soft palate
Particles makes air in the makes it vibrate
irritate throat vibrate
nerve
endings

Mucus and Air forced
particles spray out from lungs

Sneezing Snoring

This reflex aims to remove irritants from A partial collapse of the upper airway during

the nasal cavities, and can be triggered sleep will cause snoring. The tongue falls back

by inhaled particles, infection, or allergies. and the soft palate vibrates as you breathe.

1 Irritants 2 Involuntary
enter lungs intake of breath
Coughing is triggered
when special cough The brain sends a nerve
message instructing
receptors in the lining the lungs to fill with air.
of the airways are Cough
irritated by inhaled receptors in There follows a sharp,
airway irritated deep intake of breath.
particles, chemicals, by particles
or excess mucus. LUNG
Air drawn in

Diaphram
contracts

Irritant particles Lungs expand
(dust, smoke)

Coughs
and sneezes

The respiratory system leaps into sudden action without our conscious
control. Its reflex actions get rid of particles in the airways with coughs and
sneezes. The functions of hiccups and yawns, however, are more mysterious.

118 119THE HEART OF THE MATTER
Coughs and sneezes

Air expelled Vocal cords
explosively open throat
Irritant particle,
trapped in mucus,
flies out of throat

4 Air explodes out
The chest muscles
forcefully contract and
diaphragm relaxes. The Air bursts out
vocal cords snap open Irritants
and an explosive cough expelled
expels the irritants.
Chest muscles
3 Pressure contract
rises Pressure from
The vocal cords diaphram
snap shut and the
diaphragm begins
to relax, causing air
pressure in the
lungs to rise.
Vocal cords
close throat Air rushes
Pressure from lungs
builds up
in lungs

Diaphram YAWNING
relaxes and
Amazingly, experts still don’t know
curves up why we yawn. Because yawning is
contagious, some scientists suggest
Air drawn in that in our evolutionary past,
yawning was used to signal fatigue
Epiglottis to other members of the troop
snaps shut or herd, and may even
have helped
Hiccups Sound radiates synchronize the
group’s sleep
A rapid, involuntary contraction Lungs patterns.
of the diaphragm, sometimes two or expand
more in rapid succession, causes Wide-open
mouth of yawn
air to rush into the lungs. A flap of does not increase

cartilage in the throat called the Diaphragm oxygen intake
epiglottis audibly snaps shut—this is a spasms

hiccup, and it is unknown why we do it.

The many tasks Fluid of life
of your blood Plasma is a straw-coloured
fluid containing water
Your heart and blood vessels contain around
10 ½ pints (5 liters) of blood, which transports plus dissolved salts,
everything your cells need or produce, such as hormones, fats, sugars,
oxygen, hormones, vitamins, and wastes. Blood and proteins, as well
carries nutrients from food for processing and as tissue wastes.
toxins for detoxification to the liver, and
transports wastes and excess fluid to the 45% red blood cells
kidneys, which expel it from the body. 1% white blood cells and platelets

What is blood made of? 54% plasma

Blood consists of a fluid called plasma, in which float 5 MILLION
billions of red and white blood cells, plus platelets—the cell
fragments involved in blood clotting. Blood also contains THE NUMBER OF RED
wastes, nutrients, cholesterol, antibodies, and protein BLOOD CELLS IN A
clotting factors that travel within the plasma. The body DROP OF BLOOD
carefully controls blood temperature, acidity, and salt
levels—if these vary too much, blood and body cells could
not function properly.

Oxygen transport

Most oxygen is carried within the red blood cells. A small amount

of oxygen is also dissolved in plasma. After a red blood cell collects

oxygen from the lungs, it takes around a minute to complete one circuit

around the body. During this circuit, oxygen diffuses into the tissues

and carbon dioxide is absorbed into the blood. Oxygen-depleted blood

cells are then taken back to the lungs, where

the blood releases carbon dioxide and the

WHERE IS cycle starts again.

BLOOD MADE? Double circulation LUNGS
Oxygen-depleted blood is
Strangely, blood is actually pumped from the right side of
manufactured in bone marrow the heart to the lungs. Blood
in your flat bones (such as the rich in oxygen from the lungs
is pumped from the left side
ribs, sternum, and shoulder of the heart out to the body.

blades)—millions of blood

cells are produced every Lungs absorb
single second! oxygen from the air
and releases it into

the blood

120 121THE HEART OF THE MATTER
The many tasks of your blood

What the body needs FOOD

All the living cells throughout the INTESTINES
body need various things to help
them function properly. Blood GLUCOSE
carries these vital supplies, such
as oxygen, salts, fuel (in the form of LIVER HEART
glucose or fats), and protein building BLOOD CELLS HEART
blocks—amino acids—for growth
and repair. Blood also carries BONE MARROW
hormones, such as epinephrine,
which are chemicals that affect the EPINEPHRINE TISSUES
behavior of cells. ADRENAL GLAND
KIDNEY BLADDER
What the body LIVER WASTE Liver converts lactic
doesn’t need MUSCLES LACTIC ACID acid into glucose
GLUCOSE
Wastes, such as lactic acid, are Lactic acid from BACK INTO
produced as by-products of normal exercising muscles travels BLOOD
cell function. Blood quickly carries
the wastes away to prevent in the blood to the liver,
imbalances. Some wastes may be which recycles it back to
transported to the kidneys, to be
expelled in urine, or can be carried glucose using oxygen
to the liver to be converted back
into something that the cells need.

T SIDELEF Blood now carries carbon
OF dioxide and travels via right
side of the heart to the lungs

T SIDE OF THE HEA RT THE HEART
HEART TISSUES

RIGH Every cell in the
body receives
oxygen via tiny
blood vessels

Blood carries
oxygen to the rest

of the body

How the R Ventricles
heart beats contract

The heart is a fist-sized muscular organ Second contraction
that contracts and relaxes around 70 The electric message
times a minute. This keeps blood flowing reaches the tip of the
around the lungs and body, transporting ventricles and spreads
life-giving oxygen and nutrients.
throughout the
ventricles. The large

R-wave occurs as
the powerful ventricles
reach peak contraction.

Heart cycle Q R

Your heart is a muscular pump that is divided Signal transfer Electricity travels
into two halves, left and right. Each side of the The electric signal along wall between
heart is further divided into two chambers—an then passes down
upper atrium and a lower ventricle. Valves the thick, muscular chambers
prevent backflow so that blood keeps traveling wall between the
in the correct direction. A patch of muscle acts left and right side
as a natural pacemaker, generating an electric
signal that makes the heart muscle cycle of the ventricles,
between contraction and relaxation. The creating the valley
rhythmic squeezing of the heart pumps
blood from its right side to the lungs and of the Q wave.
from the left side to the rest of the body.
P
ECG recording
Electric impulses within the heart can be recorded by
electrodes to produce an electrocardiogram (ECG). Each
heartbeat produces a characteristic trace on the ECG display.
Its shape is made up of five phases—P, Q, R, S, and T, each of
which is a sign of a particular stage of the heartbeat cycle.

P Sinoatrial Q
node (natural
Electric signals travel
pacemaker) through walls of
upper chambers
First contraction Atria
Electric activation of contract

muscle cells make Blood forced into
the atria contract, ventricles

pushing blood
through valves into

the ventricles and
creating the P wave

on the ECG.

122 123THE HEART OF THE MATTER
How the heart beats

Oxygen-rich blood from WHAT CREATES How electric
the lungs is pumped to the THE SOUND OF THE signals travel

rest of the body HEARTBEAT? The heart’s pacemaker, the
Blood from heart’s sinoatrial node, is a region of muscle
right side is pumped The heart has four valves, in the upper right atrium. It starts
and the opening and a regular electric impulse that is
to the lungs conducted throughout the heart
closing, in pairs, of these by specialized nerve fibers. Heart
heart valves produces the muscle cells are adept at spreading
familiar lub-dub sound of electric messages rapidly, so the
heart muscle contracts in an orderly
the heartbeat. sequence, first the two atria
followed by the two ventricles.
Atrium relaxed S
Electricity Natural pacemaker
travels back
The S wave and flat Specialized cells
ST segment occur as Natural pacemaker cells
the ventricles are in the heart are “leaky”
contracting and emptying and allow a flow of ions
of blood. The atrial muscle (charged particles) in and
cells have recharged, ready out. This generates a
for the next contraction. regular electric impulse
that causes the heart to
Electricity beat. Heart (cardiac)
travels back up muscle cells have
towards atria branched fibers that let
electric messages
spread quickly to the
neighboring muscle cells.

Ventricles still T
contracted

HEART MUSCLE T Electric current
CELLS RECHARGE
Heart recharges Cardiac
S The final T wave of the muscle
ECG trace occurs as the cell

ventricular muscle cells WITH EACH BEAT,
recharge, or repolarize. EACH LOWER
The heart rests as CHAMBER PUMPS
the muscle cells get 2 1⁄3 FL OZ (70 ML)
ready for the next OF BLOOD—
contraction. NEARLY 1⁄5 OF A BLOOD
DONATION BAG

How blood travels

Blood travels through arteries, capillaries, IN Blood flow
and veins. Arteries have muscular,
elastic walls to even out surges Artery wall DILATES
in pressure as the heart is relaxed Artery wall
pumps. Veins have thinner contracts
walls and can distend to
help lower blood pressure.ARTERY
If blood pressure rises
too high, damage
increases the risk
of a heart attack
or stroke.

Vessel narrows to limit CONSTRICTS
blood flow locally

Capillaries that Middle layer (tunica Arteries
nourish blood media) consisting of Arteries carry blood away from the heart.
smooth muscle Most arteries, except those going to the
vessel walls Elastic connective tissue lungs, carry oxygenated blood. Their thick,
Lining (tunica (lamina propria) elastic walls can cope with high pressure and
widen or narrow to regulate blood flow.
intima)

Outer layer (tunica externa)

Blood pressure

The arteries pulse with blood in time with the heartbeat,

and so the pressure inside them rises and falls in waves. Artery splits into
Arterial pressure is greatest just after the heart contracts narrower arterioles

(systolic blood pressure) and is lowest when the heart

rests between beats (diastolic blood pressure).

Pressure is much lower in the capillaries as

they are so numerous they spread the force Blood pressure (mmHg) ARTERIES Maximum, or
widely. Once blood reaches the veins, its 120 systolic, pressure
pressure is minimal. 100

80 Minimum, or diastolic,
pressure (heart relaxed)

Ranges of pressure 60 CAPILLARIES VEINS
Blood pressure is measured in millimeters of HEARTBEAT
mercury (mmHg) and typical blood pressure
varies rhythmically between 120 and 80 40
mmHg. Although the pressure is lower in 20
both capillaries and veins, blood pressure 0
never reaches 0 mmHg.

124 125THE HEART OF THE MATTER
How blood travels

Blood flows Open valve OUT Route through the body
forward
Blood pulses away from the heart in large

arteries, which divide to form smaller

arterioles. From the arterioles, blood

enters a network of capillaries. In

lung capillaries, blood collects

VALVE OPEN oxygen and releases carbon
dioxide gas. In body

capillaries, blood releases

Blood cannot oxygen and collects
Closed valve flow back carbon dioxide. Blood

VEIN then flows into

venules, which join

up to form veins

returning blood

VALVE CLOSED Layer of to the heart.
smooth muscle
Valve

Veins Lamina propria Tunica intima
Veins carry blood back to the heart. Pressure in Capillaries
them is very low (5–8 mmHg) and the long veins Capillaries form an extensive
in the legs have a one-way valve system to prevent network that branches finely
backflow due to gravity. through body tissues. The
entrance to some capillaries
CAPILLARIES is protected by muscle rings
(sphincters), which can shut
down that part of the network.

Small venules join up to
form a larger vein

Small venules

Air pump WHY IS HIGH
BLOOD PRESSURE
Measuring blood pressure Pressure gauge
To measure your blood pressure, a Cuff SO HARMFUL?
nurse inflates a cuff around your arm
until the pressure is high enough to High blood pressure damages
stop arterial blood flow. Pressure is
then slowly released until blood can artery linings. This can trigger
just squirt past the cuff, producing a
distinct sound that pinpoints systolic a buildup of cholesterol-laden
blood pressure. As cuff pressure
continues to fall, sounds suddenly plaque, which hastens
stop at the point where blood flow
is no longer constricted, which hardening and furring
pinpoints diastolic blood pressure.
up of the arteries.

Broken blood vessels

Blood vessels permeate the tissues of the body. Their WHY DO
thin walls allow oxygen and nutrients to pass but are PEOPLE GET DEEP-
easily damaged. Repair systems allow blood to clot so VEIN THROMBOSIS ON
that any damage is quickly fixed, but sometimes
unwanted clotting causes a blockage. LONG FLIGHTS?

Bruising Blood can clot by mistake
inside a healthy vessel due to
When a part of the body is knocked, tiny blood vessels may rupture and sluggish blood flow, especially
leak blood into surrounding tissues. Some people bruise more easily than
others, especially the elderly. This is sometimes related to blood-clotting when someone sits still for
disorders or nutrient deficiency such as lack of vitamin K (needed hours. Such a clot, or
to make clotting factors) or vitamin C (needed to make thrombosis, can
the protein collagen). block a vein.

Leaking blood Capillary broken, Hair
visibly pools in the causing blood to leak

upper skin layer

BRUISE Blood leakage
Circulating blood is under pressure
and readily escapes from a broken
vessel into surrounding tissues.
Released chemicals activate the
clotting response and attract
scavenger cells (macrophages).

Clotting

A damaged blood vessel must be sealed quickly to activate and plug the damage. The blood vessel may
prevent blood loss. A complex sequence of reactions constrict to slow blood flow and reduce blood loss
causes inactive proteins dissolved within the blood to from the circulation.

Platelet Blood vessel wall broken Platelets collect at opening Platelets bound together
by fibrin protein fibers

1 Initial opening 2 Forming a clot 3 Holding the clot
Exposure of proteins such as collagen Platelets stick together and release A sticky web of fibrin fibers forms a
in a broken blood vessel wall immediately chemicals that trigger fibrin—a protein net that binds platelets together. The web
attracts cell fragments called platelets. circulating in the blood—to form fibers. traps red blood cells to form a clot.

126 127THE HEART OF THE MATTER
Broken blood vessels

How bruises heal Varicose veins
Bruises start purple—the color of oxygen-
poor blood cells seen under the skin. Varicose veins are a price we pay for walking
Scavenging macrophage cells recycle the on two legs rather than four. Valves in the long
spilled red blood cells as they clean up the leg veins let blood travel up against gravity. In
area, converting the red blood pigments into surface veins, these valves can collapse, and
first green, then yellow pigments. blood pools, forming bulges. Varicose veins
may be hereditary and may also result from
BRUISE Oxygen-rich red increased pressure during pregnancy.
COLOUR blood cell with
hemoglobin

Oxygen- Blood restricted HEALTHY VEIN
poor red from flowing
blood cell backward
containing
deoxy-
hemoglobin

Macrophage Healthy valves VARICOSE VEIN
MACROPHAGE A series of valves stops Widened,
ABSORBS BLOOD CELLS blood from flowing twisted vein
backward. This allows
Cell fragment blood to flow up the
containing length of the leg against
hemoglobin the pull of gravity.

Green Valve turned inside
pigment out, allowing blood
(biliverdin)
MACROPHAGE BREAKS to leak backward
DOWN HAEMOGLOBIN Pressure builds
When weak valves give
Yellow way, gravity causes
pigment blood to fall backward
(bilirubin) and pool in the veins.
Increased pressure causes
MACROPHAGE RELEASES the veins to become
YELLOW PIGMENT dilated and twisted.

Blocked blood vessels Red blood cell Plaque collects
in artery wall
Raised blood pressure or high
Clot broken up and glucose levels slowly damage
dispersed by enzymes artery walls. Platelets stick to
injured areas to fix the damage.
Blood vessel If blood cholesterol levels are FATTY DEPOSIT Dead blood cells
wall repaired also high, this seeps into affected and fat
areas, causing a buildup that
4 Clot dissolves narrows the artery and restricts BLOCKED BLOOD VESSEL
Cells that repair the wound also release blood flow. If arteries supplying Limiting blood flow
enzymes that slowly break down the platelet/ heart muscle are affected, it can Fatty deposits may collect in damaged
fibrin clot—a process called fibrinolysis. cause a heart attack. When blood areas in arteries to form plaques. These
flow to the brain is reduced, the deposits cause the arteries to narrow
memory is affected. and stiffen, restricting blood flow.

Heart problems

The heart is a vital organ—if it stops Constricted
pumping blood, cells will not receive blood flow
the oxygen and nutrients they need. A narrowing of a
Without oxygen or glucose, brain coronary artery may
cells cannot function and you be caused by a
lose consciousness. buildup of fatty
deposits (see p.127).

Vulnerable vessels AORTA Blood Plaque
cell in artery
Heart muscle needs more oxygen than
any other muscle in the body and the C
heart has its own coronary arteries to ERY
supply its needs because it cannot absorb ORONARY ART
oxygen from the blood in its chambers.
The left and right coronary arteries
are relatively narrow and prone to
hardening and furring up (narrowing) –
a potentially life-threatening process
known as atherosclerosis.

IS LAUGHTER
REALLY THE BEST

MEDICINE?

It may very well be true—

laughter can increase

your blood flow and

relax your blood Damaged heart muscle
vessel walls. Poor blood supply means
heart muscle cells do not get all
Decreasing oxygen supply the oxygen they need. This leads to a
The heart has specialized tight chest discomfort called angina.
cardiac muscle cells whose DEAD HEART MUSCLE
branched fibers spread REDUCED BLOOD SUPPLY DEATH OF HEART MUSCLE
electrical messages quickly. HEALTHY HEART TISSUE
Characteristic changes on
an ECG (electrocardiogram) Fibers bright red Dark fibers Only a few bright-
help doctors diagnose and oxygenated lack oxgyen red fibers remain
whether chest pain is due throughout
to poor blood supply
(angina) or muscle cell
death (heart attack).

NORM AL HEARTBEAT ANGINA HEART ATTACK

128 129THE HEART OF THE MATTER
Heart problems

Heart rhythm problems Electrical activity
Rogue activity arising
If the heart is beating too fast, too slowly, in the electrical
conduction system
or irregularly, medics say that it has can block the normal
pacemaker impulses
arrhythmia, or abnormal heart rhythm. made by in the
sinoatrial node,
Most arrhythmias are harmless, such stopping the signal
reaching the next node.
as premature extra beats that feel like Sinoatrial
a flutter or skipped heartbeat. Atrial node Rogue electrical
activity can arise
fibrillation is the most common type Atrioventricular in either atrium
of serious arrhythmia, in which the node
Irregular
two upper chambers of the heart (atria) Rogue electrical activity
beat irregularly and fast. This can cause electrical Rogue electrical activity
dizziness, shortness of breath, and blocks impulses
activity

fatigue, and also increases the risk of

suffering a stroke. Some arrhythmias

can be treated with drugs. Some need

defibrillation to reset and normalize

electrical activity.

Sinoatrial node generates
regular heartbeat

NORMAL HEART BEAT IRREGULAR HEARTBEAT
Electrical interference
The coordinated beating of the heart relies DEFIBRILLATION
on a clear signal reaching the ventricles
from the sinoatrial node. If rogue electrical Some life-threatening arrhythmias can be treated by defibrillation.
activity gets in the way, the heart’s rhythm
of contraction is disturbed and can A burst of electricity is delivered to the chest in an attempt to
become erratic.
re-establish normal heart electrical activity and
THE HUMAN
HEART BEATS contraction. Defibrillation only
MORE THAN
36 MILLION TIMES works if a “shockable” rhythm
A YEAR—ABOUT
2.8 BILLION TIMES IN is present, such as ventricular Defibrillator Defibrillator
AN AVERAGE LIFETIME paddle paddle
fibrillation. It cannot restart the

heart if no electrical activity is

detected (asystole).

Cardiopulmonary resuscitation

can trigger electrical activity so

that defibrillation can be tried.

DEFIBRILLATOR
PADDLES
APPLIED TO
THE CHEST

Exercising and its limits

When you go for a jog or a sprint, extra blood is pumped to your muscles,
providing you with the vital ingredient to make energy—oxygen. Deep,
regular breaths replenish your muscles with oxygen and set your pace.

STARTING Oxygen Aerobic jog
YOUR JOG consumption
FINDING When exercising at a moderate pace, your
YOUR PLACE body is relying on the oxygen you breathe and
energy stores in your muscles. Oxygen burns
glucose, a sugar that is stored in your body,
for energy. Muscle cells use energy to contract
and ultimately move your body. This process is
aerobic respiration, and is the most stable form
of exercise. Aerobic exercises include jogging,
cycling, rowing, swimming, and dancing.

Steady rise Slowing down EXHAUSTION
Your breathing Breathing rate soon
rate will rise COMING returns to normal
with your pace. TO A STOP after a jog.

Lactic acid
level rises

Steady breaths OUT OF Replenishing breaths
Rhythmic breathing allows a BREATH Immediately after the race,
steady flow of oxygen to keep you continue to use your
lactic acid at bay. Lactic acid lungs’ vital capacity and
level starts you are taking your
Lactic acid level deepest breaths.
soon returns to fall
to normal Oxygen debt
Minutes after
30-MINUTE JOG the race, deep
Jogging breaths continue.
A slower pace allows you to exercise for This is neccessary
longer periods of time. Your body can
make energy much more efficiently because oxygen is still
from its glucose stores. needed to neutralize the
buildup of lactic acid.

130 131

All systems go EXERTING Crouch Reaching your limit
Lactic acid builds up YOURSELF You prepare to take
quickly in the muscles. deeper breaths. A buildup of lactic acid in your
Oxygen intake lags behind. body is the reason why you get
ON tired during exercise. Lactic acid
REACHING YOUR interferes with muscle contraction,
YOUR MARKS which results in physical exhaustion.
LIMIT Oxygen is needed to get rid of lactic
High level 30-SECOND SPRINT acid, which is why you breathe heavily
of lactic acid after exercise. This buildup of lactic
Sprinting acid happens during both aerobic and
Breaking point Exerting yourself in a short space of anaerobic exercise, but it occurs quicker
You become dizzy and feel time causes your body to make energy in the latter. Brain cells can only burn
the “burn.” Lactic acid will inefficiently, which releases a lot of glucose for fuel and as exercising muscles
eventually reach a level where lactic acid, causing the “burn.” deplete the body’s available glucose
your muscles simply cannot supplies, mental fatigue also sets in.
contract. The breaths you Anaerobic sprint
take are as deep as possible EFFECT OF LACTIC ACID IN MUSCLES Actin
to maximize the amount of During strenuous exercise, your body
oxygen you absorb. demands energy more quickly than you Muscle cannot
can provide oxygen to make it. Muscles contract
can continue to break down glucose MUSCLES
without oxygen in a process known
as anaerobic respiration. It is great for Lactic acid
short bursts of energy, but it generates
excessive lactic acid in your muscles and Myosin
is unsustainable. Now, oxygen is needed,
not to help burn glucose, but to convert HYDRATION

the build up of lactic acid into glucose— Drinking water during exercise helps
for future energy. This is known as regulate body temperature through
paying the oxygen debt and leaves sweating and flushes away lactic
you out of breath for some time after acid. Water in blood plasma is
an intense sprint. sweated out, so your blood thickens
and your heart works harder to
pump blood around the body.
This is called cardiac drift, and it’s
one reason why you can’t respire
aerobically and jog forever.

FULLY LIMIT OF SAFE
HYDRATED: 75% DEHYDRATION: 70%

Fitter and Scalene muscles Internal
stronger contract to raise intercostal

upper ribs muscles
contract
Exercise that makes your heart race and your lungs and tilt ribs Lung volume
downward reduced as
breathe hard and deep is called cardiovascular—it muscles contract
and ribs tilt

strengthens the heart and improves stamina. In

contrast, exercise that forces you to contract

muscles repetitively is called resistance

training, and it can build and

strengthen your muscles. COLLARBONE

Cardiovascular exercise Chest muscles LUNG STERNUM
Muscles within the neck,
When you perform cardiovascular chest wall, abdomen, RIB
exercise, such as jogging, swimming, and back coordinate to
bicycling, or brisk walking, you expand and reduce the
train your cardiovascular system. size of your ribcage, so
Your heart rate climbs, beating that the volume of air
faster in order to pump more blood your lungs inhale and
around your body, especially to exhale increases.
the chest muscles that influence External intercostal
the depth of your breaths. As muscles contract and tilt
your body’s demand for oxygen
increases, your breathing rate ribs upward
and depth rises accordingly.
Your blood is saturated with as
much oxygen as possible to
provide your body with the
energy it needs.

TIDAL Deep breaths Rectus
VOLUME include red and abdominis
VITAL CAPACITY blue areas muscle pulls

Lung capacity ribcage
Your tidal volume is the downward
volume of air that flows into Lung volume
your lungs during a relaxed increases due to
breath. If you try to breathe ribs tilting upward
all the air out of your lungs,
RESIDUAL Relaxed some air remains as your INHALING External oblique muscles
VOLUME breathing residual volume, and cannot contract and shorten to
Air that remains be breathed out. Your vital pull ribs downward
in lungs after capacity, the deepest breath
deep breath you can take when training, EXHALING
is the rest of your lung
volume excluding the
residual volume.

132 133THE HEART OF THE MATTER
Fitter and stronger

WHICH Resistance training
TYPE OF EXERCISE
BURNS MORE FAT? Weight training builds your muscle, but so does dancing,
gymnastics, and yoga – they are all forms of resistance training.
It depends on the individual, A repetition (rep) is one complete motion of exercise. A set is a group
but a combination of both of consecutive reps that will contract a particular muscle, or multiple
cardio and weight training muscles, repeatedly. You can target muscles to grow by choosing to
will result in greater fat loss perform a selection of sets and reps over a period of time. The fewer
than just doing one reps you are able to do per set, the tougher your workout.
or the other.
Cell nuclei Muscle tear Satellite cell

MUSCLE FIBER MUSCLE FIBER MUSCLE FIBER
BEFORE EXERCISE AFTER EXERCISE DURING REST

DOING A REP Muscle growth process SHRINKING BULKY MUSCLE RE-TRAINING
Exercise tears muscle fibers, which FIBER WITH MANY
Rectus abdominis are then repaired by satellite cells.
muscle Although muscle fibers are single NUCLEI
Bow pose body cells, they have many nuclei, MUSCLE FIBER AFTER
Yoga is a good way to grow and they incorporate the satellite MONTHS OF NO EXERCISE
muscle steadily. The bow pose cells, along with their nuclei—growing
forces the rectus abdominis as they do so. During a break from
muscle to contract and tear exercise, your muscle fibers shrink,
slightly. Repeating this as a “rep” but they retain the nuclei from the
will start the muscle growth process. satellite cells and regain their size
quickly after retraining.

RATES OF EXERCISE 100 Maximum training

Exercise intensity can be expressed as the Percentage of 80 Anaerobic training
percentage of your maximum heart rate. maximum heart rate Cardio training
When you go for a jog, you are working
your heart at about 50 percent of its 60 Weight control
potential power. Athletes who have Moderate activity
reached their peak fitness can work
their heart at maximum strength—100 40
percent. A fitness instructor can give
you a target heart rate to reach when
training (which varies with age) while
achieving your fitness goals.

WHEN YOU SLEEP, HORMONES
THAT STIMULATE MUSCLE
GROWTH ARE RELEASED

Maximizing BRAIN
your fitness

While exercising is necessary to maintain health,
regular training can improve your overall fitness.
Your body will adapt to tough training regimes;
muscles get thicker, breaths get deeper, and
your state of mind is enhanced.

Positive results of regular exercise

If you exercise regularly, you will see widespread

improvements across your body. Adults benefit from just HEART
LUNG
30 minutes of brisk exercise on most days, while children
LIVER
need at least 60 minutes of running OXYGEN INTAKE

around. Keeping yourself active is
vital for improving your organs

and muscles, and by exerting

yourself in steady sessions Exercise strengthens
your body systems will your chest muscles,
become more efficient which allows greater
and eventually will start
to function at the best lung expansion. So,
of their ability. the amount of air
your lungs can hold
Depth of each increases, and your
breath increases
breathing rate rises,
with exercise resulting in a greater
amount of oxygen
absorbed when
exercising and
also at rest.

AMETER INCREASE SYSTEMS IMPROVE
ARTERY DI
METABOLIC Metabolic process
When exercising, nerve occurring in liver
signals cause arteries Your metabolic rate is the speed at which
to dilate, or widen, chemical processes, such as digestion or
increasing blood flow. the burning of fat, take place in your body.
This delivers more Exercise generates heat, which speeds
oxygenated blood to the up these processes in your organs,
muscles. If you exercise even after you finish exercising.
Artery regularly, the diameter
widens that your arteries dilate

to when you exercise
becomes wider,
maximizing the amount
of oxygen that reaches
your muscles.

COGNITIVE IMPROVEMENT 134 135THE HEART OF THE MATTER
Regular exercise Maximizing your fitness
increases the delivery USCLE Maximum oxygen intake
of blood, oxygen, and Reaching your maximum
nutrients to the brain.
In turn, this stimulates During a training program, for most people, the
new connections effort you put in reaps great benefits at first, as
between brain cells, your fitness increases from your untrained level.
improving general mental Further improvements become ever harder to
abilities. Exercise also boosts the achieve as you approach your own physiological
levels of neurotransmitters such limits, which depend on age, gender, and other
as serotonin in the brain, genetic factors. You reach your maximum
raising your mood. more quickly with a higher-intensity training
program. The best athletes explore their limits,
STRONGER CARDIAC M looking for opportunities to extend them.
Cardiac muscle fibers grow in size,
but not via satellite cells as is the case Limit
in muscles in the rest of your body.
Instead, their existing fibers grow MHoigdehr-aitnetteranisniitnyg training
stronger. Your heart’s contractions
become stronger too, and it distributes 0 3 6 9 12
blood more thoroughly around the Time (weeks)
body, lowering your resting heart rate.

STRONGER MUS CLES RESTING HEART RATES
Having strong
muscles increases Athletes have low heart rates at rest because
your physical training enhances the strength of their cardiac
strength, strengthens muscle. Compared to those who are untrained,
your bones, improves athletes’ heart contractions are stronger, and
posture, flexibility, and blood is distributed more efficiently with every
how much energy you burn heartbeat. A trained athlete may have a pulse rate
during exercise and while as low as 30–40 beats per minute at rest.
at rest. Strong muscle is also
more resilient to exercise- Slow
induced injury. heartbeat

Fast
heartbeat

UNTRAINED TRAINED



IN AND
OUT

Feeding the body WHAT IF I DON’T
GET WHAT I NEED?
Although the body can manufacture many vital Your body systems will start
to fail and you may be afflicted
chemicals, a lot of the materials we need must be with deficiency diseases. For
example, if you do not have
acquired by eating. The energy needed to fuel the enough minerals in your diet,
your bones will not
body is gained entirely through the food we
grow properly.
consume. Once nutrients are absorbed
Proteins
into the bloodstream, they are then Proteins are the major structural
components of all cells. Healthy
transported to different parts of protein sources include beans,
lean meat, dairy, and eggs.
the body, where they are put to Carbohydrates
innumerable tasks. Carbohydrates
are the main energy
Water source for the brain.
Some 65 percent Whole grains and fruits and
of the body is made vegetables that are high in
up of water. This is fiber are healthy sources
constantly being lost of carbohydrates.
through breathing
and sweating, and
it is critical that it
is replenished.

Sugars

What the body needs Amino
acids
There are six essential types of
nutrient that the body needs to get DIGESTIVE TRACT Fats
from the diet in order to function Fats are a rich source of energy
properly: fats, proteins, carbohydrates, Fatty and help in the absorption of
vitamins, minerals, and water. The acids fat-soluble vitamins. Healthier
last three are small enough to be fat sources include dairy, nuts,
absorbed directly through the lining fish, and vegetable-based oils.
of the gut, but fats, proteins, and
carbohydrates need to be broken Vitamins
down chemically into smaller Vitamins are
particles before they can be needed to
absorbed. These particles are make things
sugars, amino acids, and fatty in the body. Vitamin C, for
acids respectively. example, is needed to build
collagen, which is used in
various tissues.

Minerals
Minerals are vital for building
bones, hair, skin, and blood
cells. They also enhance nerve
function and help turn food
into energy.

Building an eye 138 139IN AND OUT

Every tissue in our body is built and maintained by the Feeding the body
nutrients we absorb from our food. The tissues of the human
eye, for example, are built from amino acids and fatty acids, THE LIVER CAN
and fueled by sugars. The membranes and spaces are filled STORE UP TO
with fluids, and vitamins and minerals are needed to convert 2 YEARS’ WORTH
light into an electrical impulse—the basis of vision itself. OF VITAMIN A

Cell membranes Energy
All the cells of the eye (and the The eyes are an
rest of the body) are surrounded extension of the
by membranes that are built brain, and just like
using fatty acids and proteins. the brain, they need
the sugars we get
The food of sight from carbohydrates
Like all organs of the for energy.
body, the eye utilizes all six
of the essential nutrients.
These give it structure and
enable it to send visual

information to
the brain.

Fluids Tissue structures Vision Red blood cells
The eye is filled Eyelashes are made Vitamin A is bound The tissues of the
with fluid, which up of the protein to proteins in the eye are oxygenated
maintains the keratin, which is built eye known as visual by the red blood
pressure in the from amino acids. pigments. When light cells, which need the protein
eye and provides Other tissues of the hits the cells, the hemoglobin and the mineral
nutrients and eye are made of vitamin A changes iron in order to carry oxygen.
the protein collagen. shape, sending an
moisture to the inner electrical impulse
eye tissues. This fluid to the brain.
is 98 percent water.

How does eating work?

Eating is the process of breaking down food into molecules that

are small enough to be absorbed into the bloodstream. For the food, BEFORE EATING Hunger
this involves a 30 ft (9 m) journey through a series of organs known A few hours after
collectively as the gut, or the gastro-intestinal tract. eating, the hormone
ghrelin is secreted
The journey of food by the stomach. This
sends a signal to the
Food begins as a (usually) appetizing brain, which readies
the gut for food.

meal, and ends with us taking trips to the

toilet. Between these stages, the food has Ghrelin’s signal makes
done its job—released its nutrients in a us feel hungry

four-stage process involving the mouth, the “I’M HUNGRY” Leptin’s signal makes
stomach, the small intestine, and the large “I’M FULL” us feel full
intestine. The liver and pancreas also
play roles, as do the hormones leptin and

ghrelin. On average, it takes 48 hours

for food to pass through the body. Satisfaction
When we have eaten
Nutrient absorption HYPOTHALAMUS AFTER EATING enough, the hormone
Some nutrients take longer leptin is released by
to be absorbed than others, MOUTH our fat tissues. This
but most are absorbed in signals the brain to
the small intestine. put the gut back on
“standby” mode.
Vitamins
Sugars 1 Mouth and Hunger and satisfaction
Amino acids esophagus
Minerals Stage one starts with the We eat when we feel hungry, and stop when we
Fatty acids mechanical breakdown of feel full. However, we are not responsible for
food by chewing. This mixes these feelings. When we are low on nutrients,
Water the food with saliva, which the hormone ghrelin is released by the stomach,
Blood flow begins to digest it chemically. making us feel hungry—and when we are full,
The food is then swallowed, ESOPHAGUS the hormone leptin is released by our fat tissues,
inhibiting our appetites.

which drops it into the
esophagus (see p.142).

BLOODSTREAM

1 minute in
the mouth and
esophagus

Duct carrying LIVER
enzymes from PANCREAS

the pancreas

STOMAC 2 The stomach
Muscular contractions
in the esophagus propel the
WHAT IF H food into the stomach. Here it is
THINGS GET BLOCKED?
doused in gastric juices, which
Blockages can be caused by turn it into a soupy mixture
stress, bad diet, or infection. called chyme (see p.143).
One remedy is a laxative – a
medication that is taken to 2½–5 hours in the IN AND OUT
stomach
smooth the passage of 3 hours in the How does eating 140 141work?
food through small intestine
the gut.
LARGE INTESTINE
Duct carrying SMALL INTESTINE
bile from
the liver

4 The large intestine
Most of the water from
The small intestine the food is absorbed in this last
3 In the small intestine, section of the gut, along with

the chyme is broken down a few final nutrients. At the
further by enzymes that are
supplied by the pancreas, same time, the indigestible
parts of the food are pressed
and bile produced by the into feces and stored for
liver. Most of the food’s
nutrients are absorbed removal (see pp.146–47).

here (see pp.144–45).

30–40 hours in the
large intestine

A mouth to feed

The long and convoluted journey taken by food through the Air in Chewing
body begins with a brief stay in the mouth and an acid bath When food is in the
in the stomach. The goal of this first stage of digestion is to mouth, the epiglottis
turn food into chyme—a soup of nutrients that is then moved stands up to keep the
on to the small intestine for processing. windpipe open. This allows
us to breathe through our
Heading south noses while chewing.

The route from mouth to stomach is a vertical one, Epiglottis up
via a connecting tube called the esophagus. The food
Swallowing
is propelled by gravity and by muscular When we swallow, the
contractions in the esophagus known epiglottis folds down,
as peristaltic waves. closing off the windpipe.
At the same time, the soft
NASAL PASSAGE palate rises to block off
the nasal cavity.
Chewing creates a ball of SALIVARY
saliva-saturated food GLAND Soft palate up

The salivary gland under TONGUE Epiglottis down
the tongue produces thick
saliva containing enzymes The salivary glands Ready to chew again
in the cheeks produce When food has entered
1 Digestion begins ESOPHAGUS watery saliva the esophagus, the
As food is chewed in the mouth, the salivary WINDPIPE Another salivary epiglottis and soft palate
gland under the return to their former
jaw releases saliva position. This enables us
at the base of to breathe and chew again.
the tongue
Epiglottis up
glands increase the production of saliva, which
helps turn food into a paste. Saliva also contains How to avoid choking
an enzyme called amylase, which converts starch
into more easily absorbable sugars. Since we both eat and breathe through our
mouths, it is vital that our windpipes can
be closed off when we swallow. Luckily, our
bodies have a pair of built-in safety devices—
a small flap of cartilage in the throat called
the epiglottis, and a piece of flexible tissue
in the roof of the mouth called the soft palate.

A muscular 2 Into the stomach
wave shifts Food enters the stomach via a ring
food down the of muscle. For several hours the food is then
WHY DO WE esophagus churned by three different muscles in the
GET INDIGESTION? Chewed up stomach. In a violent procedure that we are
ball of food barely aware of, this mixes the food with the
Indigestion, or “heartburn,” Ring of muscle gastric juices that are secreted by glands
is the inflammation of the must be relaxed in the stomach lining.
stomach by the stomach’s own to let in food
acidic juices. It is commonly
Gastric juices STOMACH
caused by overeating, are released
stress, or drinking too

much alcohol.

3 Gastric juices
The stomach’s juices include the
extraordinarily corrosive hydrocholoric
acid, which kills bacteria, and pepsin,
an enzyme that converts protein into
smaller molecules called peptides. Also
released is gastric lipase, an enzyme Layers of muscle
that begins the process of breaking in the stomach wall
down fat, and mucus. Mucus forms a pull in three different
slimy layer that protects the stomach directions, flexing the
from its own digestive juices. stomach into different
shapes and churning the
Gastric juices are secreted food like clothes in a IN AND OUT
at the base of pits
washing machine A mouth to 142 143feed
Ring of muscle, open Layers of
to release chyme stomach wall

4 Moving on
After being churned
in the stomach for 3–4
hours, all the food has
been turned into
chyme. This chemical
mixture is then squirted
through another ring of
muscle at the base of the Food converted
stomach into the neck of the Chyme enters the to chyme
small intestine
small intestine. Once here,
digestion begins in earnest.

Small intestine

Gut reaction Organs in concert

Once food has been turned into chyme in the stomach, it To help it digest, the small intestine gets help
is squirted into the small intestine. Here, in a frenzy of from three other organs: the pancreas, which
chemical activity, it is broken down further and finally makes enzymes; the liver, which makes bile;
absorbed by the blood. Each day, around 24 pints and the bile-storing organ, the gallbladder.
(11.5 liters) of food, liquids, and digestive juices pass
through the small intestine.

1 Bile factory
One of the liver’s
many jobs is to produce
bile—a bitter liquid that
turns fats into more LIVER 3 Enzyme engine
The pancreas produces
readily digestible fatty Food leaves the STOMACH three main enzymes: amylase,
droplets. Once produced, stomach and enters
bile is stored in the the small intestine which turns carbohydrates into
Bile
sugars; protease, which turns
gallbladder. proteins into amino acids; and

lipase, which turns fatty droplets
into fatty acids.

2 Bile store
When food
leaves the stomach
bile leaves the gallbladder
and heads for the small
intestine. There it mixes
with incoming enzymes Bile travels
from the pancreas. GALLBLADDER down bile duct PANCREAS

AROUND 95 PERCENT OF SMALL INTESTINE The pancreatic duct,
ALL ABSORPTION TAKES filled with enzymes
PLACE IN THE SMALL
INTESTINE—THE REST TAKES Food is propelled by
PLACE IN THE COLON muscular contractions

in the intestine wall

CHEWING THE FAT Opening of ducts ENZYMES
carrying digestive juices BILE
Fats are particularly hard to digest.
Even after being drenched in FAT 4 Absorption begins
hydrochloric acid in the stomach, FATTY For 3–5 hours bile and enzymes
they are still not fit for enzyme DROPLETS work together, reducing nutrients to simpler,
consumption. This is where bile BILE absorbable forms. Absorption takes place
comes in. In a process called in the intestine wall, which is lined by
emulsion, bile turns fats into fatty thousands of fingerlike projections. These
droplets, which are then small projections, called villi, greatly increase the
enough for enzymes to attack. surface area of the intestine and thereby its
capacity for absorbing nutrients.

Lipase digests fatty Protease digests protein, Amylase digests
droplets, producing fatty producing amino acids carbohydrates,
acids producing sugars

FATTY DROPLET PROTEIN CARBOHYDRATE

Thousands of villi line
the intestine wall

Fatty acids Amino acids Sugars IN AND OUT

5 Into the blood Gut 144 145reaction
Villi absorb nutrients and channel them into
the blood. Once there, they are taken to the liver
and distributed around the body. Meanwhile, the
remaining chyme enters the final part of the gut
(see pp.146–47). Although not shown here, fat
digestion has another complication; on entering the
villi, the fatty acids take a trip through the lymph
system before finally entering the bloodstream.

Dissolved fatty acids

Dissolved amino acids Dissolved sugars

BLOODSTREAM

Up, down, and out Forming pockets
Every 30 minutes or so,
The final stage of digestion takes place in the large intestine—a 4ft- (2.5m-) bands of muscles in the
long tube that frames the small intestine. Here, bacteria set to work fermenting colon squeeze to form
carbohydrates, releasing nutrients that are vital for human health. pockets in which fecal
At the same time, fecal matter is compacted, stored, and ejected. material is churned and
mixed. It hardly moves
POCKETS forward at all.
Muscular waves
TRANSVERSE COLON With similar movements
to those in the esophagus
WAVES and the small intestine,
muscles squeeze feces in
WATER waves up from the cecum
and along the colon.

2 As the waste SODIUM Mass movements
travels on, VITAMIN B Stimulated some three
water and the minerals times per day by food
chloride and sodium entering the stomach,
are absorbed by the slow-moving waves of
ASCENDING COLON blood, along with SQUEEZING muscular squeezing shift
feces along the colon
various B vitamins into the rectum.
and vitamin K. Here,
these vitamins are
produced by bacteria,
but they can also be
found in food.

Friendly bacteria VITAMIN K Keeping regular
at home in the CHLORIDE
colon wall Waste takes 19–36 hours to move through the
large intestine, allowing time for water to be
absorbed. If the feces are rich in fiber, their
bulk carries them promptly through the system.

DESCENDING COLON Potassium and WHY DO WE
CECUM bicarbonate are HAVE AN APPENDIX?
SMALL INTESTINE absorbed by the
colon to replace The appendix is possibly
Appendix the sodium the remnant of an organ that
absorbed by helped our ancestors digest
the bloodstream foliage thousands of years ago.
Today, however, it plays no
1 Having left obvious role, except perhaps
the small
intestine, waste as a safe refuge for
gut bacteria.

material begins
a vertical climb
of the cecum.

3 Feces are WHEN NATURE CALLS
compacted
into the lower colon. When feces enter the rectum, stretch
They are kept moist receptors trigger a “need to go” reflex by
by mucus secreted sending impulses to the spinal cord. Motor
from the colon walls. signals from the spine then tell the internal
anal sphincter to relax. At the same time,
4 Feces are RECTUM sensory messages to the brain make a IN AND OUT
expelled via person aware of the need to defecate,
the rectum. Some and the person makes a Up, down, and 146 147out
60 percent of the Journey’s end conscious decision to relax
volume is made the external anal sphincter.
The large intestine has three main sections: the On a healthy diet, this
cecum, where waste from the small intestine is happens between three
collected; the three-part colon, where nutrients are times a day and once
absorbed; and the rectum, where feces are expelled. every three days.
The largest section is the colon, in which colonies of
of bacteria; the bacteria consume the starches, fiber, and sugars that
rest is mostly humans can’t digest (see pp.148–49).
indigestible fiber.

Anus contains both ANUS
inner and outer
sphincters

Bacterial breakdown

Over 100 trillion beneficial bacteria, viruses, and fungi live in Lactobacilli are common stomach
the digestive tract. Known collectively as gut microbes, they bacteria that are used in probiotic
provide us with nutrients, help us digest, and help defend us medical treatments. They fight off
against harmful microbes (see pp.172–73). other bacteria that cause diarrhea

Swallowing microbes

We receive our first microbes at birth, and more

enter our bodies every day of our lives. They enter STOM

through the nose and mouth and travel to the CHYME ACH
LARGE INTESTINE
stomach, where conditions are too acidic for
SMALL
many to take up permanent residence. The small INTESTINE

intestine is likewise too acidic, but many microbes

survive just long enough to move into the colon,

where they play a vital role in digestion. Helicobacter
pylori is a foe,
90 PERCENT OF ALL causing ulcers as it
THE CELLS IN OUR burrows into the
stomach lining

BODIES ARE BACTERIAL 70 percent
RATHER THAN HUMAN of all gut

microbes live
in the large
intestine

ANTIBIOTICS Harmful bacterium
invading the small
Antibiotics destroy or slow down
the growth of bacteria, but they intestine
aren’t able to discriminate between Wall of friendly
harmful and friendly bacteria. As a
consequence, the friendly microbes bacteria
in the gut suffer when we take
antibiotics. The diversity of gut
bacteria starts to decrease as soon
as the antibiotic course starts and
reaches a minimum about 11 days
later. The populations soon bounce
back after treatment, but overuse
of antibiotics can cause them
permanent damage.

Substances released
by friendly bacteria
to ward off invaders

In it together
Although many of the bacteria that get into our
bodies are harmful, most of them protect us
against microscopic enemies. They do this both by
taking up space (lining the intestine walls), and
releasing substances that kill harmful bacteria.