The Complete Human Body Dr Alice Roberts

349

LYMPHATIC AND IMMUNE SYSTEM

Antigen- Invading bacterium
presenting cell Approaches lymph node
Carries an antigen
fragment toward ANTIBODY-MEDIATED RESPONSE

lymph node This immune response is directed toward extracellular IMMUNOLOGICAL
MEMORY
Lymph node pathogens—those that infect tissue fluids or the blood, such as
The formation of memory cells during
Helper T cell bacteria. Pathogens are conveyed via lymph to a lymph node adaptive immune responses is central
Emits signals to the development of immunological
to confirm Blood (or via blood to the spleen) where recognition by a memory for T and B cells. The disadvantage
antigen vessel of initial responses by these lymphocytes
recognition B cell results in a series of reactions that trigger an immune is that they are relatively slow to develop,
and activate reflecting the time needed for adaptive
killer T cell response. B cells recognize antigens directly via their receptors, cells to proliferate and differentiate into
effector cells and memory cells. Innate
Effector they do not require APCs. With the aid of immunity is thus of key importance during
T cells an initial infection. If a pathogen reinfects
Clones of Antigen on helper T cells, recognition leads to the body, however, it will activate an
killer T cell bacterium surface activation of the B cell and the already enlarged population of specific
able to deployment of antibodies. cells (the memory cells), which results
recognize in a far more rapid secondary response.
and attack Helper T cell B CELL RECOGNITION
virus Sends signals Once a bacterium enters the 104
to activate B cell node, it encounters B cells. If an Peak
Migration antigen is recognized, a helper
Effector cells T cell must also confirm this and 103
move to site of signal to the B cell to activate.
infection via lymphatic 102
and blood vessels B cell recognizes antigen 1st exposure
to antigen
Homing in Activated B cell BLOOD ANTIBODY CONCENTRATION
Killer T cells locate area Undergoes clonal selection 101

of infection through CLONAL SELECTION 2nd exposure
tissue recognition, as The activated B cell produces to antigen
well as through tissue effector B cells that secrete 100
permeability caused by molecules called antibodies— 0 7 14 21 28 35 42 49 56
localized cell damage these match the specificity of
their parent cell and target the DAYS
Virus particles infection. Memory cells are also Primary and secondary
break up produced: these will respond immune response
if the same pathogen reinfects. This graph illustrates the difference between
Shriveled initial and subsequent exposure to the
membrane Antibodies Effector B cells Memory cells same pathogen. The secondary response
Produce antibodies Remain in lymph node is markedly quicker to develop and much
APOPTOSIS OF CELL greater in magnitude.
Cell degenerates and dies, but the Antibodies Complement proteins
infected contents remain contained Flood circulation to Activated by bound antibodies
within the membrane. locate and target infection
and mobilize to attack target

Attachment
Antibodies bind

to surface of
bacterium

Bound antibodies ELIMINATION IMMUNIZATION
Attract phaogcytes and Invading bacterium
promote phagocytosis destroyed either by A vaccine provides an individual with
complement system immunity to a disease that has not yet
ANTIBODY TARGETING or phagocytosis been encountered. It works by mimicking
Once antibodies have located the infection, an infection, but doing so safely, in order
they do not directly eliminate target cells, but Phagocytes to generate memory cells that are specific.
bind tightly to their surface. This provides a Perform phagocytosis This may involve utilizing microbes that
focus for the attack mechanisms of the innate to kill target bacterium have been killed or attenuated (rendered
immune system. Bound antibodies activate harmless), or an antigen derived from
the complement system (see p.347), leading component parts of the pathogen.
to complement-mediated killing of bacterium These may be given with other chemicals
that might otherwise escape detection. The (adjuvants) to make the immune response
presence of bound antibodies also attracts stronger. This ensures that the primary
phagocytes that perform phagocytosis and response develops without the other less
eliminate the bacterium (see p.347). desirable aspects of natural infection. If
the pathogen is subsequently encountered,
then a ready-made memory response,
equivalent to a secondary response, is
generated, and rapidly clears the infection,
often before symptoms develop.

MOUTH STOMACH SMALL INTESTINE

Three pairs of salivary glands Acid and enzymes make an The highly folded interior of this
secrete 3.1 pints (1.5 liters) of environment hostile to bacteria tube provides a huge surface
saliva every day, which helps but perfect for the physical and area of about 3,100 ft2 (290 m2),
moisten food and makes it easier to chemical breakdown of food. ideal for absorbing nutrients.
swallow.

LIVER GALLBLADDER LARGE INTESTINE
AND PANCREAS
This wedge-shaped organ stores The colon transports indigestible waste
certain nutrients and regulates the Secretions from these organs help break from the small intestine—removing
levels of nutrients in the blood, so that down foods during the first part of water and salts along the way—to the
cells receive uninterrupted supplies. digestion in the small intestine. rectum, ready for defecation.

Hunger and thirst prompt us to eat and drink, but DIGESTIVE
after that our digestive system takes care of SYSTEM
everything else automatically. As food travels on
its journey of digestion, which takes up to two
days, it is broken down to release essential nutrients.

352

HOW THE BODY WORKS

MOUTH AND THROAT

Unlike some other animals, humans cannot swallow large chunks of food. It must first be chewed
into smaller pieces, an activity that takes place in the mouth. Once chewing has turned food into
a slippery pulp, it is pushed into the throat and swallowed, an action that propels it to the stomach.

BITING AND CHEWING Incisors Canine MANEUVRING FOOD
Premolars
Anchored in sockets in the upper and lower UPPER TEETH First molar Occupying the floor of the mouth, the tongue is a highly flexible, muscular
jaws, four types of teeth grasp food by biting LOWER TEETH Second molar organ that can change shape and also be protruded, retracted, and moved
it, then chew it into pieces small enough to be Premolars from side to side. During chewing, the tongue maneuvres food between
swallowed. Chisel-shaped incisors bite and slice; Third molar the teeth, without—usually—being bitten itself, and mixes food particles
more pointed canines grip and pierce; broad- Canine (wisdom tooth) with saliva. The tongue’s upper surface is covered with tiny bumps called
crowned premolars chew and crush; and Incisors Second molar papillae that enable the tongue to grip food and contain receptors
broad molars with four cusps (raised edges) bite First molar that detect tastes, heat, cold, and touch. When food has been thoroughly
with great force to grind food into small particles. chewed, the tongue compacts it into a mass, or bolus, by pushing
Biting and chewing is made possible by powerful it against the roof of the mouth. The tongue then initiates swallowing

muscles that elevate by pushing the bolus
the lower jaw to bring backward into the throat.
opposing sets of teeth
into contact. Adult teeth Surface of the tongue
A full set of 32 adult Spiky papillae on the tongue’s
Dentine teeth consists of four surface grip food; rounded
This bonelike tissue forms the inner incisors, two canines, papillae house taste buds that
framework and roots of each tooth, four premolars, and detect sweet, sour, salty, bitter,
and supports the outer enamel. six molars in each jaw. and umami (savory) tastes.

10 SALIVARY GLANDS and teeth. The taste, smell, sight, or thought
of food triggers the release of copious
The number of seconds Three pairs of salivary glands—parotid, amounts of saliva when hungry. Water and
it takes food to travel from sublingual, and submandibular—are mucus in saliva moisten and lubricate food,
the mouth to the stomach. connected to the mouth cavity by ducts making it easier to chew and
through which they release saliva. This swallow. Salivary amylase
EXTREME HUMAN is also produced, in small amounts, by breaks down starch in food
tiny glands in the mouth’s lining. Saliva is into the sugar maltose.
SWORD SWALLOWING 99.5 percent water, but also contains
mucus, the digestive enzyme salivary Inside a salivary gland
This involves introducing a sword at least amylase, and bacteria-killing lysozyme. This acinus inside a salivary gland
15 in (38 cm) long into the upper digestive It is released continuously in amounts is a cluster of glandular cells that
tract, and requires years of practice. Although sufficient to moisten and clean the mouth release saliva into a central duct.
the sword takes the same path as food
traveling from mouth to stomach, this is PERISTALSIS Movement of food
different from swallowing food. Practitioners Smooth muscle in the
learn to suppress the natural gag reflex that In the last part of swallowing, food is pushed actively esophagus wall contracts
prevents anything, apart from food, entering down the esophagus, from the throat to the stomach, behind the food bolus to
the throat. They also inhibit involuntary by a wave of muscular contraction called peristalsis. This push it downward, and
contractions of muscles that push food down is the main means of propulsion in the digestive tract. relaxes around and in front
the throat and esophagus and learn how to The wall of the esophagus contains layers of smooth of it to allow easy passage.
extend the neck to align the mouth, throat, muscle that are under involuntary control. During peristalsis,
esophagus, and stomach entrance. alternate waves of contraction and relaxation pass down Relaxed muscle
the esophagus to squeeze the bolus of food toward its
Art of the sword swallower destination. So powerful is peristalsis that it will propel Contracted
This X-ray of the upper body shows that there food to the stomach even if someone is standing on his muscle
is no trickery involved in legitimate sword or her head. At the lower end of the esophagus, the lower
swallowing. The head is tilted backward as the esophageal sphincter, normally closed to prevent Moving food bolus
sword passes down the throat and esophagus. backflow of food, relaxes to allow food into the stomach.

Parotid gland
This section through
the parotid gland,
which lies in front of
the ear, shows masses
of secretory cells,
which produce saliva.

Soft palate Epiglottis
Rises during swallowing Blocks entrance to larynx
to close off entrance to
during swallowing
nasal cavity

Nasal cavity

Hard palate
Provides rigid
surface against which
tongue pushes food

Bolus
Compacted mass of food
particles moistened by saliva

Parotid duct
Connects the parotid
gland with the mouth

Throat (pharynx)
Connects the mouth to

the esophagus

Tongue
Maneuvres and mixes food

during chewing before
pushing bolus into throat

Sublingual gland
Lies under the tongue and

releases saliva through
several ducts into the floor

of the mouth

Submandibular gland
Located next to the lower

jawbone; its duct opens
at the base of the tongue

Inside the mouth and throat
The first part of the digestive system
includes the mouth cavity, teeth, tongue,
and salivary glands, and the throat, through
which food passes during swallowing.

SWALLOWING Pharyngeal stage Larynx
As the bolus is pushed down the throat into Part of the respiratory
The process of swallowing involves the coordinated actions of the esophagus, the epiglottis folds backwards system that links the throat
the tongue, soft palate, pharynx (throat), epiglottis, esophagus, to prevent food going down the wrong way to the trachea (windpipe)
and several muscles. It has three phases—oral, pharyngeal, and into the larynx and trachea.
esophageal. The last two phases are involuntary (cannot be Esophagus
consciously controlled) and are controlled by the brain. During Connects the throat to the
the oral phase, the bolus of chewed food is pushed into the stomach, and is normally flattened
throat by the tongue, triggering the pharyngeal stage. Food unless food is passing down it
is squeezed down the throat and into the oesophagus by
muscular contractions. The soft palate rises to prevent access
to the nasal cavity; the tongue stops food reentering the
mouth; and the epiglottis cuts off the airway so that breathing
ceases temporarily. During the esophageal phase, the food
bolus is moved to the stomach by peristalsis (see opposite).

354

HOW THE BODY WORKS

STOMACH

The widest part of the alimentary canal, the stomach is a J-shaped bag linking the
esophagus to the first part of the small intestine. It begins the digestive process,
churning food and dousing it in gastric juice that contains protein-digesting enzymes.

STOMACH FUNCTIONS A healthy stomach
This colored, contrast X-ray of the stomach
The stomach expands by a considerable smooth muscle in the stomach wall, which shows its upper and lower curves, and the
amount as soon as food enters it. Two contract to create waves of peristalsis (see duodenum (top left).
types of digestion happen at the same right). This process mixes food with gastric
time in the stomach and together produce juice, churns it into a liquid, and pushes Inside the stomach (below)
a soupy mix of part-digested food called it toward the pyloric sphincter (muscular The highly elastic stomach wall has three muscle
chyme. Chemical digestion is carried opening) at the stomach’s exit. The stomach layers arranged at angles to each other. Deep
out by the enzyme pepsin, contained also stores food, releasing chyme through folds in its lining appear when the stomach is
in acidic gastric juice, initiating the the pyloric sphincter in small amounts to shrunken and empty.
breakdown of protein. Mechanical avoid overwhelming the small intestine’s
digestion is carried out by three layers of digestive processes (see pp.356–57). Pyloric sphincter
Ring of muscle that
GASTRIC JUICE controls the exit to
the duodenum
The gastric mucosa, or stomach lining, is dotted with
millions of deep gastric pits that lead to gastric glands. Duodenum
The first short
section of the
small intestine

Different types of cells within these glands secrete the

various components of the digestive liquid, gastric juice. Mucus
Mucous cells in the neck of the gland release mucus. Coats mucosa
Parietal cells release hydrochloric acid, which makes the and protects it
stomach contents very acidic, activates pepsin, and kills
bacteria ingested with food. Zymogenic from acidic
gastric juice

cells release pepsinogen, the inactive Mucous cell Gastric
form of pepsin. Enteroendocrine cells Secretes mucus mucosa
(stomach
release hormones that help control lining)

gastric secretion and contraction. Zymogenic cell
Secretes pepsinogen

Stomach lining Hydrochloric acid
Makes gastric
This magnified view Parietal cell juice acidic Peptide

of the stomach lining, Secretes Pepsin
or mucosa, shows its hydrochloric acid enzyme

closely packed epithelial

cells and the gastric pits Enteroendocrine
(dark holes) that lead
to gastric glands. cell
Secretes hormones

Protein Protein digestion
by pepsin
Gastric glands Muscularis Gastric pit Secreted as inactive
A section through the stomach wall Contains three Opening to gastric gland pepsinogen—to prevent it
shows deep gastric glands in the layers of smooth from digesting the stomach
mucosa lining and different secretory Gastric gland lining—and activated by acid,
cells within those glands. The muscle Produces gastric juice pepsin splits proteins into
submucosa connects the three- short chains of amino acids
layered muscularis to the mucosa. Submucosa Mucosa called peptides.
Underlies mucosa

Cardiac sphincter Longitudinal muscle layer 355
Prevents the Runs the length of the stomach
backflow of gastric DIGESTIVE SYSTEM
juice into the
esophagus

FILLING AND EMPTYING

The stomach expands enormously as it fills with recently
chewed food arriving through the esophagus. This
food is mixed with gastric juice by peristaltic waves of
contractions generated by the three smooth muscle layers
in the stomach wall. These waves of contractions gather
strength as they push food toward the closed pyloric
sphincter, where they become powerful enough to
churn food into creamy chyme. Once chyme is liquid
and lump-free, the stomach gradually releases it in
squirts through the relaxed pyloric sphincter.

Circular muscle layer
Wraps around stomach

Oblique muscle layer
Runs diagonally around
stomach

Closed pyloric sphincter
This endoscopic view shows a
pyloric sphincter tightly closed
to prevent the exit of food into
the duodenum while digestion
takes place inside the stomach.

Food mixed Muscular Chyme in
with gastric contraction duodenum

juice Pyloric Pyloric
sphincter sphincter
open
closed

1 During a meal 2 1–2 hours after a meal 3 3–4 hours after a meal
As the stomach fills, waves Food churned by powerful The pyloric sphincter opens
of muscular contraction mix muscular contractions and slightly at intervals to allow
food with gastric juice released part-digested by gastric juice small quantities of chyme into
by gastric glands. is turned into chyme. the duodenum.

Rugae REGULATION WHY DO WE Vomiting center
Folds that disappear in the brain
as the stomach The release of gastric juice and the VOMIT?
expands with food contraction of the stomach wall are Soft palate blocks
regulated by the autonomic nervous Vomiting can be caused by many factors entrance to nasal
3 Chyme system and by hormones released by the but is often the result of the stomach being cavity
Creamy liquid alimentary canal. Regulation happens in irritated by bacterial toxins. Irritants are
produced by three overlapping phases: cephalic (head), detected by receptors in the stomach’s Epiglottis blocks
digestion of food gastric (stomach), and intestinal. Before lining that send impulses to the vomiting entrance to larynx
in the stomach eating and during chewing the cephalic center in the brain stem (the base of
phase gives the stomach advance warning the brain). This triggers the vomiting Esophagus
The number of hours that food is on its way. The sight, thought, reflex in order to forcibly remove
food spends in the smell, and taste of food stimulates gastric the irritant. During vomiting, the Diaphragm
stomach before entering glands to release gastric juice and triggers diaphragm and abdominal muscles contracts
the small intestine. peristalsis. When food arrives in the contract, compressing the stomach
stomach, the gastric phase begins. Gastric so that semidigested food is forced Pyloric
juice secretion increases greatly and the up the esophagus and throat and sphincter
waves of peristalsis become much stronger. out of the mouth. closes
When semidigested food is released into
the duodenum, the intestinal phase Vomit reflex Abdominal
inhibits the release of gastric juice and the The closed pyloric sphincter, soft muscles
muscular contractions of the stomach wall. palate, and epiglottis ensure that contract
food is vomited out through the
mouth and does not enter the Small
esophagus or small intestine. intestine

356

HOW THE BODY WORKS

SMALL INTESTINE

The longest and most important part of the digestive system, the small intestine’s coiled tube
fills much of the abdomen. This is where, with the help of the pancreas and gallbladder, food
digestion is completed, and where simple nutrients are absorbed into the bloodstream.

HOW THE SMALL INTESTINE WORKS GALLBLADDER AND PANCREAS

Extending from the stomach to the large intestine, the small intestine These two organs play a key part in digestion in the
duodenum, the first part of the small intestine, when
has three parts. The short duodenum receives food from the stomach. semidigested chyme arrives from the stomach. Tucked
under the much larger liver, the gallbladder is a small,
The jejunum and ileum, together the longest section of the small muscular bag that receives, stores, and concentrates
bile, produced by the liver, then releases it along the bile
intestine, is where the final stages of digestion occur and food is duct into the duodenum where it aids fat digestion. The
pancreas produces pancreatic juice, which contains a
absorbed. Digestion occurs in two phases in the small intestine. First, number of digestive enzymes, and is released along the
pancreatic duct that merges with the bile duct before
pancreatic enzymes work inside the small intestine, digesting nutrient emptying the enzymes into the duodenum.

molecules as intestinal wall muscles contract to propel food onward by

peristalsis. Then enzymes attached to the surface of villi, the millions of Bile duct
This micrograph image shows a section through
Muscularis fingerlike structures that project from the the bile duct that carries bile from the gallbladder
Contains two intestinal lining, complete digestion before to the duodenum, absorbing water from the bile.
muscle layers the villi absorb digested nutrients.

Mucosa DIGESTION AND ABSORPTION
Lining of small
intestine As food is moved along the jejunum and ileum, digestion
continues by the enzymes on the surface of the villi. These
Pancreas tiny projections increase the inner surface area of the
Secretes pancreatic juice and small intestine for digestion and absorption by thousands
releases it into the duodenum of times. Embedded enzymes such as maltase and
peptidase break down, respectively, maltose and peptides
Duodenum to their simplest units, glucose and amino acids. These are
absorbed into blood capillaries inside the villi and carried
Small intestine wall Gallbladder to the liver. Meanwhile, fatty acids and monoglycerides,
The wall of the small intestine Stores bile and releases it into the result of pancreatic enzyme digestion, are passed into
has two layers of smooth muscle a lacteal or lymph capillary, and despatched to the liver
that mix and propel food along the duodenum when food by way of the lymphatic duct and circulatory systems.
it. Its lining is covered with tiny, arrives from the stomach
fingerlike projections called villi. Ileum
Jejunum The longest
The middle section of the section of the
small intestine between the small intestine
duodenum and the ileum
Villus projecting
23feet Middle from the
digestive tract
The length of the The small intestine, intestinal wall
small intestine. pancreas, and
gallbladder make Lacteal (lymph
up the central part of capillary)
the alimentary canal
—also known as the Capillary network
middle digestive tract.

PANCREATIC ENZYMES Monoglyceride Amylase Maltose Protease
Lipase Fatty acid Starch Protein Peptide

Acidic, semidigested liquefied food called

chyme arrives in the duodenum, causing the

intestinal wall to secrete hormones. These

trigger the release of pancreatic juice and

bile through a common opening into the

duodenum. Alkaline pancreatic juice contains Artery
Vein
over 15 enzymes, including lipase, amylase,
Wall of intestine
and proteases, that catalyze the breakdown Direction of
blood flow
of a range of food molecules. Bile contains Fat breakdown Carbohydrate breakdown Protein breakdown
bile salts that emulsify large fat and oil After “treatment” with bile
droplets into tiny droplets that present a salts, fats (triglycerides) are Pancreatic amylase breaks Pancreatic proteases break
bigger surface area for digestion by lipase. broken down by pancreatic
After digestion by pancreatic enzymes, lipase into free fatty acids down complex long-chain down proteins into short

carbohydrates, such as chains of amino acids called Absorption across the villi
The villi of the small intestine provide a massive
starch, into disaccharide peptides. Peptidases break surface area for the absorption of digestive
products. These are shown accumulating
nutrients move to the surface of villi for and monoglycerides (a fatty sugars, such as maltose (two down peptides into in the bloodstream from left to right.

further digestion and absorption. acid joined to glycerol). linked glucose molecules). individual amino acids.

X-ray of the small intestine
The convolutions of the longest part
of the digestive tract in the abdomen
are revealed by this color-enhanced
X-ray. The small intestine is visualized
by introducing radiopaque barium
sulfate into it.

358

HOW THE BODY WORKS

LIVER

The liver is the body’s largest internal organ. It plays a key role in maintaining
homeostasis—a stable environment inside the body—by carrying out many metabolic
and regulatory functions that ensure the constancy of the blood’s composition.

ROLE OF THE LIVER STRUCTURE AND BLOOD SUPPLY

The deep red color of the liver is an external indicator Hepatocytes, the functioning units of the liver, are arranged into highly Kupffer cell
of what it does—process large volumes of blood to control ordered functional units called lobules, each the size of a sesame seed. Removes bacteria,
its chemical composition. Most of the liver’s functions, apart Within a lobule, sheets of hepatocytes radiate from a central vein.
from the work carried out by debris-removing Kupffer The liver is unusual in having two blood supplies. Oxygen-rich blood debris, and old
cells, are performed by hepatocytes, the multitasking cells delivered by the hepatic artery makes up around 20 percent of its supply. red blood cells from
that are the workhorses of the liver. As blood flows past The rest consists of oxygen-poor blood, rich in nutrients and other
hepatocytes, they take up nutrients and other substances substances, including drugs, absorbed during digestion, which the blood
to be stored, used in metabolic processes, or broken down, are transported to the liver along the hepatic portal vein. Inside
and also empty into the blood secretory products and each liver lobule, blood from both supplies mixes together and Exterior of lobule
nutrients released from storage. The liver’s only direct role is processed as it flows past the massed hepatocytes.
in digestion is the manufacture of bile, which is stored in
the gallbladder and released into the duodenum. However, Central vein Cross section
once digestion is complete, it “intercepts” nutrients of lobule
arriving from the intestines and processes them.
Hepatic
Liver portal vein

SOME LIVER FUNCTIONS Spleen Bile duct
Stomach
Apart from making bile, controlling the metabolism of Large intestine Artery
carbohydrates, fats, and proteins from food, and storing
minerals and vitamins, the liver also, among other things, Vein
makes a range of proteins that circulate in blood plasma;
breaks down drugs and other dangerous chemicals from Structure of liver lobules
the bloodstream; destroys worn-out red blood cells, In section, the tiny liver lobules appear to be
recycling the iron inside them (see p.334); and removes six-sided. Running vertically up each corner
pathogens and debris in the blood. of the lobule is a threesome of vessels—a tiny
vein, artery, and bile duct—that either deliver
Bile production Hepatic portal system Sinusoid blood to, or remove bile from, the lobule.
Hepatocytes produce up to 2 pints (1 liter) of this greenish A portal system consists of blood Receives blood from
fluid daily. Bile contains a mixture of bile salts, and wastes, vessels with capillary networks at each hepatic portal vein and Branch of portal vein
such as bilirubin (from the breakdown of hemoglobin), which end. Here, veins from digestive organs, Supplies nutrient-rich
are excreted with the feces. Bile salts aid fat digestion in the including the intestines and stomach, hepatic artery blood to lobule
duodenum, after which they are returned to the liver and converge to form the hepatic portal
secreted again in bile. vein that enters the liver. Branch of bile duct
Carries bile away
Protein synthesis Inside a liver lobule from the hepatocytes
Liver cells secrete most of the plasma proteins found in blood Blood flows along sinusoids past that make it
plasma, using amino acids from digested food or hepatocytes. hepatocytes to the central vein;
These proteins include albumin, which helps maintain water bile travels in the opposite direction. Branch of
balance in the blood; transport proteins, which carry lipids and hepatic artery
fat-soluble vitamins; and fibrinogen, for blood-clotting. Hepatocytes Supplies oxygen-rich
Process blood and make bile blood to lobule
Hormone production
The body’s chemical messengers, hormones work by changing Central vein KEY
the activities of target tissues. Once a hormone has exerted its Carries away processed blood movement of
effect it is destroyed; otherwise, it would continue to operate out nutrient-rich
of control. Many hormones are broken down by liver cells. Their to be returned to the heart blood
breakdown products are usually excreted by the kidneys in urine. movement of
500 oxygen-rich
Heat generation blood
The vast numbers of metabolic processes occurring in The number of movement
hepatocytes generate, as a by-product, a considerable amount different chemical of bile
of heat. This heat, together with that from working muscles, functions the
is distributed around the body by the blood, keeps the body liver performs.
warm, and enables it to maintain a constant temperature.

359

DIGESTIVE SYSTEM

Sinusoid
Channel that carries mixed
venous and arterial blood
between the hepatocytes

Bile duct PROCESSING NUTRIENTS
Carries bile
When nutrients—particularly glucose, fatty acids, and
Hepatocyte amino acids—flood into the bloodstream following
Liver cell digestion, the liver processes them. Glucose is the
body’s main fuel source, and its level in the blood must
Lymph be kept steady. Liver cells gather glucose; they store it as
vessel glycogen if blood glucose levels rise and release it from
store if levels drop. They also convert excess glucose to
fat. The liver breaks down fatty acids to release energy or
stores them as fat. It also manufactures packages
called lipoproteins to transport fats to and from body
cells. It breaks down excess amino acids, using them to
release energy and converting their nitrogen into waste
urea, which is excreted in urine.

STORING VITAMINS
AND MINERALS

Several vitamins, notably vitamin B12 and the fat-soluble
vitamins A, D, E, and K, are stockpiled by the liver and

released when required. The liver can store up to 2 years’

supply of vitamin A, and 4 months’ worth of vitamins

D and B12. Since they are stored, and any excess cannot
be excreted, it is important not to overdose on vitamin

supplements because the presence of excess fat-soluble

vitamins can damage the liver. The liver stores iron,

needed to make hemoglobin (see p.327) and copper,

which plays a part in many metabolic reactions.

Branch
of hepatic
artery

Stellate cell Branch of Crystals of vitamin D
Stores vitamin A hepatic This is one of the vitamins stored by liver cells. It is essential
portal vein for normal absorption of calcium ions, which is needed for bone-
building and many other functions, from the small intestine.
Detail of a lobule
In each lobule, vertical sheets of hepatocytes RED BLOOD CELL REMOVAL
separated by capillaries called sinusoids,
radiate from the central vein. Hepatocytes
absorb, process, and release substances as
blood flows along the sinusoids.

White blood cell Central vein Red blood Defunct red blood cells are destroyed by Kupffer cells,
Destroys pathogens Receives which are macrophages that form part of the lining of
processed blood cell sinusoids (red cells are also destroyed in the spleen). Iron
from sinusoids Carries oxygen is retrieved from one part of the blood cells’ hemoglobin
molecules, stored by hepatocytes, and reused when
DETOXIFICATION required; another part of the hemoglobin molecule is
broken down into the bile pigment bilirubin and excreted
While ingested or injected drugs may be helpful to the body in bile (see opposite). Kupffer cells also remove bacteria
in the short term, they are harmful if they remain in the and other debris from blood, and intercept some toxins.
bloodstream. The liver plays a vital role in detoxification by
breaking down drugs, bacterial toxins, manmade poisons, and Liver cirrhosis Kupffer cell
pollutants. Hepatocytes detoxify these harmful substances by This section through the liver of an alcoholic This micrograph shows a Kupffer
converting them into safer compounds that can then be person with cirrhosis, shows in liver lobules cell (yellow) trapping and “eating”
excreted. However, over time, excessive detoxification may, (white) surrounded by fibrous scar tissue (red) worn-out red blood cells (red)
as in the case of alcohol, cause fibrous tissue to develop, caused by excessive detoxification. contained in blood (blue) flowing
which stops the liver from working properly. between liver cells (brown).

Large intestine
This color-enhanced contrast
X-ray shows the main parts of
the large intestine from the
cecum bottom left round the
shieldlike path of the colon path
up, across, and down the
abdominal cavity to the rectum.

361

DIGESTIVE SYSTEM

LARGE INTESTINE WHY DO WE

This final stretch of the digestive tract is twice the width of the small intestine, HAVE AN APPENDIX?
although only one-quarter the length. Consisting of the cecum, colon, and rectum,
the large intestine processes indigestible waste to form feces. The worm-shaped appendix projects from
the cecum, the baglike pouch that is
Longitudinal FUNCTION OF COLON AND RECTUM located beneath the point where small
muscle and large intestines connect. For many
years it was assumed that the appendix was
At 5 ft (1.5 m) long, the colon is the sodium and chloride ions—through its a vestigial organ, one that had a function in
longest part of the large intestine. Every lining into the bloodstream. This our ancient ancestors but is now without
day it receives around 3 pints (1.5 liters) of reabsorption of water helps the body purpose, apart from becoming inflamed
watery, undigested waste from the small maintain its normal water content and during appendicitis. More recent research
intestine. The colon’s primary functions avoid dehydration, and also converts the suggests that it contains lymphoid tissue
are to move this waste so that it can be watery waste into solid feces that are that forms part of the immune system, and
eliminated from the body, at the same easier to move and dispose of. In addition that it contains a reservoir of “good” bacteria
time reabsorbing water and salts—mainly to food waste, feces also contain dead to repopulate the colon’s gut flora should it
cells, scraped from the intestinal lining, be flushed away or otherwise destroyed.
Layers of the colon wall and bacteria, which can make up to 50
Circular This section shows the longitudinal and circular percent of fecal weight. At the end of the
muscle muscle layers that produce movements. The mucosa colon, the rectum stores feces and then
releases mucus to lubricate the passage of feces. contracts to expel them through the anus.

Mucosa Submucosa

COLONIC MOVEMENT

Three types of colonic movement— 1 Segmentation 2 Peristaltic contractions 3 Mass movements
segmentation, peristaltic contractions, When its bands of longitudinal muscle contract, These contractions are similar to peristaltic Around three times per day, stimulated by the
and mass movements—occur during the the colon forms pouches that churn and mix movements elsewhere in the digestive tract. Small arrival of food in the stomach, these slow-moving,
12 to 36 hours it takes indigestible waste fecal material but generate little propulsion. waves of muscular contraction and relaxation pass powerful waves of peristalsis force feces from the
to travel from the small intestine to the Segmentation happens around every 30 minutes. along the colon, pushing feces toward the rectum. transverse and descending colon into the rectum.
rectum. These movements are produced
by the contractions of a layer of circular
muscle and of the three bands of
longitudinal muscle. They are generally
much more sluggish and short-lived than
those found in other parts of the digestive
tract, giving time for water to be reabsorbed
effectively. The strength and efficiency
of colon contractions increases when the
diet contains more fiber or roughage.

ROLE OF BACTERIA DEFECATION Spinal cord KEY
Motor nerve fibers
The colon is colonized by microorganisms, principally Normally, the rectum is empty and the internal Involuntary Sensory nerve fibers
bacteria, known as the gut flora. They are harmless unless anal sphincter, under involuntary control, and external motor nerve
allowed to spread elsewhere in the body. Bacteria digest sphincter, under voluntary control, are contracted to Cerebral cortex
nutrients, such as cellulose in plant fiber, that cannot be keep the anus closed. When a mass movement pushes fibers Sensory
digested by human enzymes. Bacterial digestion releases feces into the rectum, its walls are stretched. This nerve fibers
fatty acids, as well as B complex vitamins and vitamin K, is detected by stretch receptors, which initiate the Voluntary
that are absorbed through the colon wall and used by defecation reflex by sending impulses along sensory motor nerve Rectum
the body. It also releases waste gases including odorless nerve fibers to the spinal cord. Motor signals from the
hydrogen, methane, and carbon dioxide, and odorous spinal cord instruct the internal sphincter to relax and fibers Defecation reflex
hydrogen sulfide. Colon bacteria control pathogenic make the rectal wall contract, building up pressure Stretching the rectum
bacteria that enter the large intestine by preventing their inside the rectum. Sensory messages to the brain make Internal anal walls causes impulses
proliferation. They aid the immune system by promoting a person aware of the need to defecate, and a conscious sphincter to travel to the spinal
the production of antibodies against pathogens and the decision is made to relax the external sphincter so that cord triggering the reflex
formation of lymphoid tissues in the intestinal lining. feces can be pushed out through the open anus. External anal that causes the rectum
sphincter to contract and the
sphincters to relax.

362 Liver

HOW THE BODY WORKS

NUTRITION AND GLUCOSE
METABOLISM
FATTY ACIDS
The process of digestion produces a range of simple AMINO ACIDS
nutrients that provide the raw materials for metabolism,
the collection of chemical reactions that together
bring cells to life. Before they can be used, however,
most nutrients are processed by the liver.

FATE OF NUTRIENTS that are essential to the body to provide Liver and metabolism
energy and building materials, or to make The liver stores, modifies, and despatches
During digestion, complex carbohydrates, the metabolism work efficiently. Nutrients nutrients that are needed by cells for their
fats, and proteins are broken down by are absorbed from the small intestine metabolic processes, while maintaining
enzyme action into, respectively, glucose, and most travel through the hepatic relatively constant levels of those nutrients
fatty acids, and amino acids. These simple portal vein to the liver; fatty acids reach in the bloodstream.
molecules, along with vitamins and the liver by way of the lymph system and
minerals, are nutrients—food substances then the bloodstream. According to the Cell division takes place with
body’s immediate needs, and in order the help of amino acids, fatty
Blood vessel Capillary network to maintain constant levels of nutrients
in the blood, the liver stores some acids, and glucose
nutrients, breaks others down, or simply
allows them to continue their onward
journey to be used by body cells.

Blood vessels of the small intestine Growth, renewal, and repair
This cast shows the fine networks of blood Inside cells, amino acids are built into
capillaries that infiltrate the wall of the small proteins used for cell division (shown
intestine and collect newly absorbed nutrients. here), construction, and repair. Fatty
acids form cell membranes and
CATABOLISM AND ANABOLISM supply energy for cell maintenance.

Thousands of chemical reactions take place components: catabolism and anabolism. ENERGY BALANCE
inside every body cell at any one time, Catabolism involves the breaking down of
most of them catalyzed by enzymes. These complex molecules to simpler ones, often The chart below shows energy requirements activity. A teenage boy, for example,
reactions make up the body’s metabolism. to release energy. In the digestive tract, in kilocalories (kcal) and kilojoules (kJ) for requires large amounts of energy because
This has two closely interlinked catabolic reactions break down foods. different ages, genders, and activity levels. his body is growing rapidly. Food energy
Anabolism is the opposite of catabolism. The amount of energy each person needs obtained should balance energy expended
Breaking down and building up It involves processes where smaller depends on age, gender, and level of because any excess is stored as fat.
During metabolism, nutrients such as glucose, molecules are used as building blocks to
amino acids, and fatty acids that are absorbed construct larger ones, such as linking AVERAGE DAILY ENERGY REQUIREMENTS
following digestion are broken down or built up. together amino acids to make proteins.

Simple molecules from digested food Child 8 years 1,853kcal (7,760kJ)
Girl 15 years 2,207kcal (9,240kJ)
Catabolic processes Anabolic processes Boy 15 years 2,875kcal (12,035kJ)
Many catabolic processes involve breaking The enzyme-catalyzed reactions involved in Woman (inactive)
down fuel molecules such as glucose to anabolic processes use energy to join simple Woman (active) 1,917kcal (8,025kJ)
release their energy. Catabolism provides molecules to construct larger ones, such as Man (inactive) 2,150kcal (9,000kJ)
energy for other chemical reactions. multipurpose proteins or glycogen. Man (active) 2,515kcal (10,530kJ)
3,000kcal (12,560kJ)

Energy Complex molecules 0 500 1,000 1,500 2,000 2,500 3,000

KCALS PER DAY

363

DIGESTIVE SYSTEM

HOW FOOD IS USED IN THE BODY

Glucose is either taken up by liver cells to adipose tissue (body fat) for storage for growth and maintenance. Excess amino
inside the liver (see pp.358–59) and stored as fat, providing the body with both an acids cannot be stored and are converted
as the complex carbohydrate glycogen, energy reserve and insulation. Some amino by liver cells to glucose or fatty acids.
or it remains in the bloodstream to provide acids are broken down by liver cells; others
body cells with a ready source of energy. are used by the liver to manufacture Energy release
Fatty acids may be stored in the liver, used plasma proteins, such as fibrinogen, which Like all body cells, this skin cell needs energy
by liver and muscle cells to supply energy, is involved in blood clotting. Most amino to make it work. The primary source of energy is
or picked up by cells to construct the acids, however, remain in the bloodstream glucose, although muscle fibers and liver cells also
membranes inside and around them. to be used by cells throughout the body to use fatty acids. Under starvation conditions,
However, most fatty acids are despatched build the wide range of proteins needed amino acids may be used.

KEY Fat cells Muscle cells Liver cells
Glucose leaves the Energy-rich fatty acids are stored as fat inside fat Like liver cells, muscle cells can store glucose as Inside liver cells, surplus glucose is stored as
liver to be used cells, then released when required into the glycogen. Glucose is released from store to provide glycogen granules (brown), then released as required.
bloodstream and used by some cells as an energy energy for muscle contraction, or released into the Multiple mitochondria (green) generate the energy
Glucose released source. Excess glucose is also converted to fat. bloodstream if blood glucose levels fall. needed to power the cell’s functions.
from storage

Fatty acids leave the
liver to be stored

Fatty acids released
from storage

Amino acids leave
the liver to be used

VITAMINS AND MINERALS Healthy hair and skin WHY DO WE
Vitamin A
Essential for normal body functioning, Bone formation Vitamin B2 FEEL HUNGRY?
most vitamins and all minerals can only Vitamin A Vitamin B3
be obtained from food. Vitamins are Vitamin C Vitamin B6 The feeling of hunger, which motivates
organic (carbon-containing) substances Vitamin D Vitamin B12 us to eat, is generated by the brain’s
that act as co-enzymes, which assist many Fluorine Biotin hypothalamus in response to a range of
enzymes that control metabolic processes. Calcium Sulphur signals received from the body, including
They are classified according to whether Copper Zinc those delivered by various hormones. For
they dissolve in fat (A, D, E, and K) or water example, the hormone ghrelin, released by
(B complex and C). Minerals are inorganic Phosphorus Heart functioning an empty stomach, activates parts of the
substances needed for enzyme function Magnesium Vitamin B1 hypothalamus that make a person feel
and in roles such as bone formation. Some, Vitamin D hungry. The hormone leptin, released after
including calcium and magnesium, are Boron Inositol eating by the body’s fat
needed in larger amounts; trace minerals, Calcium stores, causes the
including iron and zinc, in tiny amounts. Blood clotting Potassium hypothalamus to
Vitamin K Magnesium inhibit hunger
Use of vitamins and minerals in the body Calcium Selenium and create a
Some key roles played by vitamins and minerals Iron Sodium feeling of satiety
are shown here. A persistent dietary lack of certain Copper (fullness).
vitamins or minerals impairs body function, Blood cell formation
resulting in deficiency diseases. and functioning Muscle functioning Hypothalamus
Vitamin B (Thiamine)
Vitamins B6 and B12 Vitamin B6
Vitamin E Vitamin B12
Folic acid Vitamin E
Copper Biotin
Iron Calcium
Cobalt Potassium
Sodium
Magnesium

KIDNEY BLADDER

This bean-shaped organ cleans As it fills with urine, this
and filters all of our blood every muscular, elastic bag stretches
25 minutes. All the waste and expands. The muscles in its
products are excreted in urine. wall contract during urination.

URETER

This urine duct originates in
the kidney and channels urine
to the bladder, where it is
stored for a while.

The removal of waste produced by body cells URINARY
and maintenance of the body’s chemical balance SYSTEM
are performed by the urinary system. Blood is
filtered by the kidneys to remove toxins and any
excess substances, ready to be expelled in urine.

366

HOW THE BODY WORKS

KIDNEY FUNCTION

The urinary system plays a vital role in keeping the body’s fluid and chemical composition in
balance and in detoxifying the blood. The kidneys control fluid balance, “rinse” the blood by
removing waste products and toxins, and regulate blood pH, or acidity.

INSIDE A KIDNEY Glomerulus

The cortex (outer part) of each kidney contains about one Renal cortex
million nephrons. These are filtration units, each made
up of a glomerulus and a tubule. The glomerulus consists Tubule
of a capillary network surrounded by the glomerular
(Bowman’s) capsule. The tubule is a looped tube Cortical nephron
connected to the glomerulus. Together, they filter up
to 380 pints (180 liters) of blood plasma each day, Blood supply
reabsorbing most of the water and valuable chemicals Blood flows around
from the filtrate and producing 21/8–41/4 pints (1–2 liters) of each lobe to supply
urine as an excretory product. Loops from the nephrons
dip down into the medulla (inner part of the kidney), the glomeruli
where the amount of salt and water in the urine is
controlled. About 85 percent of nephrons are cortical Kidney lobe (above) Juxtamedullary nephron
(short-looped), the rest are juxtamedullary The kidney is subdivided into In this type of nephron the
(long-looped). Collecting ducts carry the lobes. Each lobe’s nephrons glomerulus is near the medulla
outflow of the nephrons to the renal feed into a urine-collecting
pelvis, from where urine flows into the duct, which drains into the Capillaries
ureter and the bladder for excretion. renal pelvis. Blood flows in capillaries around
In addition, the kidney has secondary each loop of the nephrons
hormonal functions (see p.391).
Urine-collecting duct
Renal cortex Urine travels down this duct through
Outer part of the kidney, the medulla to the renal pelvis
containing the nephrons
Renal capsule BREAKTHROUGH
Renal pelvis Outer shell of white,
Funnel-shaped tube narrowing fibrous tissue REPLACING A KIDNEY

into upper end of ureter The first successful human kidney transplant
was performed in 1957, between identical
Renal artery twins. Since then, immunosuppressive
Supplies blood drugs have made transplants from unrelated
for filtration in donors a routine procedure that offers new
life to people with kidney failure. When a
the nephrons replacement kidney is not available, kidney
dialysis (where the blood is cleansed
Renal vein artificially) is the only alternative. Kidney
Removes repair using stem cells, transplants from
animals, or replacement with a cloned
filtered blood human kidney may offer additional
treatment options in the near future.
Renal medulla
Inner part of
the kidney

Ureter
Conveys urine
to the bladder

Kidney cross section
The kidney is enclosed within
a capsule and comprises the
cortex, medulla, and renal
pelvis. Blood supply enters
through the renal artery and
leaves via the renal vein.

367

URINARY SYSTEM

Glomerulus Proximal Urine-collecting duct Proximal
Substances such as salts, Urine from many convoluted tubule
water urea, and glucose convoluted nephrons collects
Bowman’s capsule
are filtered into tubule here to travel to the
the space within the Carries solution renal pelvis Glomerular
capillary
Bowman’s capsule from the
glomerulus Fenestration
(pore)
Blood enters
Podocyte
nephron Filtration slit
Blood containing
between
glucose, salts, podocytes
proteins, and urea

travels to the
glomerulus

Afferent
arteriole to
glomerulus

Distal Efferent arteriole
convoluted tubule from glomerulus

Water content of Glomerulus
urine is fine-tuned Each glomerulus is a cluster of
blood capillaries surrounded by a
here and in the Bowman’s capsule. Blood enters the
urine-collecting duct glomerulus and pressure forces fluid out of
the blood through the filtration slits, creating
Thick ascending a cell-free fluid that enters the renal tubule.
limb of loop
of Henle Filtered blood Foot process
Salts are
leaves the nephron Podocyte
reabsorbed and the When filtration is
body’s chemical complete, blood Glomerular cross section
balance adjusted leaves the nephron Cells have projections called foot processes that wrap
to join the renal vein around the glomerular capillaries. Filtration slits are
Nephron created by the gaps between podocyte foot processes.
The nephron is the functional unit of the kidney.
Blood entering the kidney contains urea, a waste Thin HOW URINE IS MADE
product formed in the liver as a result of the
metabolism of body cells. The purpose of filtration descending The glomerulus of each nephron is a ball of capillaries that receives
in the kidney is to remove the urea and other toxic blood at high pressure from the renal artery. The pressure squeezes
chemicals, along with excess salts and water, while limb of loop the blood through its sievelike membranes so that water and small
leaving blood cells, important proteins, and molecules pass through, but larger cells and proteins are retained
chemicals in the bloodstream. of Henle in the blood. Each glomerulus sits inside the Bowman’s capsule, which
Salt is removed conveys the plasma filtrate to the proximal (nearest) convoluted tubule.
Thin ascending across the loop This tubule is the first part of a twisted tube that then runs down into
limb of loop of Henle wall here and the medulla in a loop—the loop of Henle—and back up the distal
Here, water is lost from transferred into (farthest) convoluted tubule to join tubules from other nephrons
the surrounding passing into the collecting ducts. In the proximal tubule, glucose is
the tubule, leaving the solution and reabsorbed and replaced into the bloodstream. In the loop of Henle,
urine more concentrated capillaries most of the water is reabsorbed back into the capillaries that surround
it. In the distal tubule, most of the salts are reabsorbed. What remains
is concentrated urine, containing urea and other waste products.

3,600 Urine contents 3.5% Urea 0.15% 94% Water
Water, urea, and other waste 1% Sodium
The number of pints products are the main 0.5% Chloride Creatinine
of blood received by the components of urine. The exact 0.25% Potassium
kidney every 24 hours. content varies depending on fluid 0.25% Phosphate 0.1%
and salt intake, environmental 0.25% Sulfate
conditions, and health. Uric acid

368

HOW THE BODY WORKS

BLADDER CONTROL

The bladder is a muscular bag that expands to store urine and contracts to expel
it. The ability to inhibit spontaneous urination is acquired in early childhood and
is vital to maintaining continence. This can be lost as a result of damage to the
pelvic floor or to the nerves supplying it.

DISCHARGE OF URINE Bladder fills Bladder lining
As urine flows into the bladder the detrusor Colored micrograph showing the internal surface folds
Waves of muscular contractions in the walls muscle in the wall relaxes and the bladder of the wall of the bladder when empty. The bladder
of the ureters help propel the urine to the stretches. The sphincters remain closed. expands and contracts as it fills and empties.
bladder from the kidneys. At the point where
they enter the bladder, valves prevent urine Two ureters carry urine from Bladder empties
reflux back up the ureters. This is important in the kidneys to the bladder The sphincters relax and open and the
preventing microbes from traveling up the detrusor muscle contracts, squeezing
ureters and infecting the kidneys. At the exit to Openings of the urine out through the urethra.
the bladder there are two sphincters that prevent the ureters
the urine from draining into the urethra. The have valves Internal sphincter Detrusor muscles
internal sphincter at the bladder neck opens and remains closed in the bladder walls
closes automatically but the external sphincter, As the bladder contract, voiding
located lower down, is under voluntary control. fills the detrusor The urethra leads the bladder
When the bladder is empty, the detrusor muscle from the bladder
in its walls is relaxed and both sphincters are muscles relax, to the outside of
closed. As the bladder fills, the walls become allowing the the body
thinner and stretch, prompting a small reflex bladder to
contraction in the detrusor muscle and triggering stretch Both internal and
the urge to urinate. This can be resisted external sphincters relax,
voluntarily by keeping the external sphincter External urethral sphincter
closed until an appropriate time. When it is remains closed allowing urine to exit
convenient to urinate, the external sphincter and
pelvic floor muscles are consciously relaxed, and
the detrusor muscle contracts, propeling urine
out of the bladder.

BLADDER SIZE NERVE SIGNALS Spinal cord segments S2
S2, S3, and S4
The size and shape of the bladder changes with the Control of micturition (urination) involves S3
amount of urine it is storing. When empty, the bladder nerve centers in the brain and spinal cord, Spinal reflexes travel from S4
is flattened into a triangular shape. As it fills, the wall thins and peripheral nerves supplying the here to the bladder
and it gradually distends and expands upward into a bladder, sphincters, and pelvic floor. where they trigger
more spherical shape protruding out of the pelvis into the As the bladder fills, its internal pressure
abdominal cavity. Its length may increase from 2 in (5 cm) increases. Stretch receptors in the wall bladder contraction and
to 5 in (12 cm) or more. transmit signals to the sacral micturition sphincter relaxation to
center in spinal cord segments S2 to S4, allow urination
FEMALE MALE which triggers reflex contraction of the
detrusor muscle. Signals sent to the Pudendal
Different bladder sizes KEY Prostate micturition center in the brain allow nerve fibers
The female bladder is generally Uterus voluntary control, so the need to urinate Control external
smaller than the male with less Bladder is consciously recognized, but the sacral
room to expand on filling. Urethra reflex is inhibited. When the decision to sphincter
urinate is made the detrusor muscle in
the bladder wall contracts, the internal Pelvic nerve fibers
sphincter relaxes, and the external sphincter Have both
is relaxed voluntarily. Once urination
begins, further reflexes from the urethra parasymathetic
also cause detrusor muscle contraction and sympathetic
and sphincter relaxation.
components
(see p.297)

Bladder nerve impulses
This schematic shows the
connection between segments
S2–S4 of the spinal cord with the
bladder via the pudendal and
pelvic nerves.

369

URINARY SYSTEM

Control in the brain 17 fl oz
The micturition center in
the brain inhibits the sacral The capacity of the average bladder
micturition center until a of an adult male.
conscious decision is made
to urinate. The pontine
micturition center, lower
in the brain, enables the
internal sphincter to relax
at the same time.

FLUID BALANCE

The body’s fluid content is maintained by balancing intake with excretion.
The osmolarity (concentration) of body fluids is detected in the brain by
nerve cells called osmoreceptors. If osmolarity rises, signaling dehydration,
antidiuretic hormone (ADH) is secreted from the pituitary gland and
acts on the kidney to increase reabsorption of water and decrease urine
output. If water intake is increased, osmolarity falls and ADH output
is reduced, leading to decreased fluid reabsorption in the kidney and
increased urine volume. When the body is sufficiently hydrated, urine is
a pale straw color. Darker urine signals a need for increased water intake.

The process of thirst Fluid balance upset by loss of water
Although the kidney can
conserve body water, it Water is lost from the
cannot replace it. Thirst, body through urination,
prompted by increased respiration, sweating
osmolarity, reduced (shown here), vomiting,
body fluid volume, and diarrhea, burns, or
symptoms such as a dry bleeding. This affects the
mouth, signals the need balance of fluids, setting in
to increase fluid intake. motion a series of events.

Osmoreceptors Concentration of Thirst
in the body fluids
As the body Increased intake
hypothalamus loses fluid, of water
activated plasma osmolarity
(concentration of
ADH released body fluids) increases,
triggering thirst and
Water is activation of
retained and osmoreceptors
reabsorbed
Dilution of
body fluids
As fluid levels in
the body increase,
plasma osmolarity
(concentration
of body fluids)
decreases

Release of ADH Inhibition of thirst
inhibited

Loss of water and return to fluid balance

BREAST UTERUS

Both men and women have A muscular sac that sheds its
breasts containing mammary lining during menstruation. Inside
glands. In women these are larger, the uterus, a fertilized egg can
and produce milk after childbirth. develop into a fetus.

OVARY

Two organs, one either side of
the uterus, house and mature
eggs (ova). One egg is released
each month during ovulation.

PENIS TESTIS REPRODUCTIVE
SYSTEM
The structure and blood supply of the penis Sperm grow, develop, and mature in a
allow it to become engorged and remain firm maze of tubules in each of a man’s two
enough to deliver sperm during intercourse. testes before traveling to, and then out
of, the penis during ejaculation.

The only system that differs greatly between
the male and female bodies, the reproductive
system is designed to fulfill the purpose of
producing offspring—the ultimate biological
goal of the human body and all living things.

372

HOW THE BODY WORKS

MALE REPRODUCTIVE SYSTEM

The reproductive organs of an adult male manufacture and supply sperm (spermatozoa), together
with the secretions of various glands that make up the semen, or ejaculate. In addition the testes,
which are the site of sperm production and storage, produce the male sex hormone testosterone.

SPERM PRODUCTION Sertoli cell Nucleus of Sertoli cell Membrane of
seminiferous
The production of sperm cells (spermatozoa) in the testes Lumen of tubule
is known as spermatogenesis. Each testis contains about seminiferous
500 tightly packed tubes called seminiferous tubules, tubule SPERMATOGONIUM
containing the immature male germ cells (spermatogonia).
The germ cells initially multiply by normal cell division, Mitotic division
or mitosis (see p.21), to produce spermatocytes. These Produces multiple primary
undergo a special reproductive division called meiosis spermatocytes with diploid
(see p.396), in which the number of chromosomes in chromosome number
each cell is halved from 46 to 23. These cells, carrying
half the genetic material needed to create a new human, PRIMARY
are called haploid cells (all other body cells are diploid). SPERMATOCYTE
Further divisions form sperm precursors (spermatids),
which develop into mature spermatozoa, completing First meiotic division
the process. Sperm are produced at a rate of several One primary spermatocyte
hundred million per day, from puberty into old age. splits into two haploid
secondary spermatocytes
Seminiferous tubule
Sperm heads are buried in SECONDARY
Sertoli cells (orange). Tails (blue) SPERMATOCYTE
project into the tubule’s lumen.
Second meiotic division
Both cells divide again (but
remain haploid), producing
two spermatids each

Vas deferens EARLY SPERMATIDS
Long, wide tube
Maturation
that conveys The four spermatids each
sperm from contain the haploid cell
epididymis count of 23 chromosomes

during LATE SPERMATIDS
ejaculation
Spermiogenesis
Epididymis Spermatids mature and
Site of sperm develop tails to form
mature sperm cells
maturation
and storage. Head Tail MATURE SPERM
Here they gain Contains nucleus with Will provide
motility and 23 chromosomes motility once Release into lumen
the ability to fully mature Mature sperm are not yet
fertilize an egg motile, so are transported
Acrosome via testicular fluid
Rete testis Enzymes in the
Mature sperm caplike coating Sperm surplus
(acrosome) help It takes around 65 days for a
enter this spermatogonium to complete
network of penetrate egg spermatogenesis and become
ducts that feed mature. A man may produce up
sperm into the to 12 trillion sperm in a lifetime.
epididymis

Seminiferous
tubules

Tightly coiled
tubes where
spermatogenesis

takes place

373

REPRODUCTIVE SYSTEM

Spermatic TESTES AND SCROTUM SPERM PROTECTION Sertoli cells
cord Sertoli cells (blue) nourish developing sperm in
The seminiferous tubules make up about Tight connections between the Sertoli cells the coiled seminiferous tubules, and offer them
Network 95 percent of testicular volume. They in the seminiferous tubules form what protection via the vital blood–testis barrier.
of blood contain male germ cells, from which sperm is known as a “blood–testis barrier.” This
vessels develop, and Sertoli cells, which provide separates the tubules from the blood
supplying the developing sperm with nourishment. vessels to prevent harmful substances
testis Fibrous tissue between the tubules contains in the blood from damaging developing
Leydig cells, which produce testosterone. sperm. If this barrier is breached, sperm
Testis Each testis has a tough coat called the cells can seep into the blood and may
tunica albuginea and sits within a pouch of provoke an immune response if the body
Tunica skin and muscle called the scrotum. Scrotal mistakes them for foreign invaders.
albuginea muscles are vital for thermoregulation of Antibodies may then enter the tubules
sperm, which must stay 3.5–5.5° F (2–3° C) and attack the sperm, impairing fertility.
below core body temperature to survive.
Cremaster The scrotum moves the testes to and away Seminal Bladder Vas deferens PATH OF SPERM
muscle from the body in response to fluctuations vesicle
contracts of air temperature, to promote fertility. Sperm make up less than 5 percent of
Dartos to raise semen volume. As they pass from the
muscle testis Temperature regulation seminiferous tubules into a long duct
wrinkles toward When it is cold, scrotal muscles contract to wrinkle called the epididymis, they undergo
scrotal skin body the skin and elevate the testes, conserving further maturation to become motile and
to avoid temperature. When warm, they relax, smoothing fertile before entering the vas deferens,
heat loss scrotal skin and lowering the testes to cool them.
a muscular tube that joins the duct
HORMONAL CONTROL (also in the brain) to release luteinizing of the seminal vesicle (behind the
hormone (LH) and follicle-stimulating bladder) to form an ejaculatory duct.
The hypothalamus (a gland in the brain) hormone (FSH), which both act on
secretes gonadotropic-releasing hormone the testis. LH stimulates Leydig cells to The seminal vesicle adds a fructose-
(GnRH). This triggers the pituitary gland produce testosterone (responsible for rich solution that provides energy
spermatogenesis and male secondary and nutrients for the sperm, and
sexual characteristics). FSH prompts Sertoli Cowper’s contributes around two-thirds
cells to support developing spermatozoa. gland of the total semen volume.
Feedback loops reduce GnRH secretion It is highly alkaline (to counteract
in response to rising levels of testosterone. Prostate gland Sperm leave Urethra vaginal acidity) and contains
epididymis prostaglandins, which dampen vaginal
Micrograph of testosterone immune responses to semen. As semen
Testosterone promotes spermatogenesis in the Toward ejaculation enters the urethra, the prostate gland
testes, and maintains male sexual characteristics, Sperm are propelled through the vas deferens into contributes a slightly alkaline fluid that
such as a deep voice, and facial and body hair. the ejaculatory duct, where added secretions form makes up around a quarter of the seminal
semen. This continues into the urethra, aided fluid. Finally, Cowper’s gland secretes
by contractions of the muscular prostate gland. a fluid (comprising less than 1 percent of
the total volume) to lubricate the urethra
and flush out any urine before ejaculation.

ERECTILE FUNCTIONS Veins drain Compressed veins
blood normally cannot drain blood
The penis has a dual role in the urinary and reproductive
systems, by conveying both urine and semen through Dorsal vein Arteries
the urethra. The urethra is contained within a tube called dilate
the corpus spongiosum, which runs the length of
the penis. On either side are two larger tubes called the Central artery
corpora cavernosa, each of which has a large central
artery surrounded by an expansile, spongy tissue that fills Corpora
with blood during erections, prompted by nerve impulses cavernosa
that cause the blood vessels to dilate. This usually occurs
due to sexual arousal, but can be unprompted. Corpus Urethra Corpora
spongiosum cavernosa fill
Prior to ejaculation, contractions within the duct
system drive the semen into the urethra. Rhythmic Flaccid penis with blood
contractions of perineal muscles during male orgasm In the nonerect penis, the corpora cavernosa have minimal blood
then eject the semen from the body. flowing through them, while the veins of the penis are wide open Erect penis
and full. The penis droops forward and is soft and flexible. During an erection, the corpora cavernosa fill with blood, and
as a result the veins become compressed, hindering outflow. The
engorgement results in enlargement and elevation of the penis.

374

HOW THE BODY WORKS

FEMALE REPRODUCTIVE SYSTEM Egg travels down
fallopian tube
The female reproductive organs release a stored egg (or ovum) at monthly
intervals, with two possible outcomes each time: to allow shedding Fallopian tube
of the uterine lining at menstruation, or to enable fertilization, Provides egg
implantation, and nurture of a developing embryo. with a 4 in
(10 cm) pathway
to the uterus

OVULATION Egg to uterus
An egg is released from the ovary
The ovaries are paired, oval organs, each midway through each reproductive
one about the size of an almond, that sit cycle, and reaches the uterus 6–12
at the ends of the fallopian tubes. Female days later. Only a tiny minority
germ cells (eggs, or ova) mature in the of eggs, if any, will be fertilized.
ovaries and are regularly released in
a process known as ovulation. Released egg

Each month 10 or more follicles, the Cilia Fimbriae
protective casings surrounding each egg The fallopian tube lining has cells Help direct egg
(see below), start to ripen, but usually just bearing tiny hairs or cilia (yellow) that into fallopian tube
one releases its egg from either the right help transport the egg to the uterus.
or the left ovary—right is favored 60
percent of the time. The egg travels down
the fallopian tube to the uterus and is
shed from the body along with the uterine
lining during the woman’s next menstrual
period. If, however, the egg is fertilized in
the fallopian tube, the resulting cell mass
may implant in the wall of the uterus.

An unfertilized egg stays Fimbriae
in the reproductive tract Tiny, fringelike folds called fimbriae, located at the
for between 12 and 24 junction of the fallopian tube with each ovary, pick
hours after ovulation. up the egg and guide it into the tube after ovulation.

FOLLICULAR DEVELOPMENT Cyclical development Secondary Primary follicle

Immature ova are protected within layers of cells Each month, some primordial (developing) Enlarging primordial
called ovarian follicles. The smallest, primordial follicle
follicles, have just a single layer of cells. Each follicles enlarge to become follicles
month, some of these develop to become
mature (Graafian) follicles. Just before ovulation, primary then secondary follicles,
one mature follicle moves toward the surface
of the ovary and bursts through to release its egg. until they are fully mature. Primordial
Its remnants form a body called the corpus follicles
luteum and, if the egg is not fertilized, this shrinks These follicles continually
to a small, white body called the corpus albicans.
At birth, girls have around 1 million follicles per develop in each ovary.

ovary. These will Mature Ovarian
degenerate to (Graafian) ligament
about 350,000 by
puberty, and 1,500 follicle Ovarian blood
by menopause. vessels
Rupturing
Ovulation follicle releases Corpus albicans
A magnified image (a type of scar
of an egg (in reality egg through tissue)
the size of a period) ovary wall
shows its release Corpus luteum
from a follicle. Released egg degenerates if
egg is unfertilized

Corpus luteum

375

REPRODUCTIVE SYSTEM

UTERUS AND MENSTRUATION Basalis layer Myometrium
(muscular
A menstrual cycle is counted from the first day of wall)
menstruation and usually lasts 28–32 days. Just prior
to ovulation, which usually occurs on day 14, the uterine Lumen
lining (endometrium) gradually thickens in preparation for
a possible pregnancy. If fertilization does not occur, the Cervix
outer endometrial layer (functionalis) is shed as menstrual
blood. The inner layer (basalis) remains and regenerates Functionalis
the functionalis with each new cycle. If an egg is fertilized, layer
the whole endometrium remains to protect the embryo.
Endometrium Shedding the uterine lining
Two endometrial layers, basalis An electron micrograph shows the process of
and functionalis, are richly menstruation: the endometrium (red) breaks away
supplied with blood vessels. from the uterus wall and is released as blood.

Egg reaches
uterus opening

Ovarian CHANGES DURING MENSTRUAL CYCLE Myometrium Endometrium
ligament Muscular wall Uterine lining, part of

HORMONAL CONTROL of uterus which sheds during
menstruation
The reproductive cycle is controlled by Menstruation Preovulation Ovulation Postovulation
two hormones from the pituitary gland in
the brain (see p.386). Follicle-stimulating HORMONES FSH Estrogen LH Progesterone Path of egg
hormone (FSH) causes ovarian follicles An unfertilized egg
to ripen and produce estrogen. When is expelled from
estrogen levels are high enough, a surge of the uterus during
luteinizing hormone (LH) from the menstruation
pituitary prompts final maturation of the
egg and its release from the ovary. After ENDOMETRIUM Menstruation
ovulation, as estrogen levels fall, FSH
production increases to repeat the cycle. Menstruation Thickening

Endometrial responses 0 2 4 6 8 10 12 14 16 18 20 22 24 26 28
Estrogen stimulates endometrial thickening. This is DAYS OF MENSTRUAL CYCLE
temporarily maintained by progesterone from the
corpus luteum, but it sheds as levels fall.

FUNCTION OF THE CERVIX

The cervix connects the uterus with the vagina and forms

a vital barrier to the outside. It secretes mucus that varies

in form and function throughout the reproductive cycle.

For most of the cycle and during pregnancy, the mucus

is thick and sticky to protect the uterus from infection.

It also forms an impenetrable barrier to sperm. During

a woman’s fertile period, rising

levels of estrogen make the Healthy cervix
mucus thin and stretchy (sort of The tight cervical entrance
like egg white), to enable sperm can be clearly seen in this
to pass through the cervix and image. Fertile cervical
reach the ovulated egg. mucus protects sperm
from the acidic vagina.

376

HOW THE BODY WORKS

CREATION OF LIFE

Human reproduction involves the fusion of male and female germ cells
(spermatozoa and ova), each containing half of the genetic information
required to create a fetus that will develop into a new human being.

SEX KEY Orgasm Sexual intercourse
Male This remarkable MRI scan shows a couple having
Sexual arousal in both sexes leads to progressive Resolution sexual intercourse. The penis (blue) is bent like
a boomerang. The uterus is shown in yellow.
engorgement of the genital organs as blood flow LEVEL OF AROUSAL Female
After sex, males have a refractory
increases, along with muscle tension, heart rate, and Plateau phase period, during which they cannot
have another orgasm. Women may
blood pressure. The penis becomes erect and the experience multiple orgasms.

woman’s clitoris and labia increase in size. The vagina

lengthens and its walls secrete Arousal Resolution
lubricating fluid to enable the Sexual responsiveness TIME
penis to enter and ejaculate passes through various
semen high up in the vagina, phases, and timing differs
near the opening of the cervix. for men and women.

SPERM RACE FERTILIZATION AND IMPLANTATION

Male fertility depends on a vast overproduction of sperm compared with The first sperm to reach the egg in the fallopian tube binds to its surface, releasing enzymes from the
the single sperm cell required to fertilize an egg. An average ejaculate acrosome surrounding its head (see p.372) that help it to break through the egg’s protective coating.
contains 280 million sperm per 1/16–1/6 fl oz (2–5 ml) of semen. Only around The egg responds by releasing its own enzymes to block any other sperm from entering, and the rest
ovulation will any sperm survive the vaginal acidity and cervical mucus fall away. The successful sperm is then absorbed into the egg and loses its tail. The nuclei of the egg
barrier to take part in the competitive race to reach the released egg. and sperm fuse, enabling their genetic material to join together: conception
has occurred. The newly fertilized egg then continues to travel down the
200 sperm enter both Fallopian Egg and fallopian tube, undergoing various stages of cell division to become
fallopian tubes tube sperm meet a ball of cells called a blastocyst that implants in the uterus.

100,000 Egg is 3 Morula 4 Blastocyst
enter uterus released Cell division A fluid-filled core
continues—the cells forms. The outer cells
60–80 Ovary are confined within (the trophoblast)
million the original egg cell invade the uterine
pass the cervix membrane so get lining and develop into
progressively smaller. the placenta.
100–300 By around day 4 there
million Uterus Against the odds is a ball of about 30
sperm enter Even during a woman’s fertile cells called a morula.
vagina at Cervix period, of the 300 million
ejaculation Vagina sperm that can enter the
KEY vagina, only about 200
reach the fallopian tubes.

Path of sperm 2 Zygote
The single cell
Path of egg that results from fusion Uterus
carries the complete
Strong swimmers amount of human
Sperm swim the 4 in (10 cm) DNA and is called a
fallopian tube towards the egg zygote. About 24 hours
at about 1/8 in (3 mm) per hour. after fertilization, the
cell divides into two.
Cervical mucus
During ovulation, cervical mucus The journey 1 Fertilization
becomes clear, slippery, and The fertilized egg undergoes A single sperm
stretchy, making it easier for sperm progressive cell division, at first just burrows into the egg,
to pass through. Mucus at this increasing the number of cells in the and they fuse. The
time dries in a “fern leaf ” pattern. mass. After implantation, these cells egg is about 20 times
start to specialize, to create the the size of the sperm.
different tissues of the embryo.

Conception
An electron micrograph shows sperm
(blue) attempting to enter the egg. The
egg’s thick outer membrane has been
removed to reveal its inner structure.

378

HOW THE BODY WORKS

THE EXPECTANT BODY

Pregnancy is a time of remarkable physical change in the body, when
hormonal surges and metabolic demands affect every tissue and organ,
not just the uterus. The blood, cardiovascular and respiratory systems,
gastrointestinal organs, and kidneys are all involved in this process.

MEASURING PREGNANCY 7% 7%

Breast Uterus 26%

Weeks of pregnancy are dated from the first day Weight gain (right) 7% Body fluids

of the woman’s last menstrual period, since the A healthy woman Amniotic
actual date of conception is rarely known. will gain 24–35 lb fluid
Pregnancy usually lasts for 40 weeks, and is (11–16 kg) during
arbitrarily divided into three 12-week periods pregnancy, only a 5%
quarter of which is
Placenta
known as trimesters. The first signs of pregnancy the weight of the baby.
25%
are cessation of menstruation (or sometimes
Baby
irregular bleeding), nausea or vomiting, breast tenderness, urinary Pregnancy posture
The weight of the enlarged uterus throws a pregnant
frequency, and fatigue. As pregnancy progresses, the uterus gradually 23% woman’s center of gravity forward, causing her to lean
backward and arch her back. Backaches are common.
rises up out of the pelvis, and the level at which its top can be felt (the Fat and

fundal height) is an important guide to fetal growth and development. protein

Mammary Nipples may Restricted
lobules darken in lungs cause
enlarge response shortness
to pregnancy of breath
Liver hormones
Heavy
Waistline may Intestines are breasts
start to thicken compressed sag slightly
by enlarging
Intestines uterus Indigestion
may become
Growing fetus Placenta is increasingly
is encased in fully formed troublesome
amniotic fluid by 20 weeks
Navel may
First trimester Enlarging protrude
Nausea is common, breasts may enlarge and feel tender, and uterus
there is an increased need to urinate. Heart rate rises and the Fetus has
woman often feels unusually tired. Food transit through the Bladder reached
gut slows and heartburn or constipation may result. becomes full size
slightly
compressed Bladder is highly
compressed
Second trimester
Any sickness usually subsides and food cravings may be Hemorrhoids
experienced. The woman gains weight rapidly. Back pain are common
is common, as are stretch marks on the abdomen. Increased
circulation may cause nosebleeds and bleeding gums. Third trimester
The abdomen reaches maximum protrusion and the navel
may bulge outward. Leg cramps and swelling of hands and
feet may occur. Irregular Braxton-Hicks contractions (“false
labor”) often begin in the weeks leading up to labor.

0–12 WEEKS 13–24 WEEKS 25–40 WEEKS

379

REPRODUCTIVE SYSTEM

SUPPORTING THE FETUS NON-PREGNANT Pear
8 WEEKS Orange
The placenta develops from the trophoblast (cells within the blastocyst, 14 WEEKS
20 WEEKS Cantaloupe
see p.376) and draws a blood supply from the uterus lining to nourish melon
FULL TERM
the fetus as it develops, dispose of its waste products, and protect it from Honeydew
melon
microorganisms. Clear amniotic fluid surrounds the fetus, offering
Watermelon
protection and allowing movement

and lung development. As it grows,

the uterus increases its blood flow

and its suspensory ligaments stretch.

The woman’s whole body increases

its blood and body fluid volume

and fat reserves,

Life support system to prepare for Safe haven Relative size of uterus
The placenta is richly supplied with labor and feeding. The fetus is The above guide to uterine growth during
blood vessels, which provide essential A healthy diet, protected within pregnancy indicates the vast change that occurs.
oxygen and nutrients to the fetus. including calcium, the sac of warm The uterus may never return to its previous size.
iron, vitamins, amniotic fluid and
and minerals is nourished by
is also crucial. the placenta via
the umbilical cord.

HORMONE CHANGES maintains the uterine lining and placenta, tends to Human chorionic gonadotropin (hCG)

After fertilization, progesterone from the corpus relax the uterus. In the second trimester, progesterone BLOOD LEVELS Estrogen
luteum in the ovary prompts endometrial thickening Progesterone
in readiness to receive the fertilized egg. A few days after is produced by the placenta, and acts with the hormone
implantation, the trophoblast produces human chorionic
gonadotropin (hCG), a hormone that stimulates the relaxin to soften cartilage and
corpus luteum to produce more progesterone, and
estrogen. Estrogen keeps the uterus growing, stimulates loosen joints and ligaments, Chemical surge
fetal development and breast enlargement, and boosts
blood circulation. It also prompts uterine contractions, aiding pelvic expansion, in The huge surge of the 04 8 12 16 20 24 28 32 36 40
along with the hormone oxytocin. Progesterone, which preparation for birth. hormone human chorionic AGE OF EMBRYO/FETUS (WEEKS)
Human placental lactogen gonadotropin (hCG) during
(HPL) and prolactin both early pregnancy is what
prompt milk production. causes a pregnancy test
to register as positive.

CHANGES IN THE CERVIX BREAST CHANGES MULTIPLE PREGNANCIES

In order for the muscular cervix to dilate before birth, it must first soften During pregnancy, the breasts gradually expand and may Twin pregnancies may result from a single
and then efface, a process where the tissue thins, or shortens. During feel tender. The nipples and areolae (the surrounding fertilized egg that splits in half early in cell
pregnancy, the cervix also produces extra-thick mucus that forms a plug circles) enlarge and darken due to pregnancy hormones, division, resulting in monozygotic, or
in the cervical canal. This helps protect the fetus from infection. and small bumps called Montgomery’s tubercles appear identical, twins. The fetuses have exactly
around the areolae. Increased blood supply can make the same DNA and are genetically identical.
Cervical softening veins under the skin more prominent. As birth More often twins are nonidentical
In late pregnancy, substances approaches, the nipples may leak a yellowish fluid called (dizygotic), resulting from the fertilization
called prostaglandins in the colostrum, or “pre-milk,” that is rich in minerals and of two separate eggs by two different
blood cause the cervical sperm. They are no more alike than any
tissue to soften and become antibodies to nourish and protect two siblings. Multiple pregnancies place
malleable (like the lips). the baby. Breast-feeding after a greater strain on the woman’s body and
birth stimulates the release there is a higher risk of adverse outcomes.
Cervical tissue forms of oxytocin, which
a necklike canal promotes uterine
contractions and
Mucus plug helps to birth the
placenta.
Cervical effacement
As it softens, the cervix begins Mammary Milk production
to thin (efface) and is drawn lobules Milk glands and ducts
in toward the lower part multiply and expand
of the uterus. from early pregnancy,
and are able to produce
Cervix gradually retracts milk even during the
and fuses with the uterus second trimester.

Softening cervical tissue
begins to thin (efface)

380

HOW THE BODY WORKS

LABOR AND BIRTH

Labor, the process by which a baby is delivered, can be both a joyful and painful Oxytocin
experience. The mother undergoes huge physiological and emotional stress, from This light micrograph shows crystals of oxytocin, the
the first contractions of the latent phase through to the delivery of the placenta. hormone secreted by the pituitary gland to instigate
labor. The trigger for its release is still unknown.

CONTRACTIONS Cardiotocograph (CTG) Heart rate increases
The CTG shows two corresponding lines: the strength of uterine with each contraction
Labor involves strong contractions of the uterine contractions and the correlating fetal heart rate. Normal fetal heart
muscle that open up the cervix and expel the baby rate is 110–160 beats per minute, and abnormal patterns, such 160
through the birth canal. Irregular, short-lived as deceleration, indicate fetal distress during contractions.
“tightenings” known as Braxton-Hicks contractions
may be felt much earlier in the pregnancy. As labor Regular uterine contractions
progresses, contractions become stronger, last longer, INTENSITY OF 140
and occur at regular, increasingly short, intervals— CONTRACTIONS
most women require analgesics. Contractions and 120
fetal response are monitored by a cardiotocograph FETAL HEARTBEATS
(see right) via sensors on the abdomen and on the PER MINUTE 100
baby’s head as it presents through the opening cervix.
TIME (MINUTES) 5 10 80 5 10
TIME (MINUTES)

STAGES OF LABOR

Labor begins in response to the release of oxytocin third stage ends with delivery of the placenta. During the to progress, abnormal presentations such as “breech,”
hormone, which stimulates uterine contractions. It divides second stage, pushing, or bearing down, by the mother tearing of the birth canal and perineum, and difficult
into three stages: the latent stage occurs when the cervix is synchronized with the contractions to help expel the placental delivery (see pp.478–79). Forceps or vacuum
starts to dilate; the first stage is defined by dilation of the baby. Maternal pain, particularly during the second and suction may be used to help pull out the baby, while
cervix from 11/2 to 4 in (4 to 10 cm); the second stage, third stages, may be managed by oral or injected analgesics cesarean section (delivery through the abdominal wall)
from full cervical dilation to delivery of the baby; and the or epidural anesthesia. Common problems include failure is used when either the baby or the mother is at risk.

Placenta Uterus Bladder Umbilical Contracting uterus Presenting part
Attached to Strong contractions Compresses as baby cord Contractions are combined Crowning head flexes
uterine wall push baby forward moves through with active pushing backward as it emerges
birth canal

Head Spine Cervix Rectum Vagina
Rotates toward Fully dilated Compresses under Widens to allow
passage backward
spine pressure of head

1 Dilation of the cervix 2 Descent through birth canal
In the first stage of active labor, the cervix dilates from 1½ to 4 in (4 to 10 cm), which can The presenting part, usually the head, is pushed forward by repeated contractions and
take hours. Delivery can only begin when the cervix is fully dilated. The baby usually faces pushing. The head progresses from the open cervix, through the vagina, until visible at the
its mother’s back, so the widest part of its head passes through the widest axis of the pelvis. perineum (“crowning”). It begins to flex backward to allow the rest of the body to follow.

381

REPRODUCTIVE SYSTEM

DILATION OF CERVIX RUPTURE OF MEMBRANES

Once labor has begun, cervical effacement (see p.379) gives way to Shortly before labor is due to begin, the membrane Placenta
dilation, when the cervix begins to open in order for the baby to be
delivered. Dilation usually begins during the latent phase of labor. of the amniotic sac that surrounds the fetus ruptures, Uterine wall
Contractions in the upper part of the uterus cause it to shorten and
tighten, consequently pulling up the lower part of the uterus and allowing amniotic fluid to leak out into the birth canal. Amniotic sac
retracting the cervix. In the latent phase, dilation does not exceed 11/2 in This is known as the water breaking and most women
(4 cm), but it can be long and uncomfortable, with irregular contractions.
go into spontaneous labor within 24 hours. 1 The show
Eventually, uterine activity continues into active labor, where regular, If it occurs before 37 weeks, it is considered As the cervix starts
increasingly powerful contractions lead to the progressive dilation of the premature rupture of the membranes, to open, either before
cervix up to a maximum of 4 in (10 cm), at which stage it is wide and may put the fetus at risk of infection or during labor, the
enough to accommodate the baby. The cervix moves from a posterior or premature delivery. Conversely, if the mucus plug, which
to an anterior position, and once it is fully dilated the fetal head rotates, membranes have not ruptured naturally, or if has sealed the cervical
flexes, and molds, before descending into the birth canal. labor is being induced, they may be ruptured canal until now,
artificially to speed up labor and allow a fetal loosens and is passed
monitor to be attached to the baby’s scalp. out. This is known
as the “bloody show.”

2 Contractions Fundus contracts 3 Water breaks Mucus plug
Muscular The amniotic sac is ejected
contractions start stretches and
Effaced At 4 in (10 cm) in the upper part eventually ruptures
cervix dilates wide, the of the uterus (the under the pressure
fundus) causing of the contractions,
cervix is fully the cervix to thin, releasing the amniotic
dilated stretch, and fluid and allowing
dilate, further descent of
Beginning to dilate Fully dilated preparing the fetus’s head.
The effaced cervix begins to dilate in As the contractions become stronger the way for
response to uterine contractions. For and more painful, their frequency and the fetus. Continuing
first-time mothers, the cervix dilates at regularity also increase. The cervix contractions
an average speed of 1/3 in (1 cm) per hour. dilates further under this strain as well Dilating cervix
The rate is faster for subsequent births. as under the pressure of the fetus’s head. Amniotic fluid drains
Bulging out through the
amniotic sac birth canal

Contracting uterus Shoulder Placenta Abdominal
Continuing strong First shoulder emerges Begins to pressure
contractions propel separate from
baby forward uterine wall

Birth canal
Starts to regain
normal dimensions

Body Birth canal Uterus Rectum
Rotates to release May tear as baby Contracts to seal Widens as
passes through pressure eases
shoulders blood vessels

3 Delivery of the baby 4 Delivery of the placenta
As the head is delivered, the doctor ensures that the baby’s airway is clear of mucus, Further contractions compress the uterine blood vessels, preventing blood loss.
and that the umbilical cord is not wrapped around its neck. The baby turns in the birth The doctor eases the placenta out by pulling the umbilical cord and applying pressure to the
canal to allow the shoulders to be delivered. The rest of the body then slips out easily. lower abdominal wall, or an injection of oxytocin hormone may be given to induce delivery.

HYPOTHALAMUS THYROID GLAND TESTIS

The hypothalamus links the The butterfly-shaped thyroid The testes produce sex hormones,
nervous and endocrine systems; produces hormones that help which stimulate sexual development
it secretes hormones that spur to regulate the body’s and sperm production.
the pituitary into action. metabolism and heart rate.

PITUITARY GLAND ADRENAL GLAND PANCREAS

Often known as the “master gland,” The distinct parts of this gland This gland has a dual purpose:
the pituitary controls the activities (medulla and cortex) produce secreting the hormones insulin
of many other glands. It is closely hormones that help us deal with and glucagon as well as
connected to the hypothalamus. stress and that attain homeostasis. digestive enzymes.

OVARY ENDOCRINE
SYSTEM
Each ovary makes the sex
hormones progesterone, which
thickens the uterine wall, and
estrogen, which ripens eggs.

The body’s internal environment is monitored
and regulated by a chemical communication
network. Working alongside the nervous system,
endocrine glands produce hormones that
control and coordinate many bodily functions.

384

HOW THE BODY WORKS

HORMONES IN Traveling hormones Endocrine tissue
ACTION Hormones are secreted into
the bloodstream by endocrine THYROID
Hormones are powerful chemicals that work by glands, such as the thyroid in GLAND
altering the activity of their target cell. A hormone this example, and travel to their
does not initiate a cell’s biochemical reactions, but target cells—which may be at Fat-soluble
adjusts the rate at which they occur. Endocrine cells some distance from the gland. hormone in
secrete their hormones into the fluid surrounding bloodstream,
them; hormones then travel through the bloodstream Water-soluble such as
and affect cells and tissue in distant parts of the body. hormone in thyroid
hormone
bloodstream,
such as calcitonin

Blood vessel

HOW HORMONES WORK

Although hormones come into contact although the signal reaches everyone WATER-SOLUBLE FAT-SOLUBLE
with essentially all cells in the body, they within range, you need to be tuned to HORMONES HORMONES
produce an effect on only certain cells, the right frequency to be able to hear it.
called target cells. These target cells have These hormones are unable to pass through Hormones that are fat soluble are able to
receptors that the hormone recognizes A hormone can have several different the cell membrane, which has fatty layers. pass through the cell membrane. They
and binds to, triggering a response inside target cells. However, these do not all Therefore, to have an effect on target cells, produce their effect by binding with receptors
the cell. Each hormone can only affect react in the same way to the hormone. they bind to receptors on the surface of the in the cell. Fat-soluble hormones include
specific target cells that possess the right For example, insulin stimulates liver cells cell. Most hormones are water-soluble. the sex hormones and thyroid hormone.
kind of receptor for that hormone. For to store glucose but prompts adipose cells
example, thyroid-stimulating hormone to store fatty acids. Once hormones reach Receptor on Hormone Hormone Binds to
only binds with receptors on cells of the their target cell, there are two different cell membrane binds to passes through receptor
thyroid gland. The mechanism is similar mechanisms by which they bind to the receptor in cell
to the way a radio broadcast works— cell’s receptors and produce a reaction, membrane
depending on whether a hormone is
water soluble or fat soluble (see right). 1 Receptor binding 1 Binding in cell
Water-soluble hormones are built from The hormone recognizes a receptor The hormone diffuses through the cell
amino acids (the building blocks protruding from the surface of the target membrane and binds to a mobile receptor
of proteins), while most fat-soluble cell and binds to it. The mechanism works within the cell itself, which is activated by
hormones are made from cholesterol. in a similar way to that of a key in a lock. the process of binding.

Cytoplasm

Cell nucleus

Secretory granule

Endocrine cell Cell Biochemical Complex
This micrograph shows a parafollicular cell in the nucleus reaction enters
thyroid, which produces and secretes the hormone triggered
calcitonin. Dots in the cytoplasm (colored red) are nucleus
secretory granules, where calcitonin is stored. Enzyme
activated
DNA of cell

SCIENCE Prostaglandin crystals 2 Activation 2 Genes triggered
Crystals of prostaglandin BI are seen in this Enzymes inside the cell are activated, The hormone–receptor complex makes
PROSTAGLANDINS micrograph, taken in polarized light. There altering the biochemical activity of the cell its way to the nucleus, where it binds to a region
are more than 20 types of prostaglandin. —either increasing or decreasing the rates of DNA. This triggers genes to switch on or off
Chemicals called prostaglandins act in a of normal cell processes. enzymes that alter the cell’s biochemical activity.
similar way to hormones, by stimulating
activity in target cells. However, they act
locally, near where they are produced, rather
than traveling in the blood. Prostaglandins
are released by nearly all cell membranes
and have many different effects, including
lowering blood pressure and increasing
uterine contractions during labor. They are
also involved in inflammation, and their
release contributes to the sensation of pain.

385

ENDOCRINE SYSTEM

TRIGGERS FOR HORMONE RELEASE Gonadotropins
from pituitary
Factors stimulating the production turn stimulate other glands; for example, Blood Nerve Suprarenal gland
and release of hormones vary. Some adrenocorticotropic hormone stimulates vessel fiber medulla
endocrine glands are stimulated by the the cortex (outer part) of the suprarenal

presence of certain minerals or nutrients gland to produce corticosteroid hormones.

in the blood. For example, low blood Hormonal stimulation leads to the Parathyroid Sex gland
levels of calcium stimulate the parathyroid rhythmic release of hormones, with Thyroid (testis)
glands (see p.388) to release parathyroid hormone levels rising and falling in a

hormone, while insulin, made in the particular pattern. In a few cases, release of Hormone Sex
pancreas, is released in response to rising hormones is triggered by signals from the release hormone
glucose levels. nervous system. An example is the medulla release
(inner part) of the suprarenal gland, which Epinephrine
Many endocrine glands respond to release

hormones produced by other endocrine releases epinephrine (also called adrenaline) Blood level response Nervous stimulation Response to hormones

glands. For example, hormones produced when stimulated by nerve fibers from the Low blood calcium prompts the Nerve fibers of the sympathetic Gonadotropin hormones from
by the hypothalamus stimulate the sympathetic nervous system. With this parathyroid to release parathyroid nervous system, signaled by the pituitary gland stimulate the
anterior pituitary gland to produce its type of stimulation, hormone release hormone, which raises calcium the hypothalamus, stimulate the sex glands (ovaries and testes)
hormones. These pituitary hormones in occurs in bursts rather than rhythmically. levels. The release of calcitonin suprarenal medulla to release to secrete more sex hormones.
from the thyroid is also inhibited. epinephrine in times of stress. In the testes, this is testosterone.

HORMONE REGULATION HORMONAL RHYTHM

Hormones are powerful and affect target When the desired temperature is reached, Hormone secretion The blood levels of some hormones vary
organs at low concentrations. However, the control unit triggers the boiler to go off. Thyroid hormone (yellow) is secreted from the according to the time of the month or day.
the duration of their action is limited— In a hormonal feedback system, the blood thyroid gland, following stimulation by hormones Levels of female sex hormones follow a
from seconds to several hours—so blood levels of a hormone (or chemical) are from the pituitary. The hormones enter the monthly cycle (see p.375), regulated by
levels need to be kept within limits, tailored equivalent to the air capillaries (blue) and travel in the bloodstream. the rhythmic release of gonadotropin-
to the specific hormone and the body’s temperature and the releasing hormone (GnRH) from the
needs. Many hormones are regulated by thermostat is often the hypothalamus. GnRH regulates release
negative feedback mechanisms. These hypothalamus–pituitary of hormones from the pituitary gland:
work like a thermostat-controlled heating complex. If the blood follicle-stimulating hormone, which causes
system. The thermostat is set at the desired levels of a hormone (or egg follicles to develop, and luteinizing
temperature and its sensor monitors the air. chemical) drop lower hormone, which triggers egg release.
If the temperature drops, a control unit in than is optimal, this Growth hormone (GH), cortisol from the
the thermostat triggers the boiler to go on. triggers the endocrine suprarenal gland, and melatonin from
gland to “turn on” and the pineal gland follow diurnal (daily)
Negative feedback loop release hormones. Once cycles. GH and melatonin are highest
Hormone blood levels are kept within an optimal blood levels have risen, at night, while cortisol peaks in the
range (known as homeostasis) by negative feedback the endocrine gland is morning. Diurnal hormone rhythms are
mechanisms. Levels are monitored and if they get triggered to “turn off.” linked with sleep–wake or light–dark cycles.
too high or low production switches off or on.

Rising blood levels of Hormone production by Gland decreases secretion CORTISOL CONCENTRATION (μg/dL) 22
hormone detected gland turned down of hormones 20
18 5 10 15 20 24
Gland releases more Homeostasis Low blood levels of 16 TIME OF DAY (HOURS)
hormone into the blood hormone detected 14
Hormone production by 12
gland turned up 10

8
6
4
2

0

Cortisol levels
The hormone cortisol affects the metabolism
and is controlled on a 24-hour cycle. Maximum
concentration is achieved between 7 and 8 am
each day, with a nadir at about midnight.

386

HOW THE BODY WORKS

THE PITUITARY GLAND Hypothalamus

The tiny pituitary gland, at the base of the brain, secretes hormones that
stimulate other glands to produce their own hormones. It is often called
the master gland because of its wide-ranging influences, but the real
master is the hypothalamus, linking the endocrine and nervous systems.

HORMONE CONTROLLERS

The pituitary gland consists of two anatomically Anterior lobe Macrophage Secretory cell
and functionally different parts: an anterior lobe and Secretory cells, which manufacture hormones, can be seen
a posterior lobe. The anterior lobe forms the bulk of the around the edge of this color scanning electron Pituitary gland
pituitary, and consists of glandular tissue that manufactures microscope picture. Controlling hormones from the
hormones. The posterior pituitary is really part of the hypothalamus reach the secretory cells through LOCATOR
brain and is derived from hypothalamic tissue. It does not capillaries, one of which is visible toward the
make hormones itself, but stores and releases hormones bottom of the image. The inside of the Portal system
produced by the hypothalamus. capillary contains a macrophage, a type The system of
of cell that helps fight infection.
The two lobes link to the hypothalamus differently. blood vessels that
The anterior lobe is linked by a system of interconnected 9 carries regulatory
blood vessels called a portal system. In a portal system,
blood from arteries and veins connects directly rather The number of hormones from
than traveling through the heart first. This system allows hormones made the hypothalamus
hormones from the hypothalamus to be delivered to the by the pea-sized
anterior pituitary rapidly. The posterior lobe is linked to the pituitary gland. to the anterior
hypothalamus by a nerve bundle, the hormone-producing pituitary
neurons of which originate in the hypothalamus. The axons
of these neurons extend into the posterior lobe and carry Capillary wall
their hormones there for storage. Nerve signals from these
neurons prompt release of their hormones “on demand.”

ANTERIOR LOBE HORMONES The release of hormones from the anterior pituitary is Capillary
regulated by the hypothalamus, which secretes releasing Hypothalamic
Seven hormones are produced in the anterior pituitary. or inhibiting hormones. Although different hormones hormones enter
Four of these, known as tropic hormones, target other from the hypothalamus reach the anterior lobe, secretory the anterior lobe
glands, prompting them to release their hormones. They cells recognize those directed at them and secrete or via capillaries
are thyroid-stimulating hormone (TSH), adrenocorticotropic release their specific hormones accordingly. The hormones
hormone (ACTH), follicle-stimulating hormone (FSH), are secreted into capillaries that drain into veins and into Secretory cell
and luteinizing hormone (LH). The others—growth the general circulation to reach their target organs. Cells of the
hormone (GH), prolactin, and melanocyte-stimulating
hormone (MSH)—act directly on target organs. anterior lobe
make and
release
hormones

Anterior lobe

Adrenal gland Testis Ovary

Skin Adrenal glands Thyroid gland Bone, skeletal, Sex glands Breast Venule
MSH targets skin cells ACTH stimulates the TSH stimulates the thyroid to muscle, and liver LH and FSH trigger the sex Prolactin helps stimulate Small veins called
called melanocytes, which cortex of the adrenal secrete hormones that affect GH promotes the glands to make hormones. milk production by the
produce the hormone glands to secrete steroid metabolism and body heat enlargement of bones, In females, they cause egg mammary glands. Levels venules carry
melanin. If produced in hormones that help the production, and promote increase of muscle mass, cells to ripen and stimulate rise before menstruation, hormones from the
excess MSH can cause body resist stress; they normal development of and tissue building and ovulation; in males, they which may account lobes of the pituitary
the skin to darken. also affect the metabolism. many body systems. renewal. prompt sperm production. for breast tenderness.
gland into the
blood stream

387

ENDOCRINE SYSTEM

Neurosecretory Posterior lobe HEALTH
cell In this color electron micrograph, the hormone-filled end of an
axon (nerve fiber) terminates on a blood vessel in the posterior lobe. GROWTH HORMONE
Pituitary stalk Hormones produced in the hypothalamus travel down the length of
Connects both lobes the axons to be stored in the axon terminals (at the ends of the axons). During childhood and the teenage years, growth hormone
of the pituitary to Signals from the hypothalamus stimulate the release of the hormones (GH) is essential for normal growth. In adults, it is needed to
the hypothalamus from the axon terminals into the adjacent blood vessels for transport maintain muscle and bone mass and for tissue repair. If too
around the body, when they are needed. much GH is produced during childhood, the actively
Axon growing long bones are affected and the person becomes
Nerve fibers that abnormally tall, but with relatively normal body proportions.
carry hormones Too little GH during childhood results in slowed growth of
from the long bones and short stature. An overabundance of GH after
neurosecretory the growth of the long bones is complete results in enlarged
cells in the extremities because bones of the hands, feet, and face
hypothalamus remain responsive to the hormone. Too little GH in
to the posterior adulthood does not usually
pituitary cause problems. If a lack
of GH is identified before
Blood puberty, treatment with
vessel synthetic growth hormone
means that affected
children will reach a nearly
normal height.

Nucleus

Neurosecretory Granule

cell Somatotroph
Specialized nerve Growth hormone is
cells in produced in cells called
hypothalamus somatotrophs in the anterior
produce hormones lobe of the pituitary gland.
This color electron
micrograph shows
numerous hormone-
containing granules within
the cell cytoplasm.

Axon terminal POSTERIOR LOBE HORMONES (nerve fibers) of the neurons to the axon terminals, where
Hormones made they are stored until needed. Nerve impulses from the
by the Two hormones—oxytocin and antidiuretic hormone same hypothalamic neurons where they were produced
hypothalamus (ADH)—are stored in the posterior lobe of the pituitary trigger the release of the hormones into capillaries. From
are stored and gland. These hormones are not made in the gland but by the capillaries, they pass into veins for distribution to their
released here the cell bodies of neurons located in two different areas target cells. Oxytocin and ADH are almost identical in
of the hypothalamus. After production, the hormones are structure: each is made of nine amino acids, only two of
Posterior lobe packaged in tiny sacs and transported down the axons which differ between them. However, each has different
effects. Oxytocin stimulates smooth muscle to contract,
Muscle especially that of the uterus, cervix, and breast. ADH
stretches influences the balance of water in the body (see p.369).

Pituitary gland anatomy Breast Uterus Kidney tubules Cuddle hormone
The pituitary gland consists of two lobes Oxytocin prompts the Oxytocin stimulates ADH causes water to be Oxytocin is produced naturally
and a stalk, or infundibulum, which connects release of milk from the contractions in labor. returned to the blood during childbirth and is thought
the lobes to the hypothalamus. Traveling mammary glands in Stretching of the uterus by the kidney’s filtering to play an important role in
through the stalk are blood vessels and breast-feeding. The baby’s triggers the hypothalamus tubules, making urine promoting nurturing maternal
nerve fibers that transport hormones suckling triggers this to make oxytocin, which more concentrated. ADH behavior. Oxytocin may also
from the hypothalamus. hormonal response. the posterior lobe releases. also affects blood pressure. be responsible for feelings of
satisfaction after intercourse.

388

HOW THE BODY WORKS

HORMONE PRODUCERS

The thyroid, parathyroid, adrenal glands, and pineal gland are all organs of the endocrine system
that exclusively produce hormones. Other organs and tissues also considered part of the endocrine
system, but which are not exclusively endocrine organs, are discussed on pages 390–91.

THYROID GLAND in the body has receptors for TH, and it produces calcitonin from parafollicular Thyroid hormone regulation
has widespread effects in the body. The cells located between the follicles. An Thyrotropin-releasing hormone (TRH) from the
The butterfly-shaped thyroid gland is thyroid gland is unusual among endocrine important effect of this hormone is to hypothalamus and thyroid-stimulating hormone
composed mainly of spherical sacs called glands as it can store large quantities of inhibit the loss of calcium from bones (TSH) from the anterior pituitary stimulate the
follicles, the walls of which produce two hormones—maintaining about 100 days’ into the blood. It is most important in production and release of thyroid hormones (TH).
important hormones, T3 (triiodothyronine) supply of TH. The thyroid gland also childhood, when skeletal growth is rapid. Blood levels of TH feed back to the pituitary and
and T4 (thyroxine), collectively known as hypothalamus to stimulate or inhibit activity.
thyroid hormone (TH). Almost every cell

PROCESSES INVOLVING TH EFFECTS Stimulate Hypothalamus Inhibit
Basal metabolic rate (BMR) Stimulate Inhibit
Increases BMR by stimulating the conversion of fuels TRH
Temperature regulation (glucose and fats) to energy in cells; when BMR increases, Decreased (thyrotropin-releasing hormone) Increased
(calorigenesis) metabolism of carbohydrates, fats, and proteins increases levels in levels in
Carbohydrate and fat metabolism Pituitary gland blood stream
Stimulates cells to produce and use more energy, blood stream
which results in more heat being given off, raising TSH
body temperature (thyroid-stimulating hormone)

Promotes use of glucose and fats for energy; enhances Thyroid gland
cholesterol turnover, thus reducing cholesterol
Thyroid hormones
Growth and development Acts with growth hormone and insulin to promote normal (T4 and T3)
Reproduction development of nervous system in fetus and infant, and
Heart function normal growth and maturation of skeleton Effects
(metabolism, growth, heart rate)
Necessary for normal development of male reproductive
system; promotes normal female reproductive ability
and lactation

Increases heart rate and force of contraction of heart
muscle; enhances sensitivity of cardiovascular system
to signals from the sympathetic nervous system (see p.297)

Parathyroid stimulated PARATHYROID GLANDS impulses, so it needs to be controlled
to release PTH precisely. When blood calcium levels fall
The four tiny parathyroid glands at the too low, PTH stimulates the release of stored
(parathyroid hormone) back of the thyroid gland produce calcium from bone into the blood and
parathyroid hormone (PTH), the major reduces calcium loss from the kidneys into
Low blood calcium regulator of calcium levels in blood. The urine. It indirectly increases the absorption
correct balance of calcium is essential of calcium from ingested food in the small
Bones Kidney Kidney for many functions, including muscle intestine. In order for the intestine to absorb
release slows loss converts contractions and the transmission of nerve calcium, vitamin D is needed, but the
calcium of calcium vitamin D ingested form is inactive: PTH stimulates
in urine to calcitriol Effects of parathyroid hormone the kidneys to convert vitamin D from its
Parathyroid hormone acts on the bone, kidneys, precursor form into its active form, calcitriol.
Increased level of calcium in and (indirectly) the small intestine in order to
blood inhibits release of PTH increase the amount of calcium in the blood.

Calcitrol increases Parathyroid hormone has a relatively short
absorption of calcium life span in the blood stream, its levels falling
from food in intestines by 50 percent every 4 minutes.

389

ENDOCRINE SYSTEM

ADRENAL GLANDS

The outer and inner regions of the It also helps the body to resist stress, STRESS RESPONSE
adrenal glands differ from each other in including from exercise, infection, extreme
structure, and each produces different temperatures, and bleeding. The androgens When stress is detected, nerve impulses from the emergency. This reaction is initiated mainly
hormones. The outer adrenal cortex is produced by the adrenals are relatively the hypothalamus activate the sympathetic by hypothalamic-releasing hormones, which
glandular tissue, while the inner medulla weak in their effects, compared with those nervous system, including the adrenal medulla. trigger the anterior pituitary to release growth
is part of the sympathetic nervous system produced by the ovaries and testes during These nerves start a fight-or-flight response, hormone and other hormones that prompt
and contains bundles of nerve fibers. late puberty and adulthood. However, preparing the body for action. Hormones the thyroid and adrenal cortex to secrete their
they probably play a role in the appearance from the adrenal medulla prolong the hormones. These mobilize glucose and
The adrenal cortex produces three of underarm and pubic hair in both sexes. response. Next, the body tries to respond to proteins for energy and repair.
groups of hormones: mineral corticoids, In adult women, they are linked to the sex
corticosteroids, and androgens. An drive. The adrenal medulla produces Brain Hypothalamus
important mineralocorticosteroid is epinephrine and norepinephrine. In Blood vessels dilate Triggers fight-or-flight
aldosterone, which regulates the sodium– stressful situations, when the sympathetic response and stimulates
potassium balance in the body and helps nervous system becomes activated, the Eye adrenal medulla; releases
adjust blood pressure (see p.391) and hypothalamus stimulates the adrenal Pupil dilates hormones that stimulate
volume. The main glucocorticosteroid is medulla to secrete these hormones, which anterior pituitary
cortisol, which controls the body’s use of augment the stress response (see right). Thyroid
fat, protein, carbohydrates, and minerals. Releases T3 and T4 Anterior pituitary
Releases growth hormone,
Adrenal Adrenal Zona granulosa to increase use of
cortex medulla Secretes mineral glucose for energy which acts with cortisol to
corticoids, mainly release glucose from
Blood vessel aldosterone, which is Lungs liver; and hormones
important for regulating Airways and blood that stimulate
mineral balance and thyroid and
blood pressure vessels dilate adrenal cortex

Adrenal anatomy Zona fasciculata Liver Heart
Each adrenal gland sits on a Secretes corticosteroids, Converts glycogen Increased rate
fatty pad on top of the kidney. mainly cortisol, which and force of beat
The cortex forms the bulk of regulates metabolism into glucose
the gland. The medulla contains and helps the body Stomach
nerve fibers and blood vessels. cope with stress Adrenal cortex Digestive activity
Releases cortisol, decreases
Adrenal cortex zones Zona reticularis which prompts liver
The adrenal cortex has three Secretes weak androgens, to release glucose, Spleen
layers, or zones. Each consists of which prompt growth of adipose tissue to Contracts
a different cell type and makes pubic and undearm hair release fatty acids
its own hormones. The outer at puberty and are Kidney
zone, zona granulosa, is located responsible for the Adrenal medulla Urine output
just under the fibrous capsule female sex drive Secretes decreases
that encloses the gland. The
middle zone, zona fasciculata, epinephrine and Intestines
is the widest and has columnar norepinephrine, Movement
cells. Cells of the inner zone, which supplement of food slows
zona reticularis, are cordlike. the effects of the
Bladder
sympathetic Sphincter
nervous response muscle
constricts
Skeletal muscle
Blood vessels dilate Skin
Blood vessels constrict,
hair stands on end, and
sweat pores open

PINEAL GLAND sends signals to the pineal gland via nerve Pineal % OF AVERAGE MELATONIN 80
connections near the spinal cord. The gland 70
The tiny pinecone-shaped pineal gland is suprachiasmatic nucleus also controls 60 18:00 00:00 06:00 12:00 18:00
located near the center of the brain, behind other diurnal biological rhythms, such as LOCATOR 50 TIME IN HOURS
the thalamus. It secretes the hormone body temperature and appetite, and it is 40
melatonin, which is involved in the body’s likely that melatonin cycles influence these Melatonin levels 30
sleep–wake cycle. Pineal activity lessens in processes. Melatonin is also an antioxidant The level of circulating melatonin 20
bright light, so melatonin levels are low and may protect against damage from free rises at night or when it is dark, 10
during the day. They rise at night, increasing radicals in the body. In animals that breed creating a daily rhythm of rising
about tenfold, making us sleepy. Bright seasonally, melatonin inhibits reproductive and falling hormone levels. 0
light does not directly affect the pineal function but it is not known whether 12:00
gland; instead, input from the visual melatonin affects reproduction in humans.
pathways stimulates the suprachiasmatic
nucleus (part of the hypothalamus), which

390

HOW THE BODY WORKS

PANCREAS Blood sugar regulation
The body needs to regulate blood glucose levels so that cells receive enough energy to meet
The pancreas is a dual-purpose gland with both digestive their needs. The main source of fuel is glucose, which is carried in the blood stream—any excess
and endocrine functions. The bulk of the gland consists glucose is stored in liver, muscle, and fat cells. The pancreatic hormones insulin and glucagon prompt
of acinar cells, which produce enzymes used in digestion storage or release of glucose from cells, keeping blood levels stable.
(see pp.362–63). Scattered among these cells are about
a million pancreatic islets, or islets of Langerhans, cell High blood sugar Low blood sugar
clusters that produce pancreatic hormones. There are four After each meal, blood levels of glucose If the body is not fed for a very long time
different types of hormone-producing cell. Beta cells make increase, stimulating beta cells in the blood glucose levels fall, stimulating
insulin, which enhances transport of glucose into cells, pancreas. alpha cells in the pancreas.
where it is used for energy or converted into glycogen for
storage. In this way, beta cells lower blood glucose levels. BETA ALPHA
Alpha cells secrete glucagon, which has the opposite effect CELLS CELLS
of insulin, stimulating release of glucose from the liver and
raising blood glucose levels. Somatostatin, secreted by Insulin released Glucagon released
delta cells, regulates alpha and beta cells. There are only a Beta cells in pancreas Alpha cells in pancreas
few F cells. They secrete pancreatic peptide, which inhibits release insulin, stimulating release glucagon, causing
secretion of bile and pancreatic digestive enzymes. body to store glucose. release of stored glucose.

Pancreatic islets Glucose stored in liver Liver releases glucose
Surrounded by enzyme- Liver converts glucose to Liver breaks down its
producing acinar cells, the glycogen for storage, ready for stored glycogen to form
islets contain four types of quick release when needed. glucose, which is then released
cell: alpha, beta, delta, and F. into blood stream.
Glucose stored in muscle
Beta cell Muscle cells are stimulated to Muscles release glucose
take up glucose and convert it Muscles break down glycogen
Delta cell to glycogen for storage. to release glucose. Fats and,
in extreme cases, amino acids,
F cell Glucose stored as fat can also be used for energy.
Some excess glucose is combined
Alpha cell with fatty acids for storage as
Acinar cell triglyceride (fat).

Blood sugar stabilized Blood sugar stabilized

OVARIES AND TESTES from the anterior pituitary gland. Before puberty, FSH LH. In males it regulates sperm production and in females

The female ovaries and male testes, also known as gonads, and LH are almost absent from the blood stream, but it plays a role in the menstrual cycle. The ovaries also
produce eggs and sperm respectively. They also produce
sex hormones, the most important of which are estrogens during puberty they begin to rise, causing the ovaries produce relaxin, which prepares the body for childbirth.
and progesterone in females, and testosterone in males.
Release of these sex hormones is stimulated by follicle- and testes to increase
stimulating hormone (FSH) and luteinizing hormone (LH)
hormone production. As OVARIAN HORMONES TESTICULAR HORMONES

a result, secondary sexual

characteristics develop Estrogens and progesterone Testosterone
and the body is prepared Stimulate egg production; regulate menstrual Determines “sex” of brain in fetus;
for reproductive functions. cycle; maintain pregnancy; prepare breasts for stimulates descent of testes before birth;
The hormone inhibin lactation; promote development of secondary regulates sperm production; promotes
inhibits release of FSH and sexual characteristics at puberty development of secondary sexual
characteristics at puberty

Hormone-producing cells Relaxin Inhibin
In the testes, interstitial cells Makes the pubic symphysis more flexible Inhibits secretion of follicle-stimulating
(dark circles) secrete testosterone. during pregnancy; helps cervix to widen hormone from the anterior pituitary
In the ovaries, granulosa cells during labor and delivery
(dark purple dots), shown here
surrounding an egg follicle, Inhibin
produce estrogen. Inhibits secretion of follicle-stimulating
hormone from the anterior pituitary

TESTICULAR TISSUE OVARIAN TISSUE

391

ENDOCRINE SYSTEM

OTHER HORMONE PRODUCERS

Many organs in the body that primarily have another tract secrete a number of different hormones, most of Stomach pylorus glands
function also produce hormones, including the kidneys, which play a role in the digestive process. Some of these This micrograph shows a section through gastric
heart, skin, adipose tissue, and gastrointestinal tract. hormones, called incretins, have sparked particular glands (pink) in the stomach. These glands contain
Although not as well known as hormones from purely interest as they affect many different body tissues. endocrine cells that produce gastrin.
endocrine glands such as the thyroid, they are just as Incretins stimulate insulin production in the pancreas,
important in controlling vital functions. Hormones from enhance bone formation, help promote energy storage,
the kidneys and heart help control blood pressure and and, by targeting the brain, suppress appetite. Researchers
stimulate production of red blood cells. Skin is hope that in the future incretins may be useful in treating
responsible for supplying the body with much of its diabetes mellitus and obesity. The hormone leptin,
vitamin D by producing cholecalciferol, a precursor form produced by adipose tissue, also affects appetite, and
of the vitamin. Endocrine cells lining the gastrointestinal has provoked interest as a possible aid to weight control.

Adipose tissue is not just a passive energy reserve, but HEALTH
an active endocrine organ that may hold the key to
controling obesity and its damaging effects. HORMONAL CONTROL OF BLOOD
PRESSURE
Hormone-producing tissues Skin
Various body organs not classified as Hormone: cholecalciferol (inactive The nervous system responds to sudden changes in blood
endocrine glands contain isolated cell vitamin D) produced on exposure pressure, but longer term control is managed by hormones.
clusters that release hormones. These to UV radiation Low blood pressure prompts the kidneys to secrete renin.
hormones regulate many important Trigger: in response to parathyroid Renin generates angiotensin, which constricts arteries and
processes in the body. hormone, the kidneys convert to raises blood pressure. The adrenal glands, pituitary gland,
active form of vitamin D (calcitriol) and heart also respond to low or high blood pressure by
Kidney Effects: active vitamin D helps secreting aldosterone, ADH (antidiuretic hormone), and
Hormone: erythropoietin absorption of calcium from food natriuretic hormone respectively. These hormones alter the
Trigger: low level of oxygen in blood by intestines amount of fluid excreted by the kidneys, which affects the
Effects: stimulates bone marrow to increase volume of blood in the body and hence blood pressure.
production of red blood cells Heart
Hormone: atrial Pituitary gland ADH
Hormone: renin natriuretic ADH produced by Promotes water
Trigger: low blood pressure or blood volume Trigger: high pressure hypothalamus is stored retention by kidneys,
in heart here and secreted when which raises blood
Effects: initiates mechanism for release of Effects: signals kidneys blood pressure falls pressure
aldosterone from adrenal cortex; returns to increase production
blood pressure to normal of urine and inhibits Natriuretic Heart
release of aldosterone, Elevated blood
Stomach reducing blood pressure hormone pressure stretches
Hormone: gastrin Acts on kidneys atria of heart,
Trigger: response to food Stomach, duodenum, to lower blood stimulating atrial
Effects: stimulates gastric acid secretion and colon endocrine cells
Hormone: motilin pressure by to produce
Hormone: ghrelin Trigger: associated inhibiting renin natriuretic
Trigger: long period without eating with fasting hormone
Effects: stimulates secretion and
Effects: appears to stimulate movements of stomach promoting
appetite and eating; stimulates and small intestine excretion of

growth hormone secretion Hormone: incretins sodium and water
Trigger: fat and glucose
Duodenum in small intestine Adrenal Kidney
Hormone: intestinal gastrin Effects: stimulates Low blood
insulin secretion; glands pressure
Trigger: response to food enhances bone Produce reduces blood
Effects: stimulates gastric acid secretion and formation; promotes aldosterone flow through
energy storage; kidneys and
movements of gastrointestinal tract suppresses appetite when stimulates
stimulated by them to
Hormone: secretin Adipose tissue produce the
Trigger: acid environment Hormone: leptin angiotensin, hormone
Effects: stimulates release of bicarbonate- Trigger: released which is renin
rich juice from pancreas and bile ducts; following uptake
inihibits production of gastric acid of nutrients activated by Renin
Effects: regulates renin from Activates
in stomach appetite, energy kidneys angiotensin
expenditure, and in arteries
Hormone: cholecystokinin food intake Aldosterone
Trigger: response to fats in food Causes kidneys
Effects: stimulates secretion of enzymes in to retain sodium
pancreas, and contraction and emptying of
gallbladder to allow bile and pancreatic and water,
increasing
enzymes to enter duodenum amount of fluid
in body and
raising blood

pressure

Hormonal action
The hormones that raise or lower blood pressure
become effective over a period of several hours.
Their effects may last for days.

life cycle

Each human is unique, with an individual genetic makeup. This
section tracks the changes that take place over each person’s life
cycle, from what characteristics are inherited from their parents,
through to childhood, puberty, old age, and eventually death.

392 400 Fetal development 406 Childhood 410 Adulthood and old age
404 The newborn 408 Adolescence and puberty 412 End of life
LIFE CYCLE

394 Life’s journey
396 Inheritance
398 Developing embryo



394

LIFE CYCLE

LIFE’S JOURNEY

Like all living organisms, every human is created out of
elements from its parents. Having grown from infancy
to a mature state, where reproduction of the next
generation is possible, a gradual aging precedes the
eventual decline toward death.

CONCEPTION TO DEATH DEVELOPMENT AND AGING Signs of aging
Wrinkles form with age as the
skin becomes drier, thinner,
droopier, and less elastic.

From the moment of fertilization, through preparing the body for potential Little is understood about the aging The multiplication, regeneration, and
the resulting development of a ball of reproduction. Fertility is time-limited for process, including why and how it occurs. death of cells is a necessary part of life, but
cells that contains a new combination of women, and at menopause the female During development there is evidence at some point their ability to regenerate
genetic material, the human fetus grows reproductive system becomes less of degenerative change affecting many successfully fails. Cancers develop when
in size and complexity. By birth, its organs responsive to hormonal stimulation and cellular components. Cells are the cell regeneration is uncontrolled and cells
are functioning, yet size and proportion eventually ovulation ceases. Men produce fundamental structures that comprise multiply rapidly and abnormally; organ
continue to change as the infant grows. sperm until the end of their lives, although organs; factors known to affect ongoing failure occurs when the cells cannot
Major changes occur at puberty when, less efficiently. As the body ages, its tissues cell function, division, and repair, such as regenerate at all.
under new hormonal influences, the become less able to repair and regenerate free radicals and UV radiation, have been
secondary sexual characteristics develop, and disease develops, leading to death. shown to reduce cellular longevity and Death rates rise after the age of 30, with
hence organ function. On a macroscopic women often surviving longer than men,
By 2020, for the first time in human history, the level, disease processes can be found to probably due to the protective effects of
number of people in the world aged 65 years and have started even in children, for example female hormones prior to the menopause.
older will exceed the number of children under five. the fatty deposits that occur within blood Age-related deterioration of cell function
vessel walls in atherosclerosis. relates to many factors, but eventually
death occurs as a result of organ failure.

Young and old Spots and
The hands of babies and adults are acne may start
similar in shape and structure, yet to appear
size, muscle bulk, skin color,
texture, and surface markings can
identify the individual’s age.

Underarm hair
begins to grow

Stages of man Limbs continue Childhood Puberty
All the organs and tissues in the body continue to grow to lengthen A child can communicate This involves a physical
until the end of puberty. Brain development generates effectively, and will dress, growth spurt and
early motor skills, such as walking and dextrous tool use, Early childhood feed, and look after the development of
as well as higher functions, such as speech and logical Now walking, a toddler oneself at a basic level secondary sexual
thought. After middle age, these skills decline as the brain gains height as the of independence. characteristics.
deteriorates and body tissues, including muscles, become long bones of its limbs
weaker and less able to respond to cerebral command. grow. Dexterity and
language develop.
Skeletal and
muscular

proportions start
to change

Infancy
During the first year, an
infant develops many
motor skills, including
mobility: from crawling,
to shuffling, then walking.

LIFE EXPECTANCY Most countries in 395
western Europe have
Around the world, life expectancy varies an aging population LIFE’S JOURNEY
hugely, from an average of less than 40
years in some African countries to an With one of
average of over 80 years in Japan. Many the highest life
factors contribute to this variation, expectancies, Japan
including ethnic and individual genetic also has the highest
tendencies, gender, nutrition, and proportion of
lifestyle habits. In addition to these, other centenarians
factors, such as sanitation and the
prevalence of infectious disease, can also Loss of hair and skin
affect longevity. Historically, improved tone lead to elderly
sanitation, health care, and nutrition have appearance
increased human life spans. However, in
the developed world, the current trend
toward obesity and its many associated
illnesses, including type 2 diabetes, heart
disease, and cancer, is already a factor
in a possible reversal of this pattern.

KEY 60–70 years South America has a The countries of
50–60 years relatively wide range sub-Saharan Africa have
above 80 years below 50 years
75–80 years of life expectancy some of the lowest life
70–75 years expectancies in the world

Adult height Hair may begin to
is reached lose pigmentation

Chest and shoulders and turn gray
have broadened and
muscles have developed

Reproductive An increase of Skin may become Old age
organs are fully fatty deposits mottled and saggy Bone and disk
developed often appears degeneration leads
Late adulthood to a reduction in
Hair has grown Reduced muscle Muscle bulk is height, while muscle
over much of the bulk and tone gradually reduced, definition is lost.
body, including and skin and hair
the legs Adulthood degeneration begin
Although the period to alter appearance.
Young adulthood of least physical
At full physical and change, a gradual
emotional maturity, a loss of muscle
young adult is ready definition occurs.
to reproduce and
restart life’s cycle.

396

LIFE CYCLE

INHERITANCE MAKING SEX CELLS

The basic data of genetic inheritance is the unique Sex cells divide in a different way from normal mitotic cell division (see p.21). This process,
combination of genes lying in chromosomes within called meiosis, is distinct from mitosis and also includes a further division, so that the
our cells. Created from our parents’ genes at the point chromosomal content of the resulting gametes is halved and also mixed.
of conception, this combination forms a template for
all cellular forms and functions throughout the body. Duplicated Matching
chromosome pair of
GENERATION TO GENERATION chromosomes
Nuclear
membrane 2 Pairing
The two sets then pair up and part again;
1 Preparation genetic material may cross over within the pairs,
The cell’s DNA strands divide to form two giving a new mix for the daughter cells.
identical sets of each chromosomal pair. The
nuclear membrane starts to break down.

Chromosomes are inherited as a unique combining 23 chromosomes each from the Cell spindle Chromosome Duplicated
parental combination. Most tissues are mother and father. Traits from both parents pair separates chromosome
comprised of cells that contain two sets may or may not be expressed, depending
of 23 chromosomes (diploid cells). These on what has been inherited, and whether 3 First separation 4 Two offspring
divide by mitosis (see p.21) to make genes are recessive or dominant (see The cell spindles pull the chromosomes There are now two daughter cells, each with
replica cells with the same chromosomal opposite). The physical expression of a apart so that there is one set of each pair in a pair of the 23 chromosomes (but these are
content. However, sex cells (the egg or gene (its phenotype), such as hair color, each of the two cells that form. slightly different from the original ones).
sperm), or gametes, form with only one can be obvious, but unseen tendencies to
set of chromosomes. When an egg and disease may also be inherited. Mutations
a sperm fuse at conception, the resulting that occur during cell division can be
embryonic cells contain two sets again, passed down through generations.

Single Chromosome
chromosome Nucleus

Spindle

X and Y chromosomes 5 Second separation 6 Four offspring
The sex chromosomes provide The chromosomal content is divided The resulting four cells all have a single set
data for sexual development again so that each sex cell contains just one of 23 chromosomes, each set containing a mix of
and function. Females have two set of 23 chromosomes. the genes from the original pair of chromosomes.
X chromosomes (right) in each
cell; males have one X and one
Y (left), named because of their
basic shapes.

SCIENCE can switch particular genes on or off. Although MIXED GENES gene developing within current and future
every cell contains a full set of DNA, each cell generations. A child’s genetic material is
EPIGENETIC PROCESSES epigenetically silences some genes, leaving Sophisticated technology allows the inherited from both parents. They, in turn,
active only those it needs to do its specialized study of gene sequences within several will have inherited genetic material from
Although the human genome has been function. However, when this process is affected generations of a family. This enables their own mother and father, and so on
mapped and partly explains patterns of by external, environmental factors, abnormal scientists to understand the origin of a back through the generations.
disease inheritance, environmental factors cells may develop and grow uncontrollably particular gene as well as to predict the
also play a part. Epigenetics is the science as a tumor. As the understanding of genetics risk of a feature, or disease, linked to that
of all modifications to genes other than
changes to the DNA sequence itself. Various increases, scientists are MATERNAL MATERNAL PATERNAL PATERNAL
intracellular changes, called epigenetic learning more about how GRANDMOTHER GRANDFATHER GRANDMOTHER GRANDFATHER
processes, alter gene activity—in effect they genes are affected by their
environment, and how
resultant conditions may
eventually be treated.

Twin studies MOTHER FATHER Units of inheritance
Studies of genetically identical This diagram shows how
(monozygotic) twins have Genes shared Genes shared genes are passed down
shown that, over time, with maternal with paternal generations and shuffled—
environmental factors affect grandmother grandmother not blended—to create
genetic expression. new combinations.

CHILD

397

INHERITANCE

RECESSIVE AND DOMINANT GENES SEX-LINKED INHERITANCE

Whether the effects of the message held in a gene on one of the chromosomal pairs Because males have only one X chromosome, if recessive genetic phenotypes are carried
is expressed or not depends on whether it is recessive or dominant. If both genes are on the sex chromosomes they will show a sex-linked pattern of inheritance. Women
the same, the individual is said to be homozygous for that gene, but if they are different have two X chromosomes, so recessive phenotypes may be hidden by a dominant gene
the person is described as heterozygous. Dominant genes overwhelm the message on the other, and she will “carry” the gene. However, in males, the presence on their
in recessive genes, so that only one of the pair needs to be dominant to see its effects. single X chromosome allows that gene to be expressed whether recessive or dominant.
Recessive genes may show their effects if both of the pair are recessive, but if there
is only one recessive gene it is suppressed by the presence of the dominant gene. X-linked
dominant
Recessive gene for BLUE EYE BLUE EYE inheritance
blue eyes The “abnormal” gene
Abnormal gene Normal gene is on the father’s
Recessive and recessive X X UNAFFECTED X chromosome. This
When both parents are XY example shows an
homozygous for a recessive AFFECTED FATHER MOTHER abnormal gene
gene, here the gene for blue inherited in a
eyes, the phenotype will be dominant fashion.
expressed because there is no The gene is expressed
dominant gene to overwhelm even if there is also a
it. This means that all normal gene present.
offspring will have blue eyes.

Recessive BLUE EYE ALL INDIVIDUALS HAVE BLUE EYES XY XX XY XX
gene for UNAFFECTED AFFECTED UNAFFECTED AFFECTED
blue eyes Dominant gene DAUGHTER
BROWN EYE for brown eyes SON DAUGHTER SON

Recessive and mixed Normal gene Abnormal Affected mother,
When one parent is gene unaffected father
homozygous recessive and the XY In this case, the
other heterozygous (has one UNAFFECTED FATHER XX mother is affected.
recessive gene for blue eyes AFFECTED MOTHER There is a 50 percent
and one dominant gene for chance that a
brown eyes), the offspring daughter or son
have an equal chance of being would inherit the
blue-eyed homozygous faulty gene and have
recessive, or brown-eyed the condition.
heterozygous.
BLUE EYES BROWN EYES BLUE EYES BROWN EYES
Dominant gene
Recessive gene for brown eyes
for blue eyes

Mixed and mixed XY XX XY XX
When both parents are UNAFFECTED UNAFFECTED AFFECTED AFFECTED
brown–eyed heterozygous, DAUGHTER
the offspring have a one in SON DAUGHTER SON
two chance of being
brown-eyed heterozygous; a X-linked
one in four chance of being recessive gene
homozygous blue-eyed; or a Here, both parents
one in four chance of being are unaffected,
homozygous brown-eyed. but the mother
carries the abnormal
BLUE EYES BROWN EYES BROWN EYES BROWN EYES Normal gene Abnormal gene on one of her
XY gene X chromosomes.
Recessive gene Dominant gene Her sons have a one
for blue eyes for brown eyes UNAFFECTED FATHER X X UNAFFECTED in two chance of
CARRIER MOTHER being affected. Her
XY XX daughters have a one
Dominant and recessive This male in two chance of
With two homozygous is affected having one affected
individuals, where one is chromosome and,
homozygous recessive because therefore, carrying
blue-eyed and the other is there is no condition.
homozygous dominant matching
brown-eyed, all the offspring normal gene XY XX
will be heterozygous
brown-eyed. UNAFFECTED UNAFFECTED AFFECTED UNAFFECTED
SON DAUGHTER
ALL INDIVIDUALS HAVE BROWN EYES SON CARRIER DAUGHTER

398

LIFE CYCLE

DEVELOPING EMBRYO

From fertilization to the end of the eighth week of pregnancy, the embryo
grows rapidly from a ball of cells into a mass of distinct tissue areas and
structures, which develop into organs within a recognizable human form.

EMERGING BODY STRUCTURES morula develops a hollow central cavity, after which it is Embryo at 5 weeks
described as a blastocyst. The blastocyst then implants Already the embryo’s external features, including the eyes,
The cell mass, or embryo, that results from fertilization into the richly vascular endometrium (uterus lining). spine, and limb buds, are clearly visible, as is the umbilical
undergoes cell division (cleavage) within 24–36 hours to cord. Scans can detect a pumping heart, and rudimentary
become two cells. About 12 hours later, it divides into The embryonic cells have started to differentiate into major organs are in place, although not developed.
four cells, and continues to divide until it becomes a ball specific cell types as genes within its chromosomes are
switched on or off. Within the inner cell mass of
of 16–32 cells, which is the blastocyst, an embryonic disk forms, consisting of
called a morula. During three primary germ layers: endoderm, mesoderm, and
cell division, the ectoderm. These layers are the origins of all the structures
embryo progresses in the body. The endoderm cells will form linings of
down the fallopian tube systems such as the gastrointestinal, respiratory, and
to the uterine cavity. urogenital tracts, as well as some glands and ductal parts
Around day six, the of organs such as the liver; mesoderm cells develop into
the skin dermis, the connective tissues of muscle, cartilage
Fertilization and bone, the blood and lymphatic systems, as well as
Sperm approach the zona some glands; ectoderm cells form the skin epidermis,
pellucida (the outer layer, tooth enamel, sensory organ receptor cells, and other
or shell, that surrounds the parts of the nervous system.
egg), which must be pierced
by a single sperm in order
for the egg to be fertilized.

Yolk sac nourishes Chorionic cavity Muscle fibers have Fluid-filled amniotic Pits on the side of the Pharyngeal arches will
embryo until placenta formed a structure that sac cocoons the embryo’s head are first develop into various
will become the heart growing embryo signs of developing eyes structures in the head
starts to function and neck
Tube down the
Amniotic cavity Umbilical embryo’s back will
will become cord become the spine
amniotic sac

Embryonic Developing Tiny buds will Beating
disk placenta grow into legs heart

Differentiation Neural tube formation Major organ formation
Having embedded into the maternal endometrium, the Attached by the umbilical cord to the placenta, and suspended By 4 weeks, the1/5 in- (5 mm-) long embryo has formed
embryo at 2 weeks has already started to differentiate into in the fluid of the amniotic sac, the 1/8 in- (3 mm-) long embryo rudimentary major organs. The heart has reorganized into four
various cellular types. The outer layers are forming the has formed a neural tube that will become the spinal cord. An chambers, and now beats to pump blood through a basic
placenta, to provide nutrition via the maternal blood, but the enlarged area at one end will form the brain, while the other vessel system. The lungs, gastrointestinal system, kidneys, liver,
main source of energy comes from the yolk sac, which has end curls under in a tail-like shape. Heart muscle fibers begin and pancreas are all now present, and a basic cartilaginous
developed alongside the rapidly changing embryo. to develop in a simple tubal structure that pulsates. skeletal system has developed to provide a supportive structure.

2 WEEKS 3 WEEKS 4 WEEKS