How body works

148 149IN AND OUT

Bacterial breakdown

Digesting what we can’t WHAT’S THAT SMELL?

The microbes in the colon use the carbohydrates we can’t digest for Fermentation by gut microbes
energy. They ferment fibre such as cellulose, which help us absorb produces a number of different
dietary minerals such as calcium and iron, are used to produce gases, including hydrogen, carbon
vitamins, and have other benefits in the body. The microbes dioxide, methane, and hydrogen
themselves also secrete essential vitamins, such as vitamin K. sulfide. In large amounts, these
can cause bloating and flatulence.
LARGE INTESTINE Gases produced The most gas-producing foods
by fermentation include beans, corn, and broccoli—
but onions, milk, and artificial
Bacteria digesting sweeteners are major offenders too.
carbohydrates

Carbohydrates

CORN BROCCOLI

Nutrients being
absorbed by the
large intestine
Wall of friendly
bacteria

Vitamin K plays
a vital role in
blood clotting

AACCIEDTIC
TAMI

NK
N B VITAMI

ABCUITDYRIC PROPAIOCINDIC VI WHAT ARE
PROBIOTICS?
Acetic acid is vital BLOODSTREAM Vitamin B helps
for muscle health Propionic acid helps us convert food Probiotics are the opposite
tissues respond to insulin of antibiotics. They are live
Absorption into the into energy bacteria that are consumed—in
bloodstream yogurts or tablets—to fortify
Butyric acid produces gut bacteria that have been
energy for gut cells
damaged by antibiotics
or disease.

Cleaning the blood THE ENTIRE BLOOD
STREAM IS FILTERED
As blood travels through the body, it picks up a great deal of BY THE KIDNEYS
waste and excess nutrients. These would quickly reach life- 20–25 TIMES PER DAY
threatening levels without the kidneys, whose job
it is to flush them out of the system. Each nephron is anchored
to the middle part of the
kidney, called the medulla

Waterworks Waste in the form
of urine is collected
It takes 5 minutes for blood to pass through in the medulla
the kidneys. It enters waste-laden and leaves
clean, having passed through countless CORTEX MEDULLA
microscopic filters that turn the waste into
urine. The urine then flows to the bladder, 1 Dirty blood in
at which point we feel the need to Waste-laden
urinate. A major component of urine
is urea—a waste product formed in
the liver (see pp.156–57).

STONES IN THE BODY blood enters the kidney
via the renal artery.
So much waste passes through This artery branches
the kidneys that even the smallest out into a forest of
amount of a mineral can build up capillaries that feed
and form a stone. These so-called around a million
“kidney stones” can pass out of microfilters known as
the body without incident, but nephrons. After being
some can become filtered, clean blood
large enough to leaves the kidney via
block the ureter. the renal vein.
The causes of
kidney stones RENAL PELVIS Dirty blood
include obesity, flows in
bad diet, and
not drinking Clean blood
enough water. flows out

Kidney stones RENAL ARTERY
RENAL VEIN

Dirty blood The glomerulus, where URETER
approaching the blood is filtered

glomerulus Waste fluid

Dirty blood flowing Collecting urine Waste products,
to a neighboring The urine-collecting tubes of including urea, other
nephron the medulla join together as they toxins, and excess salts,
coverge on the renal pelvis. Here the flow out in the urine
urine flows past the renal artery and
Cleaned blood leaving the renal vein and enters a tube
to join renal vein called the ureter. The ureter connects
the kidney to the bladder.
Capillaries NEPHRON Urine leaving
reabsorbing salts the nephron

and water

The filtration process Waste disposal IN AND OUT
As the blood passes through a nephron, it is forced Muscular contractions squeeze the urine
through a tiny filter called a glomerulus, which lets urea along the ureter—which is why our bladders Cleaning the 150 151blood
and other wastes pass, but keeps blood cells and valuable fill even when we are lying down. When the
proteins in the bloodstream. On the far side, the waste bladder is full, its muscular walls squeeze the MUSCULAR BLADDER WALL
fluid passes on a long loop through the kidney, where urine further, but the urine is halted by a
its composition of salts and water is fine-tuned, before ring of muscle at the base of the bladder.
it flows into urine-collecting ducts. Learning how to control this muscle gives
us the choice about when to urinate.

WHAT IF Bladder full BLADDER
THE KIDNEYS FAIL? of urine
Urethra
If a person’s kidneys are
too weak to filter the blood, a
dialysis machine can be used

instead of the kidneys. The
person’s blood flows through
a tube into the machine, gets

cleaned and filtered, and
then returns to
their body.

Water balance LOSING BALANCE

Water levels in the blood have to be kept within A number of commonly consumed substances uspet our water
a certain range; otherwise, the body’s cells balance. Alcohol, for example, blocks the pituitary gland from
become too shrunken (dehydrated) or too bloated releasing ADH. This means that the kidneys, which are working
(overhydrated) to work. The kidneys, the endocrine hard to get rid of the alcohol in the bloodstream, send more
system, and the circulatory system work together to water out into the urine. Drinking just one glass of wine can
maintain a healthy balance in our bloodstream. cause the body to get rid of the equivalent of four wine glasses
of water. Substances that
make us produce a lot
of urine are called
“diuretics.” Caffeine
is another diuretic.

Too little water Too much water

We lose water constantly, but there are times when we lose a lot of Far rarer than dehydration is overhydration, which can be
caused by extreme water intake after exercise, drug abuse,
water quickly—through sweating, vomiting, or diarrhea, for example. or disease. This results in an increase in blood volume and
a reduction in the level of salt relative to water in the blood.
This results in both a decrease in blood volume and a rise in the level

of salt relative to water in our blood. These act as triggers for balance

to be restored. HYPOTHALMUS HYPOTHALMUS

Salt detector Salt detector

Pituitary gland Pituitary gland

1 Low water alert 1 High water alert
The hypothalamus The hypothalamus
receives signals that blood
pressure is low and salt receives signals that blood
pressure is high and salt levels
levels are high. It responds are low. It responds by
by increasing the
production of ADH BRAIN producing less ADH. Since ADH
instructs the kidneys to store
(antidiurectic hormone), water, a reduction in ADH
which is carried to the
pituitary gland, where it means an increase in urination.

is released into the blood.

Stretch receptor on blood Stretch receptor on blood
vessel warns hypothalamus vessel warns hypothalamus
of decreasing blood pressure of increasing blood pressure

Torrent Trickle
of ADH of ADH

Decreasing water levels WATER EXCESS Rising water levels
in blood vessel in blood vessel

BLOOD VESSEL WATER DEFICIT BLOOD VESSEL

Contracting “RELEASE WATER!” Relaxing
muscles “STORE WATER!” muscles
in blood in blood
vessel wall vessel wall

2 Blood vessels contract 2 Blood vessels dilate
High levels of ADH instruct Low levels of ADH
instruct the blood vessel
muscles in the walls of the blood wall muscles to relax. This
vessels to contract. This constricts
the blood vessels, which, given the expands the blood vessels
and eases the blood pressure
current reduction in blood volume, caused by the excess water.
restores blood pressure to normal.

KIDNEY KIDNEY

Water URETER Water release
reabsorption URETER accelerated in
accelerated in
kidneys kidneys

BLADDER

3 Water 3 Water release
reabsorbtion Low ADH levels
High ADH levels also signal the kidneys
also signal the kidneys to to reduce the amount of IN AND OUT
reabsorb water and to retain water that they reabsorb,
the salts that are often lost so that more water is Water 152 153balance
through sweating or vomiting. added to the urine and
passed out through
URINE URINE the bladder.

4 Concentrated urine 4 Diluted urine
With the body retaining as With less water being reabsorbed
much water as possible, the bladder fills
more slowly, so the urine is more by the body, the bladder fills quickly and
more diluted urine is produced. The more
concentrated, and darker in color. diluted the urine, the lighter its color.

How the liver works HEPATIC VEIN

Once nutrients have entered the blood—via the mouth,
stomach, and intestines—they are taken straight to the
liver. Here, they are variously stored, dismantled, or turned
into something new. At any one time, the liver holds some
10 percent of the body’s blood supply.

Liver lobule Ins and outs of the liver HEPATIC PORTAL VENULE
Blood arrives from two directions, then HEPATIC ARTERIOLE
The liver is made up of thousands of the liver outputs blood via the hepatic
tiny factories called lobules. Each of vein and bile through the bile duct. HEPATIC ARTERIOLE
these contains thousands of chemical HEPATIC PORTAL VENULE
processors called hepatocytes. These Blood from the intestines
do all the liver’s work, albeit supported
by Kupffer cells and stellate cells. Each Blood from the heart
lobule has a central outflowing vein and Blood to the heart
is six-sided, with each of its corners
supporting two incoming blood supplies Bile to the gallbaldder
and an outflowing duct for bile.

LIVER LOBULE

Lobule cut in half

DOUBLE BLOOD SUPPLY

An unusual fact about HEART
the liver is that it has two
blood supplies. Like all LIVER 1 Nutrients in
other organs, it receives Each corner of the lobule
oxygenated blood from receives nutrient-rich blood from a
the heart to give it energy, branch of the hepatic portal vein,
but it also receives blood which comes from the intestines;
from the intestines, this is called the hepatic portal
which it cleans, stores, venule. It also receives oxygen-rich
and processes. blood from a branch of the hepatic
INTESTINES artery, which comes from the heart;

this is called the hepatic arteriole.

154 155IN AND OUT

How the liver works

3 Nutrients out
After being processed, the blood is drawn up
through a central vein, which sends it away from the liver.
From there it travels to the heart, the lungs, back to the
heart, and finally to the kidneys, where toxins are flushed
away in the urine. Kupffer cell removes HOW FAST
bacteria, debris, and DOES THE
old red blood cells LIVER WORK?

INTERLOBULAR VEIN The liver filters around
3 pints (1.4 liters) of blood
every minute. It also makes

up to 2 pints (1 liter) of
bile every day.

Microcanal
carries bile to
the bile ducts

HEPATIC PORTAL VENULE
BILE DUCT
HEPATIC ARTERIOLE

CENTRAL VEIN

Rows and columns
of hepatocytes

Stellate cell is a Branches of the Branches of the hepatic 2 Nutrients processed
storehouse of hepatic portal arterioles interlace the Hepatocytes work around
vitamin A venules interlace
the entire lobule entire lobule

the clock storing, dismantling, and
reconstructing nutrients. They also
produce bile, a chemical used in the
breakdown of fat (see pp.144–45).
Bile is continually sent to the
HEPATIC PORTAL VEIN gallbladder for storage.

What the WHAT ELSE
liver does DOES THE LIVER DO?

The liver is perhaps best understood It produces blood clotting
as a factory—a processing plant with proteins, which ensure that
three main departments; processing,
manufacturing, and storage. Its raw we stop bleeding when
materials are the nutrients absorbed injured. People with
by the blood during digestion— unhealthy livers tend
but which department they go to to bleed easily.
depends on the body’s priorities.
Glucose from
carbohydrates
In a process called
gluconeogenesis, the
liver makes glucose
out of carbohydrates
when the body
is low on energy.

THE REGENERATING ORGAN Metabolizing fat Processing
Excess carbohydrates
Unlike other organs, which create scar tissue and proteins are The liver spends most of its
at sites of injury, the liver creates brand new converted into fatty time processing nutrients.
cells when it needs them. This is lucky, since it acids and released into This involves making sure
is constantly being bombarded by unhealthy, the bloodstream for that the right nutrients are
toxic chemicals. These chemicals—which energy. This becomes sent to the right parts of the
include some prescribed medications— vital when glucose body, and that back-ups are
frequently damage the liver, but it holds its is running out. provided when needed.
ground by regenerating itself. Incredibly, it Crucially, this also means
can lose 75 percent of its mass and still flushing out toxic subtances.
regrow completely—all in a matter of weeks.
Detoxifying
25% the blood
Pollutants, bacterial
toxins, and defensive
chemicals from plants
are turned into less
dangerous compounds,
then sent to the
kidneys to be flushed
out of the body.

Bile production 156 157IN AND OUT
Bile is constantly
being produced by What the liver does
the liver and sent to the Hormone production
gallbladder for storage. The liver secretes at least
It is made out of three hormones, making it a
hemoglobin, which key player in the endocrine
is released during the system (see pp.190–91). The
breakdown of old liver’s hormones stimulate
red blood cells. cell growth, encourage bone
marrow production, and aid
blood pressure control.

Manufacturing

The liver is a major Protein synthesis
manufacturing hub, turning The liver produces
simple nutrients into, among many proteins that are
other things, chemical then secreted into the
messengers (hormones), body blood. It does so
tissue components (proteins), particularly when
and a vital digestive fluid (bile). certain amino acids
Since it is always busy, the liver (the building blocks of
also produces another precious proteins) are missing
commodity—an enormous from the diet.

amount of heat.

THE LIVER
PERFORMS SOME
Storage Vitamins 500 CHEMICAL
The liver can store up FUNCTIONS
A great deal of stockpiling to 2 years’ worth of IN TOTAL
goes on in the liver, mainly vitamin A, which is vital
of vitamins, minerals, and to the immune system. LIVER DAMAGE
glycogen (the stored form of Vitamins B12, D, E,
and K are also stored
until needed.

glucose). This enables the Minerals The liver is the only organ that can
body to survive without food Two vital minerals are regenerate itself. However, repeated
for days and weeks on end, stored in the liver: iron, exposure to damaging agents, such
and ensures that any shortfall which carries oxygen as alcohol, a drug, or a virus, can
in dietary nutrients can through our bodies; and eventually injure the liver. This
quickly be corrected. copper, which keeps the happens when it is inundated by
immune system healthy. toxins and never gets a chance to
Copper is also used to regenerate. In this strung-out state,
make red blood cells. the liver is finally scarred—
a condition known
Glycogen as cirrhosis. A
Energy is stored as common cause
glycogen in the liver. of cirrhosis is
When the body runs drinking too
out of energy (see much alcohol.
pp.158–59), the liver
converts it to glucose
and releases it into
the bloodstream.

Energy balance Numerous sugar
molecules indicate
Most of the body’s cells use glucose or fatty acids
for energy. To maintain a regular supply of these, high blood sugar
the body alternates between absorbing energy level after meal
(by eating) and releasing it (after which we feel
hungry again). In ideal conditions, this cycle
repeats itself every few hours.

Filling the tanks Fatty acid molecule

Glucose and fatty acids enter our bodies through

the food we eat. As blood glucose levels rise, the Glucose molecule

pancreas releases the hormone insulin. This Fatty acids being stored in a fat cell
tells muscle and fat cells to absorb and store the

glucose and fatty acids as energy for the future.

3 Excess glucose stored
Most fatty acids are stored
in fat cells, which serve as resevoirs
of energy. These cells also absorb
excess glucose and convert it into
fatty acid molecules.

Excess glucose heading for ABSORB!
storage in a fat cell

Food rich in sugar 2 Muscle burns glucose
Muscle cells, among
others, convert glucose
into energy for contracting.
Muscle cells also absorb
fatty acids. They burn
the fatty acids when
glucose levels are low.

Glucose being absorbed ABSORB!
by a muscle cell

DOES FAT Fatty acid being
MAKE YOU FAT? absorbed by a muscle cell

Only when eaten with sugary 1 “Absorb!” signal sent
foods or carbohydrates. These After a meal, the
foods contain glucose, which pancreas detects high levels
of sugar in the blood. In
signals the body to store response, it releases insulin,
nutrients, and so which circulates in the blood.
put on weight. This readies the muscle and fat
cells to open and receive glucose,
which all cells use for energy.

PANCREAS

158 159IN AND OUT

Energy balance

Burning the fuel Sparse sugar
molecules indicate
As the body’s cells absorb nutrients, blood low blood sugar level
glucose levels start to fall. Unless more food
is digested, these levels drop to a point
where the body burns fat instead of glucose
for energy. Once again, this process is
organized by the pancreas.

Fatty acids being burned 3 Muscle cell
in a muscle cell burns fat
Here, a muscle cell
ENERGY SUPPLY AND DEMAND receives fatty acids
from a fat cell and
Food energy is measured in calories. A steak breaks them down
contains around 500 calories, as does a large for energy.
bag of potato chips or 10 apples. A person at
rest needs around 1,800 calories a day to BURN! Fatty acids released
maintain weight—more in or out tips the scales. into the bloodstream

WEIGHT MAINTAINED 2 Fat sent to
muscle
Glucagon also tells fat
cells to release their
stored fatty acids into
the bloodstream.
These fatty acids can
then be used as a
source of energy
by other cells.

BURN!

WEIGHT LOSS 1 “Burn!” signal sent
A few hours after
eating, specialized cells
in the pancreas detect a
drop in blood glucose
levels. The pancreas
releases the hormone
glucagon into the
bloodstream. This signals
the liver to release the
WEIGHT GAIN glucose it has stored

CALORIES IN CALORIES OUT in the form of glycogen
into the bloodstream
PANCREAS (see pp.154–55).

The sugar trap ARE CALORIES
BAD FOR YOU?

A calorie is the amount of

Calories are equal in terms of the amount of energy energy your body will gain from

they contain, but where they come from—fat, protein, eating the food that contains it,
or carbohydrate—determines how they are used by so no—we need energy to live!
the body. Some foods give us a steady source of energy;
others can take us on a hormone roller-coaster ride. But if you eat too many
calories, your body will

store the excess

Lingering insulin as fat.

Foods that are quickly turned into sugars cause a spike Rise and fall Glucose
in blood glucose levels (see p.158). Insulin spikes in response, The peak and crash of glucose Insulin
causing glucose levels to plummet. The sugar crash leaves and steady rise and fall of insulin
us tired and craving more sugar, while insulin lingers in levels in the blood is traced along
our blood and prevents us from burning fat. mealtimes during a morning.

1 8am breakfast 2 10:30am snack 3 1pm lunch
A carbohydrate-rich As blood glucose plummets and By lunchtime, a
breakfast—be it toasted lingering insulin inhibits fatty acid new sugar crash is
bread or cereal—gives release, we start to feel tired and upon us, which can
us a sugar rush and want a snack. Some sugary cookies tempt us to eat a
insulin levels rise. This raise blood glucose again, and high-carbohydrate
rush can be heightened insulin follows in response. lunch. And so the cycle
by the fruit juice we continues, with both
drink or the sugar we glucose and insulin
put in our coffee. levels spiking beyond
the healthy range.

8AM 10:30AM 1PM
Cell nucleus
Putting on the pounds Fatty acid being
FAT CELL absorbed
The sugar trap quickly leads to weight
gain, and being overweight can have Stored fatty acid
serious health implications. These include Storing fat
insulin sensitivity, insulin resistance, When we put on fat
type 2 diabetes (see pp.201), heart disease, we don’t increase the
some types of cancer, and stroke. To avoid number of fat cells in
obesity, it is vital to keep insulin levels our body. The same fat
low, and one way of doing that is through cells just get bigger as
a low-carbohydrate diet. they accumulate more
fat deposits.

160 161IN AND OUT

The sugar trap

HIGH-PROTEIN DIET Low-carbohydrate diets SUGAR IS NOW
THOUGHT TO BE
To cut out carbohydrates, A popular, if controversial, way out of the MORE ADDICTIVE
some diet promoters sugar trap is to limit our consumption THAN COCAINE
recommend getting calories of carbohydrates, which are otherwise
from protein and healthy fats broken down into sugars and stored as fat.
instead. You can follow a diet By doing so, we avoid the glucose-insulin
in phases designed to train roller coaster that ends in sugar cravings
your body to start burning fat and increased fat storage. Keeping
and rely less on carbohydrates.
sugar and insulin levels within a
healthy range enables fat, rather
than glucose, to be used as
an energy source.

MUSCLE CELL BRAIN CELL

Fatty acid being FAT CELL LIVER Ketone body being used
used for energy in for energy in a brain cell

a muscle cell Ketone body released
into bloodstream
Fatty acid released Ketone body
into bloodstream produced from fatty
Stored fatty acid acids in the liver

Releasing fatty acids Producing ketone bodies
When blood glucose is maintained at a Unlike other tissues, the brain can’t use fatty
healthy level, insulin levels remain low. acids as an energy source. So when blood
This allows the release of fatty acids from glucose is low, the liver begins to convert
fat cells—a process that is otherwise fatty acids into ketone bodies—molecules
inhibited by insulin. that provide energy for brain cells.

Feast or fast? VEGETABLES

Two of today’s most popular EGGS Hunted and
diets don’t involve calorie FRUIT gathered foods
counting at all. Palaeolithic Whole foods, such as
diets aim for an ancestral way fruits, vegetables, nuts,
of eating, removing the highly and seeds, are part of the
processed foods of today. paleolithic plan. The
Intermittent fasting, on the diet promotes eating
other hand, takes a more high-quality protein,
feast and fast approach, which includes eggs,
restricting when you eat wild fish, and pasture-fed
rather than what you eat. meat, which has higher
nutritional value than
Back to basics
grain-fed meat.
The theory behind paleolithic diets is that
our bodies have not evolved to consume the MEAT
highly processed, sugary, carbohydrate-rich
foods that are abundant in supermarkets NUTS AND SEEDS
today. The diet promotes foods that are
thought to have been available to our
hunter-gatherer ancestors, who lived before
the advent of farming, 10,000 years ago—
although the lifestyle doesn’t involve
reverting back to cave life. Dieters used to
getting their calcium from dairy foods need
to find calcium-rich alternatives, or they put
themselves in danger of calcium deficiency.

Intermittent fasting

The idea behind intermittent fasting is to take regular

breaks from eating, during which the body gets all its

energy from stored fat, but not for so long that it starts

to break down muscle protein for energy. There MONDAY TUESDAY WEDNESDAY THURSDAY

are two main intermittent fasting

methods; the 16:8 and the 5:2. 22 24 2

The 16:8 method 20 4 FRIDAY SATURDAY SUNDAY Fasting days
Followers of this regime eat during
an 8-hour period every day (say
noon to 8pm). The other 16 hours 18 6

you fast, but luckily a lot of this
time is spent sleeping, which
makes it more manageable. 16 8 The 5:2 method
This regime restricts your daily energy intake to about 500 calories
Key Eating Fasting 14 12 10 (about one meal) per day for two days of the week. You can eat as
much as you like (within reason) for the other five days of the week.

162 163IN AND OUT

Feast or fast?

GRAINS ONE-THIRD OF
THE WORLD’S
SUGARS ADULTS NOW PRODUCE,
THE ENZYME THAT
DIGESTS DAIRY SUGAR

Farmed and REFINED PROCESSED The glycemic index
processed foods FOODS
Sugar, processed food, The glycemic index (GI) is a measure of
grains, beans, alcohol, and how quickly carbohydrate-containing
dairy are excluded from foods increase glucose levels in the blood.
paleolithic diets because The lower a food’s GI value, the less it
they are products of affects blood sugar levels. An attraction
farming and industry. of paleolithic diets is that they focus on
However, many followers low GI foods.
do eat some dairy foods,
since many of us have Blood glucose level HIGH GI Blood glucose rockets
evolved a tolerance to up and down
milk (see pp.164–65),
unlike our ancestors. Blood glucose
rises steadily but
BEANS remains low

DAIRY LOW GI

Hours 1 2

Blood glucose levels
High GI foods rapidly increase blood sugar levels,
but this is followed by a rapid decrease, leaving us
feeling hungry. Low GI foods gradually increase
blood sugar levels, leaving us feeling full for longer.

Natural fat-burning FEASTED BRAIN HEALTH
STATE
Exercising when your body is SUGAR FASTED There is evidence that fasting improves
naturally burning fat may give STATE
your workout more punch. FAT FAT brain health. Intermittent fasting in
A run before breakfast, for
example, takes advantage MUSCLE MUSCLE particular puts the neurons under mild
of the fact that your body Evening Morning
is already burning fat after The body can run Once glucose stress—much like our muscles are stressed
fasting all night. A run in the on the glucose is used up, the
evening, however, is more from a meal for body starts to by exercise. This stress causes the release of
likely to be fueled by blood about 3–5 hours. burn fat stores.
glucose supplied by the day’s chemicals that help in
food. For this reason, morning
exercise is generally more the growth and
effective for losing weight.
maintenance FASTED BRAIN

of neurons.

Neuron

Digestive problems

Digestive problems can range from temporary discomfort after
eating to life-long persistent disorders. In most cases, the
treatment is simply to avoid the foods that cause the symptoms.

Lactose intolerance Lactose Lactase Lactose digested
enzyme by lactase
Many adults lack the enzyme lactase, 2
which is needed to break down lactose,
the sugar found in milk. All healthy babies Lactase breaks lactose into two smaller
have it, but most of us stop producing sugars—galactose and glucose.
this enzyme after weaning. Only about
35 percent of the world’s population SMALL INTESTINE Glucose
have acquired a mutation that allows
them to produce lactase into adulthood.

1 Lactose in
small intestine
When the cells that line the
WHO ISN’T walls of the small intestine Galactose
LACTOSE-INTOLERANT? encounter the sugar lactose,
they start to produce the
digestive enzyme lactase.
Countries that have a long
Galactose and
history of dairy farming tend 3 glucose absorbed
to have populations that have
These two smaller sugars
are then absorbed into
adapted to drinking milk into the bloodstream by the
adulthood. Most of these small intestine.

countries are in Europe. Bacterial fermentation
Bacteria living in the large
2

intestine (see pp.148–49) ferment
the lactose, producing gas and
acids in the process.

3 Disruption in the bowl Gas and acids released
The gas produced by by bacteria
fermentation causes bloating
and discomfort, while the acids
draw water into the bowel,
leading to diarrhea.

LARGE INTESTINE Undigested
lactose enters
the large
intestine

1 Undigested lactose
If lactase isn’t present, then
lactose can’t be absorbed and instead
passes into the large intestine.

Bacteria fermenting lactose

164 165IN AND OUT

Digestive problems

BRINGING IT UP Irritable bowel syndrome

One way the body avoids digestive IBS is a long-term condition that can cause stomach cramps,
problems is by vomiting. When
we eat something rotten or bloating, diarrhea, and constipation. It is poorly understood, but
poisonous, the stomach, the
diaphragm, and the abdominal seems to be triggered by stress, lifestyle, and certain types of food.
muscles all contract, forcing
the food back up through the Carbohydrates Bacteria Spasm
esophagus and out through Acids and gas
the mouth.

SMALL INTESTINE Water

Diaphragm LARGE INTESTINE

1 Bacterial fermentation 2 Bowel spasms
Carbohydrates that are poorly IBS causes bowel spasms, which
absorbed may increase the amount of water
in the intestinal tract. Once in the large can block the waste and gas from passing
through. Alternatively, it can cause the waste
intestine, these carbohydrates are fermented to move too quickly, preventing water
by bacteria, producing acids and gas.
reabsorption and causing diarrhea.

Gluten intolerance Small intestine INTESTINAL VILLI
before celiac
Many people experience abdominal disease
pain, fatigue, headaches, and even
numbness of the limbs when they
eat gluten—a protein found in
grains such as wheat, barley, and
rye. These symptoms are indicators
of various gluten-related disorders,
ranging from gluten sensitivity to
celiac disease.

Small intestine
after celiac
disease

RYE BREAD BEER PASTA

Gluten sensitivity Celiac disease
Lethargy, mental fatigue, cramps, and diarrhea are all Celiac disease is a serious genetic disorder that causes the body’s
symptoms of gluten sensitivity, which is only cured by immune system to attack itself when it encounters gluten. This
avoiding all gluten products—including rye bread, beer, immune response causes damage to the lining of the small intestine,
and pasta. Gluten sensitivity does not damage the inhibiting the absorption of nutrients. Left unchecked, it can totally
intestines like celiac disease does. destroy the small intestine’s little fingerlike projections, or villi.



FIT AND
HEALTHY

Body battleground

Humans are attacked on a daily basis by a host of marauding

invaders, for whom the body is an ideal place to feed and reproduce.

Ranged against them are the body’s defense forces. Any harmful

microbe, or pathogen, that breaks through the outer barriers is

met with a quick, local response at the site of the infection.

If this doesn’t work, a second team is called into action. Complement proteins
As many as 30 different proteins
Invaders circulate in the blood, ramping up
the immune response by marking
Bacteria and viruses are the major pathogens for destruction or
causes of disease in humans. causing them to burst.

Parasitic animals, fungi, and toxins

can also prompt the immune system

into action. All these microbes are

constantly adapting and evolving

to find new ways to avoid

detection and destruction

by the immune system. Dendritic cells
Fungi These phagocytes (microbe eaters)
Most are not engulf pathogens and play an
dangerous, but important role in spurring B and
some can be T cells into action.
harmful to health.

Parasitic animals
Live on or inside humans
and may carry other
pathogens into their host.

Bacteria Barricades
Tiny, single-celled organisms taken
into the body by eating, breathing, Epithelial cells are the body’s main
or through breaks in the skin. physical defence against pathogens.
Viruses The cells are tightly packed together
Viruses need other living
cells to multiply and can lie to prevent anything penetrating
dormant inside their host’s them. They also secrete liquids
cells for long periods. that act as a further barrier

Toxins SecretionsEPSIETCHREELTIUIOMNS against pathogens.
These are substances Fluids such as mucus,
capable of causing disease tears, oils, saliva, and stomach Epithelium
or a reaction that could acid can trap pathogens or break Epithelial cells form the skin and
prove deadly to the them down with enzymes. membranes that line all of the body’s
human body. openings, such as the mouth, nose,

esophagus, and bladder.

Frontline troops 168 169FIT AND HEALTHY
Body battleground
Pathogens that break through the barriers are met
with an immediate response known as the innate HOW MANY
immune system. This is a group of cells and proteins INFECTIOUS DISEASES
that respond to alarm signals from damaged or CAN THE IMMUNE SYSTEM
infection-stressed cells. Some target and mark
invading organisms for destruction, while others RESPOND TO?
(phagocytes) eat up the pathogens. It is thought that B cells alone
can produce enough different
Granulocytes
There are three types of granulocyte antibodies to deal with
that eat invading organisms and 1 billion different types
secrete chemicals that break down
the cell walls of bacteria. of pathogen.

Macrophages Killer cavalry
Their name means “big
eater” and that’s what they If the front-line response hasn’t contained the
do—surround and engulf infection within 12 hours, the adaptive immune
pathogens and dead cells, system swings into action. This system remembers
and notify other immune previous exposures to the pathogen to launch
system cells of a problem. a specific, targeted response.

Mast cells B cells
Mast cells release chemical alarms that B cells are a special
alert other immune cells to invaders. type of cell that can
They are also responsible for most be trained to produce
allergic and inflammatory reactions. antibodies in response
to the presence of a
Natural killer (NK) cells particular pathogen.
NK cells don’t attack pathogens They can multiply
directly, but instead attack cells rapidly to increase
that have become infected, the response.
causing them to undergo
apoptosis (see p.15). Antibodies
Antibodies are Y-shaped proteins
produced by B cells. They stick to the
surface of invaders and mark them out for
destruction by phagocytes.

T cells
T cells are another type of trainable
cell that directly attack infected or
cancerous cells and prompt phagocytes
to eat pathogens. Some T cells also
stimulate B cells to produce antibodies.

Friend or foe?

The immune system has to distinguish the harmful
pathogens that invade our body from the body’s own cells
and friendly microbes—in other words, recognize friends
and foes. The body puts its most potent immune cells—
B and T cells—through safety checks to prevent them
from attacking us.

Self and nonself Starting point
Both B cells (which
Every cell in the body is coated in groups of molecules produce antibodies to kill
that are unique to each individual. The main function invaders, see pp.178–79)
of these molecules is to display fragments of protein and T cells (which kill
made by the body and friendly microbes so that the invaders directly, see
immune system learns to tolerate them and recognize pp.180–81) start life
them as “self.” as stem cells in the
bone marrow.

Antigens, specific Antigen of a 1 Bone marrow
to each person, different shape. B cells mature
coat this body cell All antigens have and are tested in the
BODY CELL a characteristic bone marrow. Any that
shape known as bond with self proteins
in the marrow are
an epitope deactivated and killed
FOREIGN by apoptosis (see p.15).
CELL

BONE

Self tolerance Nonself markers B cell
All body cells carry “self” surface Foreign cells carry their own receptor
marker proteins, or antigens, surface marker proteins, which
allowing them to live in harmony trigger an immune response. B CELL
with other cells. If the immune Even the proteins you eat may
system loses its ability to be identified as foreign unless
recognize self markers, it can they are broken down first by
lead to autoimmune diseases. the digestive system.

TRANSPLANTS 2 B cell
If a B cell passes the self
Compatibility is examined before test, it is released from the bone
an organ transplant is given, marrow into the lymphatic
because if it is not a close enough system. This is a network of
match the recipient’s immune vessels that runs parallel to
system may attack the donated blood vessels and carries
tissue and start to destroy it. immune cells around the body.
Transplant recipients may have to
take immunosuppressant drugs ONLY 2 PERCENT OF T CELLS
to try to minimize this complication. PASS THEIR TRAINING—THE
REST ARE REJECTED BECAUSE
THEY MIGHT ATTACK US!

170 171FIT AND HEALTHY
Friend or foe?

DO IDENTICAL 1 Thymus Compatibility
TWINS HAVE THE SAME T cells move to the thymus
(a specialized lymph gland found Compatibility tests look at
IMMUNE SYSTEM? in front of the heart), where they the likelihood of a recipient’s
mature. Their receptors are tested immune system attacking donated
No. Immunity is shaped to make sure they don’t form tissue. Red blood cells carry extra
by what each person is strong bonds with self proteins. self markers called blood groups.
exposed to in life, so it Two of them, the ABO and Rhesus
groups, prompt an immune reaction
is very individual. to donated blood from a different
group. People with blood group O,
Tested to destruction THYMUS for example, will launch a response
T cell to blood from any other group
When the T cells and B cells of the immune receptor because they carry both anti-A
system are forming, they generate random and anti-B antibodies.
receptors and put them on their surface.
Because this process is random, it is possible Blood group A
that these receptors might bind strongly with The red blood cells
“self,” or friendly, antigens. Therefore, these display A antigens
cells go through vigorous testing before being on their surface and
released into the body. Those that bind to the antibodies to B
body’s own proteins are destroyed. antigens are found
in the blood plasma.
Bean-shaped lymph nodes, many of
which are in the armpits and groin, are A antigen Anti-B
reservoirs for B cells, T cells, and other antibody
immune cells
Blood group B
T CELL The red blood cells
display B antigens
on their surface
and the plasma
has antibodies
to A antigens.

B antigen
Anti-A antibody

LYMPH Blood group AB
NODE The red blood cells
display both A and B
T cells 2 T cell antigens on their
B cells Mature T cells are surface, but there are
released into the lymph no antibodies in the
Other immune and blood. Regulatory blood plasma.
cells T cells are a subtype that
provide an extra check B antigen
on the self-tolerance of A antigen
other T cells. Blood group O Anti-B
The red blood cells antibody
Destination display neither A nor
If invaders are present in the body’s circulation, B antigens on their
eventually, they have to pass the lymph nodes, where surface, but the blood
B cells and T cells lie in wait. The cells activate when they plasma carries both
encounter an alien antigen that matches their receptors. types of antibody.

Anti-A antibody

Germs are us Microbes are NOSE
carried through the OUTH
The microbes that live peacefully in and on our
body are a big part of staying healthy. These air, adding to the Bacteria migrate
microbes—mostly bacteria and fungi—have resident population into the mammary
benefits that range from keeping our skin glands from the
healthy by eating dead cells to helping of microbes living skin and can be
us digest food. in the nose passed to a baby
in milk
At least 600 M
different species The forearm has
more species than
of microbe live any other area of
in the mouth the skin because
of its frequent
Your local neighborhood contact with
objects
Just as towns may be built around a particular Friendly microbes produce
chemicals that suppress the
resource, microbes collect around specific growth of harmful pathogens
in the genital regions of men
areas of the body. On the skin, for example, and women

they are most abundant around sweat glands

and hair follicles, where they are more likely to It’s bacteria that ARMPIT MAMMARY
find the nutrients they need to survive. The put the O into B.O.— GLAND
conditions in each area of the body—moist,
dry, acidic—also determines which species they feed on sweat
and turn it smelly

can live there. Skin has the greatest diversity

of microbes. Those on the oily back are

different to those on the drier front. The navel is home
to unusual species
that enjoy the dry,

oilless habitat

AM I A HABITAT The gut contains a BELLY BUTTON FOREARM
FOR RARE WILDLIFE? relatively low diversity GUT

Quite possibly. In a study of of species, but by far
the greatest in quantity

90 belly buttons, researchers

found 1,400 species of bacteria

that had never been found

on human bodies before,

some of them new The community
to science. here changes

with everything HAND GENITALS
we touch and
every time
we wash

What’s living where The skin hosts large
The graphic shows the main types of organism found in or on regions quantities of microbes,
of the body. Large icons indicate species that comprise more than but most are harmless
50 percent of the population. Naturally moist hotspots
are dominated by species
BacteriaBacteriodetesMalassezia Living on bacteria that thrive in warm, wet
FungiProteobacteriaCandida Living in our cells conditions
Staphylococcaceae Aspergillus
VirusesFirmicutesOther fungi MICROBIAL
Corynebacteria CELLS
Actinobacteria SKIN OUTNUMBER
HUMAN CELLS
Beneficial microbes BACK OF BY 10 TO 1
KNEE
Science is still revealing the Feet are dominated
different species that live within Bacterium Chemical released by fungi—around
the human microbiome, let alone Epithelial cell by bacteria prompts 100 species thrive
their many benefits. Some benefits T cell into action in their cool and
are direct, such as eating dead damp environment
skin and changing the chemical T cell releases Immune cells no longer
environment to prevent harmful inhibitors trigger inflammation
microbes from growing. Others
are less obvious, such as the Happy bacteria = healthy gut SOLES FIT AND HEALTHY
calming effect some gut bacteria Eating the right foods helps good bacteria to OF FEET
have on the immune system by thrive. They produce chemicals that damp down Germs are 172 173us
reducing inflammation. Medicines, inflammation in the gut, which would allow bad
such as antibiotics, can also have bacteria to penetrate the epithelial wall.
devastating effects, wiping out the
good microbes as well as the bad.

ARE WE TOO CLEAN? Birthday presents

It’s possible that our obsession with Babies start to build their own microbiome at birth by picking up
antibacterial cleansers is taking its toll some of their mother’s microbes as they pass through the birth canal.
on friendly microbes. Some studies These bacteria start to produce chemicals that encourage other
have shown that excessive handwashing beneficial microbes to colonize. Many factors can influence the
can lead to the growth of more harmful development of the microbiome; different
microbes—but this is debatable, since species will colonize depending on how
other studies have shown the opposite. the baby is delivered (cesarian babies
have different bacteria), whether
a baby is breast fed, and who
it has contact with.

Damage limitation THERE ARE
375,000
When a physical barrier such as skin is damaged, the immune IMMUNE CELLS
system works quickly to repair it and defend the body against IN EVERY DROP
infection. The local immune cells swing into action against OF BLOOD
the first invaders, calling for more specialist reinforcements
if there are more than they can cope with.

Area becomes red Bacteria
and inflamed Pus

EPIDERMIS

DERMIS 1 Sounding the alarm
Granulocyte Cells damaged by the wound release
chemicals called cytokines, which trigger
a number of changes in nearby cells and
blood vessels. Blood flow is increased to
the area, making it red and inflamed.

Cytokines

Macrophage
eats bacteria

Blood vessel 2 On the attack
Mast cell Macrophages and granulocytes
are attracted to the wound, engulfing
any bacteria that have entered the
area and clearing away dead cells. Bacteria
Immune cells
Histamines released Blood plasma leaks squeeze
onto capillary out of capillary and through gaps
makes area swell

B cell 3 Opening the walls
The chemicals released by the damaged
Granulocyte Blood cell cells and local immune cells make the capillary

walls more permeable, which allows immune
cells in the blood to pass through more easily.

174 175

Call to arms WHY DO CUTS
TAKE LONGER TO HEAL
A number of immune cells, such as macrophages, mast cells, and WHEN WE’RE OLDER?
granulocytes, live in the dermis. If the skin is cut, mast cells detect
the injured cells and release histamines that cause nearby blood Blood vessels can become
vessels to swell. This increases blood flow to the area, making the more fragile as you get older,
wound feel hot, but it also brings other immune cells to the site quickly. which makes it more difficult
The formation of pus is an indication that bacteria have gotten into
the wound—pus is the accumulated remains of dead immune cells. to deliver immune cells
to the wound.

5 Backup team MAGGOT THERAPY
T cells called into the area
release chemicals that kill the If a wound in the skin isn’t
bacteria and encourage the healing properly or responding to
macrophages to eat them. conventional treatment, maggots
Plasma B cell may be the answer. These little fly
larvae are particularly precise in
Antibody digesting dead cells while leaving
the healthy cells alone. As they eat,
4 Antibody brigade the maggots secrete antimicrobial
If the body needs more chemicals that protect the maggot
help fighting the infection, B cells but which are also effective at
travel to the site of infection and killing bacteria, even those resistant
start producing antibodies that to antibiotics. These secretions
bind to the pathogens, marking also help inhibit inflammation of
them for destruction. the wound, contributing to the
healing process.
B cell

T cell

T cell B cell FLY LARVAE

Bacteria Viruses
Bacteria are microscopic organisms that Viruses are the smallest and simplest
are usually harmless, but can sometimes organisms of all, made up of only their
cause disease. Bacteria are responsible genetic material (DNA or RNA) in a protein
for some globally important diseases, coat. Unlike other pathogens, viruses
such as tuberculosis and pneumonia. need the host’s cells to live and replicate.

Flagellum Capsid RNA
(protein (genetic
coat) material)

SALMONELLA VIBRIO ADENOVIRUS LYSSAVIRUS
(food poisoning) (cholera) (tonsilitis, conjunctivitis) (rabies)

Surface protein

Envelope

Capsid

TREPONEMA STREPTOCOCCUS LENTIVIRUS HERPESVIRUS
(syphilis, yaws) (pneumonia, bronchitis) (HIV/AIDS) (hepatitis B, cold sores)

Antibiotics Vaccination
Commonly used for bacterial infections, The best way of preventing the spread
antibiotics break down the walls of of viral infections is through vaccination.
bacteria or interrupt their growth. A vaccine primes the immune system
However, they can’t distinguish the to recognize the virus and launch an
good bacteria from the bad. immediate attack (see pp.184–85).

Infectious Unwanted visitors
diseases
Organisms that live off the body’s cells or
Bacteria, viruses, parasites, and fungi live in tissues are called parasites. There are five
and on us all the time. Most are harmless, but main types: bacteria, viruses, fungi, and
certain species are pathogens—they can cause animals and protozoans. When they find
an illness if a change in conditions allows them favorable conditions they multiply rapidly
to thrive. Other diseases are passed to us from but may produce harmful products or
people or animals. A fever is almost always effects that make us feel sick, prompting
a sign that an infection is taking hold. our immune system to swing into action.

A SINGLE SNEEZE
CONTAINS 100,000
GERMS

176 177FIT AND HEALTHY
Infectious diseases

Animals and protozoans Fungi
We also face attacks from tiny animals Fungi are always present in and
and single-celled organisms called on the body, but sometimes
protozoans that live on or inside the pathogenic species take hold and
body. Some are large enough to see with cause diseases such as athlete’s
the naked eye, such as worms, or they foot or thrush.
may be microscopic, such as Giardia, the
protozoan that causes diarrhea.

Two COCCIDIOIDES CRYPTOCOCCUS
flagellae (valley fever) (lung or meningial
GIARDIA Arthrospores cryptococcosis)
(diarrhea)
NEMATODE Spore-bearing
(Guinea worm, Nucleus body
threadworm)
TRICHOMONAS
Flagellum (urethritis, vaginitis)

ASPERGILLUS
(lung infections)

Prevention Antifungal medications
The best strategy against this type Fungal infections are treated
of infection is to avoid activities and according to whether they are
areas where there are known health internal or external. The active
hazards, be wary of unsafe food ingredients either attack the fungus
and water sources, and take directly by breaking down its cell
recommended precautionary drugs. walls, or prevent it from growing.

How diseases spread INFECTED Animal/insect HEALTHY
PERSON Direct contact PERSON
There are many infectious diseases
but some affect relatively few Air
individuals and are local to a small Indirect
area—only diseases that spread contact
easily by person-to-person contact Food
are said to be contagious. Many
pathogens travel between people
by less direct means—through the
air or in water, on objects someone
has touched, or in contaminated
food. Zoonotic diseases are animal
infections that can spread to
humans, usually through bites.

Looking for trouble Helper T cell
stimulates B cell
If an infection becomes too great for the initial immune by releasing
system to deal with, a second, more targeted force springs chemicals
into action. B cells learn to recognize harmful microbes
that have attacked the body in the past. They can then T CELL
produce antibodies that will surround the pathogen
and tag it for destruction by other immune cells.

Macrophage Macrophage puts the
ingests microbe antigens on its outer
membrane, then presents
M
them to a B cell and
a helper T cell

ACR OP HA B CELL

GE

Foreign microbe B cell duplicates to produce
with antigens two types of clone—memory

B cells and plasma cells

1 Presenting antigens Microbe is digested and 2 Helping hand
When a macrophage ingests is broken into pieces The B cell starts to get ready when it binds to
a pathogenic microbe, it breaks it up an antigen, but it isn’t fully activated until a helper
and puts the microbe’s antigens (surface T cell recognizes and binds to that same antigen.
proteins) onto its cell wall. This is known The helper cell then releases chemicals that
as an antigen-presenting cell. prompt the B cell to produce antibodies.

Activating antibodies TESTING FOR ANTIBODIES

B cells are a type of white blood cell that Blood tests show the levels of immunoglobulins (another
constantly patrol the blood vessels or lie waiting name for antibodies) present during infections. IgM is a large
in the lymph nodes (see pp.170–171). When a antibody that the body produces at the first sign of infection,
B cell encounters an antigen it recognizes, it but it quickly disappears. IgG is a more specific, lifelong
becomes primed and ready to clone itself. antibody that is produced during a later infection. A high IgM
This can happen only when another cell of the value shows you have a current infection, whereas IgG simply
immune system, the helper T cell, recognizes means you have been infected by a pathogen in the past.
and binds to that same antigen, triggering the
B cell to clone itself and release antibodies. The IgM complex has five
times as many antibodies
A SINGLE B CELL MAY
HAVE UP TO 100,000 available to deal with
ANTIBODIES ON ITS pathogens than IgG
OUTER SURFACE
IgG IgM

FIT AND HEALTHY

178 179Looking for trouble

P ME MORY B CELL LL

Memory B cell Rounding them up
remembers the antigen Antibodies can clump
for future invasions microbes together,
reducing the number
LASMA CE Clump of of infectious units to
microbes be dealt with.
Coated Tasty morsels
microbe Coating the microbe
with antibodies attracts
macrophages and
encourages them to eat.

EPITHELIUM Nowhere to land
Antibodies prevent the
microbes from sticking
to other cells so they
can’t invade and multiply
themselves.

3 Antibody release 4 Neutralizing pathogens
The B cell clones itself. Some of these The antibodies bind to the
clones become memory cells, but most become invading microbes, neutralizing them
plasma cells, which produce antibodies that and marking them for destruction
are specific to the invader’s antigens. These by other immune cells.
antibodies are then released into the blood.

Rh-
mother

Second Rh+
fetus

First Rh+ Anti-Rh+
fetus antibodies
are made
FIRST PREGNANCY Rh+ BETWEEN PREGNANCIES SECOND PREGNANCY Mother’s
blood cells antibodies
attack baby’s
blood cells

Rhesus babies Not-so-safe haven
Antibodies produced in response to
The Rhesus factor (Rh) is a protein on the surface of red blood cells— the baby’s blood mingling with the
people who have it are called Rh+. When an Rh− mother is exposed mother’s during birth will prompt her
to the blood of her Rh+ fetus (from the father’s Rh+ gene) during birth, immune system to attack the next Rh+
she makes antibodies against it. These antibodies may attack future child she conceives. This is because
Rh+ embryos, but an injection of anti-Rh+ antibodies early in the her antibodies can actually cross
pregnancy usually reduces this danger. the placenta into the baby’s blood.

Assassination squad
The immune system can prime some cells to go out into the body and
attack the invasion one-on-one. These are known as T cells. They hunt
down infected and abnormal cells, then destroy them.

Keeping control REGULATOR T CELLS ARE
VITAL IN PREVENTING
T cells are a type of white blood cell that play a key AUTOIMMUNE
role in dealing with infections. Circulating in the blood DISEASES
and lymph, the T cells look for foreign antigens on the
surface of body cells. These characteristic proteins T cell activated
show that the cells have been invaded by a microbe
or that they have developed a dangerous abnormality.
T cells also marshal the actions of other immune cells
and prime B cells to produce antibodies.

Foreign microbe Macrophage
with antigens presents antigens

to a T cell

Microbe
is digested

1 Activating T cells
A macrophage engulfs a pathogen
and breaks it down. It then incorporates
parts of the pathogen (its antigens) into its
membrane, displaying them on its surface.
Macrophage When a T cell recognizes the antigen it binds
ingests microbe to it and becomes activated.

Cornering cancer NO RESPONSE VACCINE INJECTED

Immunotherapy is a treatment Cancer cell Vaccine
designed to help the immune
system fight cancer. There are T cell
many different ways of doing
this. All of them either make the
cancer cells more easily identified
by the immune system or boost
the immune system by multiplying
cells or cytokines in the lab before
injecting them back into the patient.

Cancer vaccines 1 No threat 2 Identifying the adversary
Vaccines form one of the methods Cancer is the uncontrolled division Cancerous cells have “self” antigens
of immunotherapy being developed. of abnormal cells. The immune system may on their surface but also produce their own
They prompt the immune system to not recognize these cells as abnormal antigens. A vaccine is designed to match the
target only cancerous cells.

because they are the body’s own cells. shape of the cancer antigen.

180 181FIT AND HEALTHY
Assassination squad

2 T cells in action Killer T cell
Once the T cell has been releases cytokines
activated it begins to clone itself.
These clones then become one T cell moves away to
of four different types of cells seek another target
in the T cell family.
Killers Pores
These are the muscle open and
of the family, actively cell starts
seeking and killing to swell
infected cells.
Helpers
Spur the B cells and
other T cells into action.
Also encourage other
immune cells to eat
invaders.

Regulators INFECTED CELL
Calm the immune
system and are critical Killer T cells CELL BURSTS
in recognizing friendly The killer T cell recognizes and binds
microbes. 3
Memory
Remember microbes to an antigen displayed by an infected cell.
that have attacked the It releases chemicals that open up pores in the
body in the past. membrane of the infected cell and cause it to
break down and be eaten by macrophages.

T CELLS PRIMED CANCER DESTROYED

T cell releases WHAT IS A
cytokines T-CELL COUNT?

Healthy cell This is a measure of the
number of T cells circulating
Healthy in your blood. Both higher
cell
and lower than normal
T-cell counts can be an

indicator of disease.

Trained to kill T cell now Targeted attack Cancer cell
The vaccine trains the T cells recognizes The T cells can now target disintegrates
cancer cell

3 4

to recognize and bind to the antigens and attack the cancerous cells,
being displayed by the cancerous distinguishing them from healthy
cells in the body. cells of the same type.

Colds and flu

The reason why you are assailed by colds again and
again is because the virus mutates each time, and
your immune system fails to recognize it when
you catch your next cold. Usually,
the symptoms you experience
are your immune system
reacting to the virus,
not directly caused
by the virus itself.

Cold or flu? Common cold Shared symptoms Flu
Frequent sneezing, a Both the common Influenza is caused
Many of the symptoms of colds mild to moderate fever, cold and flu are by virus types A, B,
and flu are similar, and that low energy, and fatigue classed as upper and C. Having the
makes them hard to are all symptoms of the respiratory tract flu may induce a
differentiate. There are common infections. Either moderate to high fever
many viruses that cause cold. There illness may cause and constant fatigue. It
the common cold, and the are more than 100 a runny nose, sore is generally caught in
influenza virus is caused viruses responsible throat, cough, the winter months and
by three virus subtypes. for the common cold, headache, an can develop into more
Generally, the symptoms which can be aching body, serious conditions
of a cold are much milder caught at any shaking, and chills. such as pneumonia.
than those of the flu. time of year.

How a virus invades a cell Virus Virus Nucleic acid
(DNA or RNA)
Viruses need to invade healthy cells
to replicate. A virus tricks the cell Cell Nucleus of cell
into making copies of it. A cell’s
nucleus is where instructions to 1 The virus attaches 2 Substances in the cell 3 Nucleic acid from
make body proteins are stored. itself to your cell and begin to strip the the virus is released,
Viruses are surrounded by a coat the cell engulfs the virus. virus’s outer coat of protein. ready to be replicated.
of protein, and the virus can hijack
cells to make these viral proteins Nucleic acid Virus has
instead of normal body proteins. enters cell nucleus been replicated
Once they have replicated, the
virus will then enter other cells in Damaged
your body and the cycle continues. cell
This process is the same for both
the common cold and the flu. 4 Your cell replicates the 5 The cell ignores its 6 The virus is released
viral nucleic acid under own chemical needs from the host cell.
This can destroy the cell, and
the false pretense that it is and switches to making new the viruses go on to invade
viral nucleic acids, which
your own DNA. become copies of the virus. other cells.

182 183FIT AND HEALTHY
Colds and flu

A change in your mood MOODINESS HEADACHES FEVER
can be brought about It is thought the chemical cocktail released
by the annoyance of RUNNY N during an immune response increases pain A rise in body temperature is
having a runny nose sensitivity in the brain, causing headaches another way that our immune
and lack of sleep system combats infection. The
Dilation (widening) of blood body’s temperature regulation
The inflammation of your vessels and mucus buildup in system is reset to a higher level
sinuses stimulates mucus the nose and sinuses leads to a to speed up immune reactions
production in your nasal congested feeling in the head required to fight
cavity. The increased mucus SINUSES infection. As long
OSE as a fever is mild,
forms a barrier against there is no cause
incoming viral cells for worry – but
persistent fevers
The release of histamines should be
triggers sneezing, which monitored.
helps clear the viral cells
Immune response
out of your nose. However,
this also leads to the The invasion of viral particles into
spread of the virus the epithelial cells found within the
mouth or nose triggers an immune
SORE THROAT response. Symptoms of the common
cold or flu are a product of this
immune response. The affected
epithelial cells release a cocktail
of chemicals including histamines,
which causes an inflammation
of your sinuses, and cytokines,
which command cells involved in
your immune response.

A reflex to clear your COUGHING An inflammation of the
airways of mucus buildup, epithelial cells in the
coughing may be triggered by throat is one of the first
inflamed cells and some of the symptoms of colds
chemicals released as part of and flu, so it is often
understood as a warning
the immune response sign for when you are
EXHAUSTION “coming down with
All of these symptoms will something”
interrupt your sleeping
pattern. Cytokines CHILLS
exacerbate the Shivering raises your body
feeling of exhaustion, temperature - rapid contractions
forcing your body from your muscles generates heat,
to slow down to helping to speed up immune reactions
fight the virus.
that fight off infection.

Vaccine action

One of the most effective ways of preventing the spread
of infectious disease is to prime the immune system through
vaccination. A vaccine trains the immune system to launch
a fast and furious attack on a pathogen.

Herd immunity Key

Vaccinating a significant portion (around 80 percent) Not immunized but Immunized and Not immunized,
of a population can help provide immunity even to those still healthy healthy sick and contagious
who have not been vaccinated. When the disease is
passed to vaccinated individuals, their primed immune
system destroys it, preventing it from spreading further.
This can help protect people who can’t be vaccinated due
to age or illness. Widespread vaccination can eliminate
diseases entirely, such as smallpox.

Safety first NO ONE IMMUNIZED CONTAGIOUS DISEASE SPREADS
Contagious diseases can be contained SOME OF THE POPULATION THROUGH THE POPULATION
if a sufficient number of people are
vaccinated. Vaccination also helps GETS IMMUNIZED CONTAGIOUS DISEASE SPREADS
people who have an existing medical THROUGH SOME OF THE POPULATION
condition that may be worsened by the
effects of the disease.

TO VACCINATE OR NOT?

Controversy exists over the use of
vaccines. Fears over possible side
effects have led some parents to
refuse to have their children
vaccinated, which has resulted in
outbreaks of preventable diseases,
such as measles and pertussis.
If only a small portion of the
population is vaccinated, herd
immunity breaks down.

MOST OF THE POPULATION SPREAD OF CONTAGIOUS
GETS IMMUNIZED DISEASE IS CONTAINED

184 185FIT AND HEALTHY
Vaccine action

Types of vaccines WHY DO VACCINES
MAKE YOU FEEL ILL?
Each vaccine is developed for a specific
pathogen and is designed to kickstart Vaccinations stimulate an
the immune system. This is done by immune response, which can
injecting a harmless version of the produce symptoms in some
pathogen that the immune system
will remember if attacked by the real people—but it means the
pathogen. This can be difficult—killing vaccine is doing what
the pathogen may make it safe, but the it’s supposed to.
vaccine may not produce an immune
response. There are also some diseases Inactivated
that progress too quickly for the immune’s The pathogen is killed using
memory system to respond in time, so heat, radiation, or chemicals.
booster immunizations are given to keep Used for influenza, cholera,
reminding the immune system. and bubonic plague vaccines.

Related microbe Alive, but not dangerous
A pathogen that causes disease The pathogen is kept alive
in another species, but few or but the parts that make it
harmful are removed or
no symptoms in humans, is disabled. Used for measles,
sometimes used. For example,
tuberculosis vaccine is made from rubella, and mumps
a bacterium that infects cattle. vaccines.

ORIGINAL DISEASE-
CAUSING PATHOGEN

DNA Tame toxins Pieces of pathogen
DNA from the pathogen is Toxic compounds released by the Fragments of the pathogen,
injected into the body, whose pathogen, which are responsible such as proteins on the surface
own cells take up this DNA and for the illness, are deactivated using of the cell, are used instead
start to produce proteins from heat, radiation, or chemicals. Used for of the whole pathogen. Used
the pathogen, which triggers an tetanus and diphtheria vaccines. for vaccines against hepatitis B
immune response. Used for
Japanese encephalitis vaccine. and human papilloma
virus (HPV).

Immune problems

Sometimes the immune system is too reactive— ANAPHYLACTIC SHOCK
launching attacks on things that aren’t harmful
and even attacking the body’s own cells. Allergies, Sometimes the immune system
hay fever, asthma, and eczema are all caused by launches an extreme panic attack
an oversensitive immune system. Alternatively, when it encounters an allergen
the immune system may not be reactive enough, such as a sting or a nut. Symptoms
leaving the body vulnerable to infection. include itchy eyes or face, followed
quickly by extreme swelling in the
ARE FOOD ALLERGIES Cartilage erodes face, hives, and difficulty swallowing
AN IMMUNE RESPONSE? and breathing. This is a medical
Macrophage emergency that needs to be
Yes. Similar to hay fever, treated with an injection
allergies to certain foods cause of epinephrine,
which constricts
an inflammatory response blood vessels to
from the mouth to the gut. reduce swelling
Severe allergies may result and relax the
muscles around
in anaphylaxis. the airways.

Rheumatoid arthritis
If the immune system attacks cells around
a joint, causing an inflammatory response,
an autoimmune disease called rheumatoid
arthritis can result. The joint swells, gets
inflamed, and is very painful. Eventually,
there is permanent damage to the joints
and surrounding tissues.

Immunity overload JOINT B cell Raised, itchy skin
Inflamed joint Allergen
Most immune problems are
a combination of genetic and Hair
environmental factors. While immune
conditions are usually triggered by Epithelium SKIN
exposure to environmental factors, such Mast cell releasing
as pollen, foods, or irritants on the skin
or in the air, some people are genetically histamine
more susceptible to developing them.
Even autoimmune diseases (when the Eczema
immune system attacks healthy body The causes of eczema are unclear, but it is thought
tissue by mistake), such as rheumatoid to be a miscommunication between the immune
arthritis, can be made worse by irritants system and the skin. It is probably triggered by an
that cause inflammation elsewhere in the irritant (allergen) on the skin that stimulates the
body. People with a hypersensitive immune system beneath to launch an inflammatory
immune system may experience several response, causing swelling and redness.
conditions; for example, many people
with asthma also suffer from allergies.

186 187FIT AND HEALTHY
Immune problems

Allergies and our modern lifestyle Allergen
More people in developed countries suffer Hayfever
from allergies, and incidences have been Many people have a
rising since World War II. The specific specific allergy to pollen
reasons are open to debate, but there is or dust called hayfever.
agreement that it is likely to do with the When allergens bind
immune system being exposed to fewer to the membranes of
microbes during childhood. immune cells just below
the epithelium of the
SINUS eyes and nose, it triggers
these cells to release
Epithelium NASAL LINING histamines. This triggers
Cytokines an inflammatory
released by response, including
immune cell itchy, watery eyes
triggers and sneezing.
swelling Mast cell
secretes histamines

Lining of Swollen bronchus
bronchus Cytokine
Allergen Constricted airway

Immune cell Mucus

LUNG NORMAL IMMUNE ASTHMA
RESPONSE ATTACK

Asthma WEAKENED IMMUNITY
An attack of asthma is a
spasm in the bronchi of the When the immune system is weakened or absent, a
lungs leading to wheezing, person is said to be immunocompromised. This can
coughing, and breathing happen because of genetic defects, as a result of HIV
difficulties. It is brought on or AIDS, certain cancers and chronic diseases, and as
by an allergic response in the a consequence of chemotherapy or having to take
lungs to some irritant in the immunosuppressant drugs after a transplant. People
environment. There is some with weakened immunity have to avoid even simple
evidence that this condition infections, such as colds, because they cannot fight them
can be inherited. effectively. Even vaccines pose a risk of causing infection.

BIOHAZARD



CHEMICAL
BALANCE

Chemical regulators SLEEP Pineal gland
When light levels decrease,
Some of the organs of the endocrine system are HYPOTHALAMUS the pineal gland releases
dedicated specifically to hormone production, PINEAL melatonin, which makes
while others, such as the stomach and the GLAND you sleepy. It works in
heart, have other more familiar functions close partnership with
too. Each receives information from the the hypothalamus.
body and responds by secreting either
more or less of a certain hormone. The NERVOUS SYSTEM Hypothalamus
hormones act as messengers, telling cells ENERGY The hypothalamus is a part
to either “keep the balance” or giving of the brain that links the
instructions to bring about short-term nervous system to the
or long-term changes, such as puberty. endocrine system. It sits
above the pituitary gland
Pituitary gland PITUITARY and works with it closely.
Despite being the size of a pea, GLAND Among other things, it
the pituitary is sometimes called controls thirst, fatigue,
the “master gland.” It controls and body temperature.
the growth and development of Thyroid gland
tissues as well as the function of The thyroid secretes
several other endocrine glands. hormones that control
growth and metabolic
GROWTH rate. It also secretes
calcitonin, which
PARATHYROIDS encourages calcium
storage in the bones.
THYROID

CALCIUM IMMUNITY

Parathyroid glands THYMUS Thymus
Four tiny glands attached The thymus secretes the
to the thyroid regulate hormone that stimulates
calcium levels in the blood the production of the
and bones. They release a pathogen-fighting T cells.
hormone that acts on the The gland is most active
kidneys, small intestine, in babies and adolescents,
and bones to increase and shrinks with the
blood calcium levels. onset of adulthood.

ACTION Heart HEART
Tissues in the
heart secrete
Adrenal glands hormones that
These produce hormones encourage the kidneys
that govern the “fight or to expel water. This reduces
flight” response, such as blood volume, and thereby Stomach
epinephrine. They also decreases blood pressure. When the stomach is full, cells
help regulate blood STOMACH in its lining secrete gastrin,
pressure and metabolism, a hormone that stimulates
and secrete a small amount ADRENAL G neighboring cells to secrete
of testosterone and oestrogen. gastric acid. This acid is needed
to break down food
(see pp.142–43).
CHEMICAL BALANCE
Pancreas DIGESTION
As well as producing Hormone 190 191factories
digestive enzymes, KIDNEY KIDNEY
the pancreas makes LAND
insulin and glucagon – PANCREAS Testes
hormones that control The testes secrete the male
blood glucose levels Kidneys hormone testosterone. This plays a
(see pp.158–59). When the kidneys role in the physical development of
detect low oxygen levels in the boys, and maintains libido, muscle
Hormone blood, they secrete a hormone that strength, and bone density in men.
factories stimulates the production of red
blood cells in the bone marrow. MASCULINITY

Molecules known as hormones FEMININITY OVARY TESTES
travel throughout the body, Ovaries
triggering changes in tissues that The ovaries produce two
regulate everything from sleep hormones that govern female
and reproduction to digestion, reproductive health—estrogen
growth, and pregnancy. They are and progesterone. These
secreted into the bloodstream by regulate the menstrual cycle,
organs that are collectively known pregnancy, and birth.
as the endocrine system.

How hormones work

Hormones are molecules that act as messengers

between the body’s organs and tissues. They are

released indiscriminately into the bloodstream,

so they travel throughout the body, Protein, produced PANCREAS
but they can only affect cells by the estrogen
that have receptors to pick trigger, in turn makes
them up—and each oxytocin, which
hormone has its own prepares the body
for childbirth

particular receptor.

Some receptors float NUCLEUS ESTROGEN
in the cytoplasm TARGET CELL
of target cells,

others line the Hormone receptor

cell membrane. CELL MEMBRANE Estrogen binds Estrogen passes through
to receptor the cell membrane
The receptor-hormone pair CYTOPLASM
floating in the cell’s nucleus, ESTROGEN
MOLECULES
where it triggers a gene to
make a specific protein

Straight to the nucleus

Some hormones can pass straight through the

outer membrane of a target cell. The receptors Estrogen
for these hormones lie in wait in the cytoplasm Estrogen is a hormone produced
of the cell. Once the hormone passes through by the ovaries. It targets most
the membrane, it binds to the receptor, and body cells, binding to estrogen
together they cross into the cell nucleus. Here receptors, which then trigger
the receptor-hormone pair binds to the DNA genes that help and maintain
and activates a specific gene. female reproductive organs.

OVARY

Hormone triggers Triggered by blood Stimulus from
Some hormones are bloodstream
Endocrine glands secrete hormones in response released when sensory cells PARATHYROIDS
to some sort of trigger. These triggers can be of detect changes in the blood
three kinds; changes in the blood, nerve signals, or other body fluids. The
or instructions from other hormones. However, parathyroids, for example,
these triggers themselves are often responses to release the hormone
messages from the outside world. When it gets PTH in response to low
dark, for example, the hormone melatonin is calcium levels in the
released to help us go to sleep (see pp.198–99). blood (see pp.194–95).

Release of PTH

TARGET CELLS 192 193CHEMICAL BALANCE
CAN HAVE BETWEEN How hormones work
5,000 AND 100,000
HORMONE RECEPTORS WHAT IS
HORMONE THERAPY?
CELL ME MBRANE
Hormones can be used to
Hormone trigger changes throughout
receptor the body. Sex hormones, for
example, can be manipulated
NUCLEUS
to change individuals to
the gender they
identify with.

CYTOPLASM LIVER CELL

GLUCAGON A second messenger protein
MOLECULES is made due to the glucagon
Glucagon binds trigger. Its job is to stimulate
the liver to make glucose
to receptor
on cell surface

Receptor triggered

Glucagon Messenger at the gate
Glucagon, released by the pancreas,
targets liver cells, where it binds to Another class of hormones can’t pass through
receptors on the cell surface. This the outer membrane of a cell. These hormones
prompts the cell’s molecular machinery bind to receptors on the surface of the cell instead.
to start converting glycogen into This triggers the cell to produce a “second
glucose (see pp.156–57). messenger” protein, which causes further
changes within the cell.

Triggered by nerves Stimulus Triggered by hormones HYPOTHALAMUS
Many endocrine glands from nerve Hormones can also be
are stimulated by nerve released in response to Growth Hormone
impulses. When we Epinephrine other hormones. The hormone stimulus
experience physical stress, hypothalamus, for example, PITUITARY
for example, an impulse is ADRENAL GLAND produces a hormone that GLAND
sent along nerves to the travels to the pituitary gland
adrenal gland, causing it to and prompts it to release a
secrete the fight-or-flight second hormone—growth
hormone epinephrine hormone—which in turn
(see pp.240–41). stimulates growth and
metabolism.

Hormone Low level of calcium THYROID
balance in the blood PARATHYROID

Hormones are released in response to

information circulating in the body.

This information-response pattern is called calcium!”D!”

a feedback loop, and it works in a similar 1 Low calcium T H YROID GLAND
The parathyroid
way to a thermostat maintaining the “Release“Release vitamin glands in the neck detect
temperature of a house. low calcium levels in

the blood and release
parathyroid hormone
(PTH) in response.
2 Bones release calcium
PTH stimulates specialized cells in
the bone known as osteoclasts, which
break down bone tissue, releasing
calcium into the bloodstream.

3 Kidneys activate vitamin D
PTH also stimulates the
kidneys to reabsorb calcium and to
produce an enzyme that converts
Increasing levels of vitamin D into its active form.
calcium in the blood “Absorb calcium!”

4 Intestines absorb calcium
The activated vitamin D travels to the
intestine, where it stimulates the formation of
calcium-binding proteins. These proteins help
the gut absorb any calcium present in food.

Calcium balance

Calcium is the most abundant mineral in the
body. It is important for most physiological
processes, including the forming of bones and
teeth. It is therefore vital to keep calcium
levels in the blood within a tight range—too
much or too little can cause serious problems.
Hormones help keep these levels in check.

194 195CHEMICAL BALANCE
Hormone balance

THYROID High level of calcium
PARATHYROID in the blood

Calcium regulation CALCITONIN REDUCES
PTH (hormone) BONE LOSS, SO IT IS GIVEN
Calcium TO PEOPLE SUFFERING
Calcitonin (hormone) FROM OSTEOPOROSIS
Vitamin D

1 High calcium
The thyroid gland
detects high levels of calcium
in the blood. In response, it “Store calciu
produces the hormone
calcitonin. At the same time, m!”
the parathyroids stop “Remove calcium!” 2 Bones store calcium
producing PTH. Osteoclasts are no longer stimulated
by PTH to break down bone. Calcitonin
stimulates other cells in the bone, known
as osteoblasts, to build bone tissue using
calcium from the bloodstream.

Decreasing levels of
calcium in the blood

3 Kidneys expel calcium
Calcitonin also inhibits
the absorption of calcium in the
kidneys, so excess calcium starts
to get excreted in the urine (see
pp.150–51). Less PTH also stops
the activation of vitamin D in the
kidneys, so calcium is retained.

4 Intestines stop absorbing
Without activated vitamin D, less
calcium-binding protein is made—so
less calcium is absorbed in the intestines.

Hormonal changes

Hormones often get blamed for our behavior when the body is Hormones and stress
undergoing significant change—the moods of a teenager, for Three hormones play a role
example. However, our daily behavior can also affect our in a cycle of behaviour that
hormones, and that in turn can have serious health implications. leads to inactivity, anxiety.
and long-term stress.
Pituitary gland Anxiety
releases ACTH, People with sedentary lives are less Cortisol
stimulating the capable of dealing with stress. This Insulin
adrenal glands to may be because they don’t have Melatonin
release cortisol a physical outlet for cortisol and
other “fight or flight” hormones
that are produced in response to
the stresses of modern life.

Smoking affects the Sleeplessness and fatigue
function of all the Exposure to bright displays such
endocrine glands as TVs and phones late at night
suppresses melatonin production.
Pancreas releases This can affect sleep quality and the
copious amounts body’s ability to control temperature,
of insulin blood pressure, and glucose levels.
Suppressed immunity
Skin Unhealthy amounts Poor diet and lack of exercise can
Untoned of fat under the skin lead to high cortisol. This hormone
High insulin levels is useful in reducing inflammation,
muscle A sedentary life leads to but over prolonged periods it can
elevated insulin levels, which suppress the immune system,
keeps the body storing fat which decreases the body’s
rather than burning it. ability to fight infection.

Unhealthy choices HUGGING RELEASES THE
HORMONE OXYTOCIN. THIS
Poor food choices and a sedentary life cause hormone REDUCES BLOOD PRESSURE SO
changes that perpetuate that same unhealthy THE RISK OF HEART DISEASE FALLS
lifestyle. Lower activity levels lead to fewer “feel
good” hormones. This can lead to poor food choices,
which affect hormones that regulate blood sugar,
leading to weight gain and less exercise.

196 197CHEMICAL BALANCE
Hormonal changes

Healthy lifestyle EXERCISE BUZZ

Regular exercise is one of the most effective ways to trigger Exercise increases the release of
changes in hormones that lead to a healthier mind and body. neurotransmitters, which are the
Some of the hormones that help equip us for physical activity chemical messengers of the nervous
by regulating temperature, maintaining water balance, and system. They transmit signals at
adapting to increased oxygen demands are also so-called junctions between nerve cells,
“feel good” hormones, which greatly improve mood. called synapses. The increase
promotes the repair and
Pituitary gland releases maintenance of the brain.
growth hormone during Some neurotransmitters,
such as dopamine, also
and after exercise provide a feeling
of happiness.
Strong bones
Transmitting
Bone density Testosterone nerve cell
Testosterone and growth release during
hormone are released during Neurotransmitter
exercise in men and women. exercise molecules
As well as promoting sex drive, Skin released
testosterone improves bone
density. Growth hormone also Receiving
promotes the laying down of nerve cell
bone and continues its work
in the night after exercise, SYNAPSE BETWEEN
encouraging the body to TWO NERVE CELLS
recover and promoting
general maintenance.

Minimal fat Good musculature,
thanks to growth
hormone and
testosterone

Hormones and health Lean muscle Muscle mass
Three hormones play a Healthy insulin levels Testosterone stimulates the
role in improving our health Insulin is inhibited during building of lean muscle mass,
and our state of our mind. exercise, forcing our cells to and increases our overall
burn fat as an energy source metabolism. Growth hormone
Growth hormone instead of glucose. Insulin levels promotes the growth of
Insulin remain suppressed for a long muscle tissue and helps
Testosterone time after exercise, meaning we the body burn fat.
burn fat even as we rest.

Daily rhythms

The body has a built-in time-keeping system that

drives our daily rhythms—particularly those of eating

and sleeping. At the core of this is the daily chemical

conversion of the wakeful hormone serotonin into the sleep 3 Hunger hormones
hormone melatonin—a process that takes about 24 hours. Hunger hormones rise and
fall throughout the day. Levels of
The daily cycle ghrelin, the appetite increaser, rise
during fasting, increasing hunger in
Many hormones go through rhythmic fluctuations every the morning. Leptin, the appetite
day. These oscillations happen independently of any suppressor, signals
when you are “full.”

external prompting. Even in a black room with no 2 Stress-managing cortisol
windows, the body gets a serotonin surge in the As you start the day, the
morning, which wakes it up. However, these body produces the steroid
rhythms are not hard-wired—they are constantly hormone cortisol, which helps
readjusted and can be changed radically when the body deal with stress by
we travel to a different time zone. increasing blood sugar levels 9AM

and kick-starting metabolism.

The circadian clock 1 Wakeful serotonin 8AM
Light stimulates the 6AM
Our bodies run on a (roughly) 24-hour hormone suprachiasmatic nucleus
cycle, known as a circadian rhythm. The biological to convert melatonin
processes that govern it are called the circadian into serotonin—a
clock, which is what governs all the body’s rhythms. hormone that helps get
One of the main cogs in this clock is a very small the brain and body going
region of the brain known as the suprachiasmatic (especially the intestines).
nucleus (SCN). Located very near the optic nerves,
the SCN uses the amount of light entering the eye The SCN orders the secretion 3AM
to calibrate the circadian clock. of melatonin or serotonin,
depending on the time of day
Internal timepiece
The SCN drives a two-way chemical conversion
between the hormone serotonin, which wakes
us up, and melatonin, which puts us to sleep.

Light rays of
varying intensity

CAN STRESS Serotonin 10 Testosterone surge
MAKE YOU ILL? Men experience a rise
in testosterone levels at night,
Stress hormones prepare us regardless of whether or not they
for fight or flight, but they also Melatonin are asleep—a fact that might explain
take a toll on some of our other
SLEEP! late-night fights at bars.
systems, particularly our WAKE!
immune system. Chronic Electrical signals
target the SCN
stress can therefore
lead to disease.