How body works

TWHOHOWERBKOSDY



TWHOHOWERKBSODY

Editorial consultant Contributors CONTENTS
Dr Sarah Brewer Virginia Smith, Nicola Temple

Project Art Editor Senior Editor
Francis Wong Rob Houston

Designers Editors
Paul Drislane, Charlotte Johnson, Wendy Horobin, Andy Szudek,
Miezan van Zyl
Shahid Mahmood

Illustrators Assistant Editor UNDER THE
Mark Clifton, Phil Gamble, Francesco Piscitelli MICROSCOPE
Mike Garland, Mik Gates,
US Editor
Alex Lloyd, Mark Walker Jill Hamilton

Managing Art Editor Managing Editor
Michael Duffy Angeles Gavira Guerrero

Jacket Designer Producer, Who’s in charge? 10
Natalie Godwin Pre-production 12
Nikoleta Parasaki Organ to cell 14
Jacket Editor 16
Claire Gell Producer How cells work 18
Mary Slater 20
Jacket Design What is DNA? 22
Development Manager Publisher
Liz Wheeler How cells multiply 24
Sophia MTT 26
Publishing Director How genes work
Art Director Jonathan Metcalf
Karen Self How genes make
different cells
First American Edition, 2016
Stem cells
Published in the United States by DK Publishing
345 Hudson Street, New York, New York 10014 When DNA
goes wrong
Copyright © 2016 Dorling Kindersley Limited
DK, a Division of Penguin Random House LLC

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001–274815–May/2016

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HOLDING IT SENSITIVE
TOGETHER TYPES

Skin deep 30 ON THE Feeling the pressure 74
Outer defenses 32 MOVE How do you feel? 76
The extremities 34 Pain’s pathway 78
Pillars of support 36 Pulling power 54 How the eye works 80
Growing bones 38 Forming an image 82
Flexibility 40 How do muscles pull? 56 Vision in the brain 84
Biting and chewing 42 Vision problems 86
The grinder 44 Working, stretching, 58 How the ear works 88
Skin damage 46 pulling, braking How the brain hears 90
Breaking and 48 Balancing act 92
mending Sensory input, action output 60 Hearing problems 94
Wearing thin 50 Catching a scent 96
The control center 62 On the tip of 98
64 the tongue
Communication hub 66 Body position sense 100
68 Integrated senses 102
Sparking into life 70 Using your voice 104
Reading faces 106
Act or relax? What you don’t say 108

Bumps, sprains,
and tears

THE HEART FIT AND
OF THE HEALTHY
MATTER

Filling your lungs 112 Body battleground 168

From air to blood 114 Friend or foe? 170

Why do we breathe? 116 IN AND Germs are us 172
OUT
Coughs and sneezes 118 Damage limitation 174
Feeding the body
The many tasks 120 How does Infectious diseases 176
of our blood eating work?
A mouth to feed 138 Looking for trouble 178
Gut reaction
How the heart beats 122 Up, down, and out 140 Assassination squad 180
Bacterial breakdown
How blood travels 124 Cleaning the blood 142 Cold and flu 182
Water balance 144 Vaccine action 184
Broken blood vessels 126 How the liver works 146 Immune problems 186
What the liver does
Heart problems 128 Energy balance
The sugar trap
Exercising and its limits 130 Feast of fast?
Digestive problems
Fitter and stronger 132 148

Maximizing 134 150
your fitness
152

154

156

158

160

162

164

THE CIRCLE
OF LIFE

CHEMICAL Sexual reproduction 204 MIND
BALANCE Monthly cycle 206 MATTERS
Tiny beginnings 208
Hormone factories 190 The generation game 210 Learning skills 230
How hormones 192 Growing life 212 Making memories 232
work Mother’s new body 214 Falling asleep 234
Inner balance 194 The miracle of birth 216 Entering your dreams 236
Hormonal changes 196 Primed for life 218 All emotional 238
Daily rhythms 198 Growing up 220 Fight or flight 240
Diabetes 200 Hormonal teenagers 222 Emotional problems 242
Getting older 224 Feeling attraction 244
The end of life 226 Extraordinary minds 246

INDEX 248
ACKNOWLEDGMENTS 256



UNDER THE
MICROSCOPE

Who’s in charge? ARE THERE ANY
BODY SYSTEMS WE
To perform any task, the body’s many parts work CAN LIVE WITHOUT?
together in groups of organs and tissues called systems.
Each system is in charge of a function, such as breathing All our body systems are
or digestion. Most of the time, the brain and spinal cord vital. Unlike some organs—
are the main coordinators, but the body’s systems are
always communicating and giving each other instructions. such as the appendix—
if an entire system fails

it usually results
in death.

Brain A matter of organization
Spinal cord
Systems are communities of body parts with a single function. However, some
body parts have more than one job. The pancreas, for example, is part of the
digestive system because it pipes digestive juices into the gut. It also acts as
part of the endocrine system, releasing hormones into the bloodstream.

Windpipe Pituitary Hypothalamus
Lungs Thyroid
Pancreas
Adrenal Testis (in
glands men)
Ovary (in
women)

Sciatic Respiratory system Endocrine system
nerve The lungs bring air into contact with blood This system of glands secretes hormones,
vessels so that oxygen and carbon dioxide which are the body’s chemical messengers,
can be exchanged. sending information to other body systems.

Liver Esophagus Bladder Kidney
Pancreas Ureter
Stomach
Small Large
intestines intestine

Rectum

Central nervous system Digestive system Urinary system
The brain and spinal cord process and act The stomach and intestines are the major The kidneys filter blood to remove
upon information received from all over the parts of this system, which turns food into unwanted substances, which are stored
body through an extensive network of nerves. nutrients needed by the body. in the bladder and expelled as urine.

Brain 10 11UNDER THE MICROSCOPE
As the body Who’s in charge?
performs a
gymnastic routine, Breathing and heart rate
the brain receives data Information from the brain
from the eyes, inner ear, and nerves prompts the release of
all over the body, which it puts hormones that equip the
together to get a sense of body for the stress it’s
balance and body position. undergoing. Breathing
Muscles and nerves becomes more rapid
Nerve impulses are sent to the muscles and heart rate increases
to make instantaneous adjustments to to carry much-needed
body position to maintain balance. oxygen to the
The nervous system interacts with the muscles.
muscular system, which in turn acts
on the bones of the skeletal system. Digestive and
urinary systems
78 The stress hormones
released by the
ONE ESTIMATE OF endocrine system act on
THE TOTAL NUMBER the digestive and urinary
OF ORGANS IN THE systems to slow them down—
BODY—ALTHOUGH energy is needed elsewhere!
OPINIONS VARY!
Everything in balance

None of the body’s systems operates on
its own—each is constantly responding
to several others to keep things running
smoothly. To balance on the rings, each
system of a gymnast’s body can make
adjustments to compensate for stress
placed on other systems, which may
require more of the body’s resources.

ONE IN 10,000 PEOPLE Organs
HAS ALL THE INTERNAL
ORGANS ON THE WRONG The organs within the body are typically self-
SIDE OF THE BODY contained and perform a specific function.
The tissues that make up that organ help it
Stomach structure OESOPHAGUS function in a particular way. The stomach,
for example, is largely made
Muscle is the main tissue of of muscle tissue that can
the stomach, but it is also lined with expand and contract
to accommodate
the intake
of food.

glandular tissue, which secretes digestive
juices, and epithelial tissue, which forms
a protective barrier on both the inner
and outer surfaces.

Organ to cell
Each organ in the body is distinct and
recognizable to the naked eye. Cut through Stomach has
an organ, however, and layers of different three layers of
tissues are revealed. Within each tissue are smooth muscle
different types of cells. They all work together
to carry out the functions of the organ. STOMACH

Entrance to Inner wall is lined
intestines with cells that secrete
mucus or acid
WHICH IS THE
LARGEST ORGAN? Outer layer is covered
The liver is the largest of with epithelial cells
the internal organs but the
skin is actually the biggest
organ of the body.
It weighs roughly

6 lb (2.7 kg).

12 13UNDER THE MICROSCOPE
Organ to cell

Tissues and cells Types of cells

Tissues are made up of a group of connected cells. Some tissues come There are around 200 different
in different types, such as the smooth muscle that forms the walls of the types of cells in the human
stomach and skeletal muscle, which is attached to the bones and makes body. They look very different
under a microscope, but most
them move. As well as cells, the tissue might contain other structures, have common features, such
such as collagen fibers in connective tissue. A cell is a self-contained as a nucleus, cell membrane,
living unit—the most basic structure of all living organisms. and organelles.

Smooth action Red blood cells
The loose arrangement Lack a nucleus and
of the spindle-shaped carry as much oxygen
smooth muscle cells as possible.
allows this type of muscle
tissue to contract in all Nerve cells
directions. It is found in the Carry electrical signals
walls of the gut, as well as between the brain and
in blood vessels and the all parts of the body.
urinary system.

Epithelial cells
Line the surfaces and
cavities of the body to
form a tight barrier.

Smooth muscle cells Adipose cells
These long, tapering cells are Store molecules of fat that
capable of operating for long help insulate the body and
periods without tiring. can be turned into energy.

Tissue types Skeletal muscle cells
Arranged into fibrous
There are four basic types of tissue found in the human body. These are bundles that contract
subdivided into different subtypes, for example, blood and bone are both to move bones.
connective tissues. Each type has different properties—such as strength,
flexibility, or movement—that makes it suited to a specific task. Reproductive cells
The female egg and male
Connective tissue Epithelial tissue sperm combine to form
Connects, supports, Closely packed cells in a new embryo.
binds, and separates one or more layers that Photoreceptor cells
other tissues and organs. form barriers. Line the back of the eye
and respond to light
Muscle tissue Nervous tissue falling on them.
Long, thin cells that relax Cells that work together Hair cells
and contract to create to transmit electrical Pick up sound vibrations
movement. impulses. being transmitted through
the fluid of the inner ear.

How cells work The nucleus is the cell’s command
centre, containing blueprints in
Your body is made up of approximately 10 trillion cells,
and each one is a self-contained, living unit. Each cell uses the form of DNA. Surrounding it is
energy, multiplies, eliminates waste, and communicates. an outer membrane, full of pores,
Cells are the basic units of all living things. which controls what goes in and out

Cell function Ribosome helps
make proteins
Most cells have a nucleus—a structure in their center
that contains genetic data, or DNA. They rely on this ROUGH ENDOPLASMIC RETICULUM
data to build various molecules that are essential NUCLEUS
to life. All of the resources they need to do this
are contained within the cell. Structures called
organelles carry out specialized functions,
similar to the organs of the body. Organelles
are held in the cytoplasm, the space
between the nucleus and the cell
membrane. Molecules are brought into
the cell and others are shipped out,
just like in an efficient factory.

1 Receiving instructions
Everything that happens in a cell is
controlled by instructions in the nucleus.
These instructions are exported on long mRN
molecules called messenger ribonucleic acid
1A

(mRNA)—these molecules travel out of the
nucleus and into the cytoplasm.

2 Manufacture 2
The mRNA travels to an organelle 3
attached to the nucleus called the rough
endoplasmic reticulum. There, it attaches to GOLGI BODY
ribosomes that stud the organelle, and the
instructions are made into a chain of amino
acids that becomes a protein molecule.

3 Packaging
The proteins travel in vesicles—little cellular
bubbles—that float through the cytoplasm to the Golgi
body. This organelle acts much like the mail room of
the cell—packaging the proteins and putting labels on
them, which determine where they are sent next.
4
4 Shipping
The Golgi body places the proteins into CELL MEMBRANE
different types of vesicles depending on their labeled
destination. These vesicles bud off, and those destined Vesicle within cell,
for outside the cell fuse with the cell membrane and packed with proteins
release the proteins outside of the cell. Vesicle fusing with
Protein released cell’s membrane and
Inside a cell by Golgi vesicle releasing protein
Numerous organelles comprise the
internal structure of cells—the specific
types vary from cell to cell.

14 15UNDER THE MICROSCOPE
How cells work

HOW DO Cell death
CELLS MOVE?
When cells have reached the natural end of their
Most cells move by pushing life cycle they undergo apoptosis—a deliberate
their membrane forward from series of events that causes the cell to dismantle
itself, shrink, and fragment. Cells can also die
the inside using long fibers prematurely due to infections or toxins. This
made of protein. Alternatively, causes necrosis, a process in which the cell’s
internal structure detaches from its
sperm cells have tails, which membrane, causing
they whip back and the membrane to
forth to move. burst and the
cell to die.

SMOOTH ENDOPLASMIC Healthy cell
RETICULUM
APOPTOSIS NECROSIS

Smooth endoplasmic Cell
reticulum produces and structures
processes fats and some expand
hormones. Its surface
lacks ribosomes, so it Fragment
looks smooth of cell
Cell shrinks and fragments
Centrosomes are the Cell swells
organization points
ESICL for microtubules— One of the body’s cleaner Burst cell
MITOCHONDRION structures that cells (phagocytes) membrane
help separate DNA
SO SO during cell division
ELY Cell fragment
V Vesicles are
ME containers that Fragments eaten by cleaner cell Cell explodes
CENTROSOME transport materials
from the cell CELL COMMUNICATION
membrane to
the interior and Cells communicate with one
vice versa another and respond to their
Lysosomes act as environment using signaling
the cell’s cleanup molecules produced by distant
crew. They contain cells, nearby cells, or even the
chemicals used to same cell. Signaling molecules
get rid of unwanted bind with receptors, which are
molecules themselves molecules, on
Cytoplasm—the space the cell’s membrane.
between organelles—is The binding event
filled with microtubules triggers changes
in the cell, such as
Mitochondria are the cell’s activating a gene. CELL 1
powerhouses, where most of Cell 1’s
the cell’s supply of chemical signaling
energy is generated molecule
Receptor
MOST CELLS on cell 2’s
HAVE A DIAMETER membrane
OF ONLY 0.001 MM
CELL 2

SOMENUCLEX CHROMO
US Y chromosome SOME
X chromosome Boy or girl?
Humans inherit one set of 23 Y CHROMO
One of 23 pairs of chromosomes from their mother and
chromosomes another set from their father. Pairs 1 to
22 are duplicates, but with a slightly
different version of each gene on each
chromosome. Our sex is determined
by our chromosome 23 pairing.
Females have two X chromosomes,
while males have an X and a Y. Few of
the X chromosome genes are repeated
on the shorter Y chromosome, which
mostly carries the genes that produce
masculine characteristics.

CELL

CHROM

Control center OSOME
DNA is stored in the nucleus of
every cell, except for red blood cells,
which lose their DNA as they mature.
In each cell nucleus, there are 6 ft (2 m)
of DNA tightly coiled into 23 pairs
of chromosomes.

Human library The DNA helix is itself Body builders
tightly coiled The genes that build our bodies may range
DNA is a long molecule that provides from a few hundred bases to more than
all the information necessary for an 2 million bases in length—longer than the
organism to develop, survive, and small section shown here. Each gene
reproduce. It is like a twisted ladder produces a single protein. These proteins
with rungs made of a pair of chemical are the building blocks of the body,
bases. These bases form long forming cells, tissues, and organs. They
sequences called genes that are also regulate all the body’s processes.
coded instructions for building The outer edge of each
proteins. When a cell needs to strand is made of sugar
duplicate its DNA or make a new and phosphate molecules
protein, the two halves of the
ladder unzip so that a copy of the gene can
be made. Humans have more than 3 billion
bases in their DNA and nearly 20,000 genes.

What is DNA?

DNA (deoxyribonucleic acid) is a chain molecule that exists in The colored bars show
nearly all living things. The chain is made up of a sequence the four bases—adenine,
of molecular components, known as bases. Incredibly, the
sequence acts as coded instructions for making an entire living thymine, guanine, and
organism. We inherit our DNA from our parents. cytosine—which are

arranged in a particular,
meaningful sequence

16 17UNDER THE MICROSCOPE
What is DNA?

Express yourself Eye color is inherited, but can Several genes control the curliness
be influenced by any of the of hair. Two curly-headed parents
The majority of genes are the 16 genes that control color may produce a straight-haired child
same in everybody because Unpredictable outcomes
they code for molecules that are Many of our physical features are under the Freckles are controlled by
essential for life. However, around control of more than one gene. This may a single gene. Variations
1 percent have slight variations— result in unexpected combinations. of the gene control the
known as alleles—that give us our number of freckles
unique physical characteristics.
While many of these are harmless
traits, such as hair or eye color, they
may also result in more problematic
conditions, such as hemophilia or
cystic fibrosis. Because alleles
come in pairs, one may override the
effect of the other so that the trait
remains hidden.

Unraveling DNA

Chromosomes help package DNA to fit into the nucleus. The DNA DO HUMANS HAVE
is wrapped around spool-like proteins that run through the center

of each chromosome. The helix is made of two strands of sugar THE MOST GENES?

phosphate linked together by a pair of bases. The bases always Humans have a relatively low
form the same pairs, but the sequences of bases along the strand number of genes. We have more
are specific to the proteins they will eventually produce. than a chicken (16,000) but fewer

than an onion (100,000) or an

The bases on one side of the strand are amoeba (200,000). This is
paired with a complementary base on the because we lose unwanted
other side—in this case cytosine (green)
bonds with guanine (blue) genes faster from our
DNA than they do.
Adenine (red) always
bonds with
thymine (yellow)

Guanine (blue) always bonds
with cytosine (green)

How cells multiply

We all start life as a single cell, so to develop specific OUT OF CONTROL
tissues and organs and enable our body to grow, our
cells need to multiply. Even as adults, cells need to be Many cancers occur when a
replaced because they get damaged or complete their mutant cell begins to multiply
life cycle. There are two processes by which this rapidly. This is because the cell
happens—mitosis and meiosis. can override the usual checks
during mitosis, enabling it to
Wear and tear 1 Resting replicate itself more quickly than
Mitosis happens whenever new cells are The parent cell gets surrounding cells and
needed. Some cells, such as neurons, are ready for mitosis by checking take up more of the
rarely replaced, but others, such as those its DNA for damage available oxygen
lining the gut or tastebuds, undergo and nutrients.
mitosis every few days.
Cancerous cell

and making
any repairs
needed.

6 Offspring Cell Preparation
Two daughter cells are Each chromosome
formed, each containing a Nucleus Four of cell’s 46 2
chromosomes
nucleus with an exact copy of in the parent cell makes an
exact copy of itself prior to
the DNA from the parent cell. Mitosis entering mitosis. The copies

Every cell enters a phase in its life cycle called join at a region
mitosis. During mitosis, the cell’s DNA is called the
centromere.
duplicated and then divides equally to form
two identical nuclei, each containing the exact Centromere
same DNA as the original parent cell. The cell
5 Splitting then divides up its cytoplasm and organelles 3 Lining up
A nuclear membrane Each of the doubled
to form two daughter cells, each containing
a single nucleus. There are a number of

checkpoints throughout the DNA replication
and division processes to repair any damaged

DNA, which could lead to permanent
mutations and disease.

forms around each group of chromosomes attaches to
special fibers, which help
chromosomes and the cell line them up
membrane starts to
pull apart to form in the middle
of the cell.
two cells. 4 Separation
The chromosomes split
at their attachment point
(centromere) and each
half is pulled to an
opposite end
of the cell.

Fiber

Centromere

18 19UNDER THE MICROSCOPE
How cells multiply

2 Pairing and
crossover
Chromosomes with similar
lengths and centromere
locations line up with one 3 First separation
1 Preparation another and undergo gene The chromosomes line
Each of the cell’s up and, just like in mitosis, are
chromosomes duplicates and swapping.
pulled to opposite ends
the copies join together at the of the cell along
centromere.
special fibers.

Cell
Nucleus
Chromosome
Centromere

Gene swapping Fiber
Meiosis features a unique process that shuffles
the DNA passing into the daughter cells. DNA is
exchanged between the chromosomes, which
creates new combinations of DNA. Some new

combinations may be beneficial.

6 Four offspring 4 Two offspring
Four cells are The cell divides, and
produced, each with half the
number of chromosomes of 5 Second separation two cells containing half the
The chromosomes line chromosomes are formed.
the original parent cell, and Each is genetically distinct
each genetically unique. up along the midline of each
cell and are pulled apart so from each other and from
the parent cell.
that each new cell receives half
of the chomosome pair.

Meiosis DOWN SYNDROME

Egg and sperm cells are produced through Sometimes mistakes can happen during meiosis. Down
a specialized type of cell division known as syndrome is caused by an extra copy of chromosome 21 in
meiosis. The aim is to reduce the number of some or all of the body’s cells. This usually happens when the
chromosomes from the parent cell by half so chromosome doesn’t separate properly during the meiosis of an
that when an egg and sperm fuse during egg or sperm cell—a condition known as trisomy 21. Having an
fertilization, the new cell has a full extra chromosome means that some genes are overexpressed by
complement of 46 chromosomes. Meiosis the cell, which can cause problems in how it functions.
produces four daughter cells that are each
genetically different from the parent cell. It is The extra 310
the process of gene swapping during meiosis genes can result
that introduces the genetic diversity that in overproduction
helps make each of us unique individuals. of some proteins.

THREE COPIES OF CHROMOSOME 21

How genes work

If our DNA is the body’s recipe book, then a gene within that DNA
is equivalent to a single recipe in the book; it is the instructions for
building a single chemical or protein. It’s estimated that humans
have around 20,000 genes that code for different proteins.

Genetic blueprint Amino acid
TRANSFER RNA
To translate a gene into a protein, the DNA is first (tRNA)
copied (transcribed) in the nucleus of a cell by enzymes,
forming a strand of messenger RNA (mRNA). The cell
will only copy those genes that it needs, not the entire
DNA sequence. The mRNA then travels outside the
nucleus where it can be translated into a chain of
amino acids, which will build the protein.

Nuclear
membrane

CELL NUCLEUS MESSENGER RNA Anticodon
Pore in nuclear
DNA membrane (mRNA)
DNA unzips at right
gene sequence

RNA polymerase 1 Starting translation
enzyme builds new The newly made mRNA travels to a
strand of mRNA protein-building unit called a ribosome, to
which it attaches. There, it attracts molecules
of transfer RNA (tRNA), each of which has an
SINGLE STRAND OF D amino acid attached to it.
mRNA contains
matching base
pairs to DNA strand

mRNA mRNA strand moves out
into the cell’s cytoplasm

NA CYTOPLASM
DNA copied in nucleus
A special enzyme binds to the DNA, where it
separates the two strands of the double helix.
It then moves along, adding RNA nucleic
acids that complement the single strand of
DNA, forming a single mRNA strand.

20 21UNDER THE MICROSCOPE
How genes work

4 Amino acids folded
into proteins
When the ribosome reaches
a stop codon at the end of
the mRNA strand, the long Making proteins
chain of amino acids is
complete. The sequence of Every three bases in the mRNA is known as
the amino acids determines a codon and each codon specifies a particular
how the chain folds up CHAIN amino acid. There are 21 different amino acids
into a protein. and a single protein may be made up of a chain
FOLDE of hundreds of these amino acids.

Chain of amino acids D INTO PROTEIN
builds as ribosome
moves along
mRNA strand

2 Ribosome attaches amino acids
As the ribosome moves along the mRNA strand,
the tRNA molecules attach to the mRNA in a specific
order. This order is determined by the matching up of
codons—a sequence of three nucleic acid bases on the
mRNA strand—and their complementary three bases—
called anticodons—on the tRNA molecule.

RIBOSOME 3 Building a chain
The amino acid detaches from
the tRNA molecule and is joined to the
previous amino acid with a peptide
bond, forming a chain.

Codon tRNA, once it has dropped
off its animo acid, floats off
into cytoplasm

LOST IN TRANSLATION

Gene mutations can cause WHAT HAPPENS
changes in the amino acid TO mRNA AFTER
sequence. A single mutation TRANSLATION?
in the 402nd base of the
gene that codes for the hair A strand of mRNA may be
protein keratin causes the translated into a protein many
amino acid lysine to be put
in place of glutamate. This times before it eventually
changes the shape of the degrades within the cell.
keratin, making the hair
look beaded. STRAIGHT HAIR BEADED HAIR

How genes make HOW DO CELLS
different cells KNOW WHAT TO DO?

DNA contains all of the blueprints for life, but cells The chemical environment
pick and choose only the plans (genes) they need. around the cell or signals from
These genes are used by the cell to build the proteins other cells tell it that it is part
and molecules that not only define what the cell looks of a particular tissue or organ,
like, but what it does within the body.
or in a certain stage
of development.

Gene expression

Each cell uses, or “expresses,” only a fraction of its genes. As it

becomes more specialized, more genes are switched off. This

process is highly regulated and happens in a specific order, usually Gene to be
when the DNA is being transcribed to RNA (see pp.20–21). transcribed
(copied to RNA)
1 Regulation REGULATOR PROMOTER OPERATOR
Transcription PROTEIN GENE SEQUENCE
Repressor protein prevents
of a required gene is polymerase binding to DNA
controlled by a series
of genes that sit in
front of it. These
include regulator,
promoter, and
operator genes.
The gene won’t be
transcribed until
conditions are right. REGULATOR
RNA
POLYMERASE

REPRESSOR

2 Repressor
protein
If a repressor protein
is blocking the gene,
transcription can’t
take place. The gene
can only be turned on
when a change in the
environment removes
the repressor protein.
Activator protein

Polymerase can now bind to the DNA
and start transcription

3 Activation
When an
activator protein
binds to the regulator RNA
protein and there are POLYMERASE

no repressor proteins
blocking the gene,
transcription can start.

22 23UNDER THE MICROSCOPE

How genes make different cells

On or off? Nerve Nerve cell gene
precursor NERVE CELL
Embryonic cells start out as stem cells—cells stem cell
Epithelial Dendrite
with the ability to turn into different cell types. precursor Axon

Stem cells initially have the same set of genes stem cell EPITHELIAL CELL
Epithelial cell gene
switched on and they simply keep growing

and dividing to produce more cells. As

an embryo develops, its cells need

to specialize and organize into

tissues and eventually organs.

So when signaled, the cells start

shutting off some genes and

switching on others to turn EMBRYONIC
into a specific type of cell. STEM CELL

Making a difference
As an embryo is developing, a stem cell destined to become
a nerve cell will turn on the genes needed to grow dendrites
and an axon, whereas another stem cell might activate
different genes to become an epithelial (skin) cell.

Housekeeping proteins TRA NSPORT BOY OR GIRL?

Some proteins, such as DNA repair STRUC Transport At 6 weeks, an embryo has all
proteins or enzymes needed for protein the internal organs needed to
metabolism, are called housekeeping be either male or female. If it is
proteins, because they are essential to On the move genetically a male embryo, a
the basic functioning of all cells. Many Special proteins are gene on the Y chromosome will
are enzymes, while others add structure needed to move turn on at this stage and produce
to cells or help transport substances in materials around the hormones that develop the
and out of cells. The genes for these the body or help male reproductive organs and
proteins are always turned on. them cross cell cause the female organs to
membranes. degenerate. The reason why
Structural protein TURE men have seemingly pointless
nipples is that these are formed in
Enzyme the first 6 weeks, but their further
development depends on whether
they are in a male or female
hormonal environment.

ENZYMES Providing support
Structural proteins
Chemical Speeding things up are found in all cells.
split by Enzymes are proteins that They give the cell its
enzyme help chemical reactions go shape and hold the
faster, such as those used in organelles in place.
the breakdown of food.

Adult stem cells WHERE DO ADULT
STEM CELLS COME FROM?
Adult stem cells have been found in the brain, bone marrow,
blood vessels, skeletal muscles, skin, teeth, heart, gut, liver, This is currently being
ovaries, and testes. These cells can sit inactive for a long time investigated, but one theory
until they are called into action to replace cells or repair damage, is that some embryonic stem
when they begin to divide and specialize. Researchers can cells remain in various tissues
manipulate these cells to become specific cell types that can
then be used to grow new tissues and organs. after development.

EXTRACTION FROM MARROW CELLS CULTURED

1 Harvest 2 Culture
Stem cell therapy may help repair damaged heart tissue The sample is filtered to remove non-
stem cell material and then taken to a lab that
following a heart attack. A small sample of the patient’s bone will identify the stem cells. The lab cultures these
marrow is taken because stem cells are more concentrated there.
cells, getting them to multiply and specialize.
Stem cells

Stem cells are unique because they can specialize into many
different types of cells. Stem cells are the foundation for the
body’s repair mechanisms, which makes them potentially
useful in helping repair damage in the body.

24 25UNDER THE MICROSCOPE
Stem cells

ADULT OR EMBRYONIC CELLS? Engineering tissues

Embryonic stem cells can develop into any cell type, but research Researchers have found that the
on them is controversial, because embryos—created using donor physical structure of the supporting
eggs and sperm—are grown specifically for the purpose of matrix (scaffold) used to grow stem
harvesting the cells. Adult stem cells are less flexible, forming only cells is critical to the way they grow
different types of blood cells, for instance, but new treatments and specialize.
can now be used to turn them into a wider range of cells.
1 Taking shape
To repair the eye’s
cornea, stem cells are
UNTREATED ADULT STEM CELL EMBRYONIC STEM CELL extracted from a healthy

tissue (the cornea of the
unaffected eye) and grown
Red blood on a dome-shaped mesh. Stem
cell White blood Skin Fat Blood cells Mesh
cell cell cell support
cell 2 Transplant
The damaged cells
on the cornea of the eye are
removed and replaced with
the mesh structure. After
several weeks, the mesh
dissolves leaving the grafted
Platelet Nerve cell Muscle cell cells, which have restored the

INJECTION TO HEART patient’s sight.

DAMAGED HEAR Potential uses of stem cells
Stem cell research has improved our understanding
T MUSCLE of embryonic development and the natural repair
mechanisms in the body. The most active area of
research is their use in growing replacement organs
and reconnecting the spinal cord so that paralyzed
people can walk again.

REPAIR Blindness
Deafness
ED MUSCLE Muscular
Missing dystrophy
teeth Wound healing

Cells grow Bone marrow
into heart transplant

muscle Spinal cord
Repaired injury

muscle Rheumatoid
arthritis
3 Inject 4 Repair
The cells are injected into After several weeks, Diabetes
the damaged heart muscle where the damaged heart muscle is
they attach to the damaged fibers rejuvenated. This process also Crohn’s disease
and begin to grow into new tissue. reduces scarring that would
restrict the heart’s movement.

Osteoarthritis

Environmental assault 20,000

Each of our cells is inundated daily by chemicals and energy THE NUMBER OF DAMAGED
that can cause damage to our DNA. Solar radiation (UV), BASES REMOVED AND
environmental toxins, and even the chemicals produced REPLACED IN EVERY
through our own cellular processes can cause changes to our CELL EVERY DAY
DNA that affect how it works, including how it can be copied
or how it produces proteins. If this damage becomes a
permanent change in the DNA, it is called a mutation.

CAN THE DAMAGE Intrastrand crosslinks
ALWAYS BE REPAIRED? make the helix unwind
and prevent it being
Our ability to repair DNA copied
diminishes as we get older.
Damage starts to accumulate Double strand breaks
are caused by radiation,
and this is thought to be chemicals, or free oxygen
one of the main reasons radicals. Incorrect repairs
can result in rearrangement
behind aging.
of the DNA, which can
lead to disease

Chemical toxins from
pollution or smoking bind
to bases, creating mutations

that can lead to tumors

Single strand breaks
can result in the loss of
a base, which leads to
mismatches when the

DNA copies itself

Abnormal bases occur
when chemicals change
the structure of the base
molecule, which leads
to mispairing

When DNA goes wrong

Every day, the DNA in cells is damaged—whether by natural
processes or environmental factors. This damage can affect
DNA copying or how specific genes function and if it can’t
be repaired, or is repaired incorrectly, it can lead to disease.

UNDER ATTACK 26 27UNDER THE MICROSCOPE
This DNA strand is shown under many When DNA goes wrong
kinds of stress. However, some types of
DNA damage can be used to advantage. Interstrand crosslinks
Many chemotherapy drugs are designed between the same bases
to cause damage to the DNA in cancerous halt DNA copying
cells. Cisplatin, for example, forms because they prevent
crosslinks in the DNA, which triggers the strands from
cell death. Unfortunately it also unzipping
causes damage in normal
healthy cells. Base mismatches occur
when an extra base has
been added or one
has been skipped in the
replication process

The insertion or deletion of Gene therapy
bases means that when the
code is being read during When DNA damage causes a mutation, it can stop
copying, the wrong proteins a gene from working properly and result in disease.
will be produced While drugs might help treat the symptoms of the
disease, they can’t solve the underlying genetic
REPAIRING DNA problem. Gene therapy is an experimental method
that’s exploring ways to fix the defective gene.
Cells have builtin safety systems that
help identify and repair damage 1 Cells with 7 Cells now
to their DNA. These systems are a defective produce the
constantly active and if they are gene are harvested correct protein.
unable to fix the damage quickly, they from the patient.
will stop the cell cycle temporarily so
they can take some extra time to work 2 A virus is
on it. If it’s not repairable, they will disabled
trigger the death of the cell by so that it can’t
apoptosis (see p.15). reproduce.

6 Altered cells
are injected
into the patient’s body.

3 A healthy 5 The virus
version of inserts the
the patient’s gene is healthy gene into the
inserted into the virus. cell’s DNA.

4 The altered
virus is mixed
with the patient’s cells.



HOLDING IT
TOGETHER

Skin deep

The skin is the largest organ of the human body. It protects us
from physical damage, dehydration, overhydration, and infection,
but also regulates body temperature, makes vitamin D, and has
an extraordinary array of special nerve endings (see pp.74–75).

Keeping cool and staying warm of our body hair and rely on clothes to keep us warm,
even fine body hair plays a role in controlling body
Humans have adapted to survive in the heat of the temperature. In hot weather, it is vital to drink plenty
of water to replace the sweat that helps keep us cool.
tropics, the cold of the arctic, and the temperate

climates in between. Although we have lost most

Hot-weather skin Cold-weather skin
Each day, the skin’s 3 million sweat glands In cold weather, the skin goes into heat-
secrete 2.1 pints (1 liter) of sweat, or up to retention mode. Tiny muscles stand
21 pints (10 liters) daily in extreme conditions. our body hairs upright, trapping
Evaporation of sweat takes the heat energy away warmth close to the skin. Meanwhile,
from the body. Ring-shaped muscles around the the capillary-network muscles stop
blood vessels also help by diverting blood to the warm blood from flowing into
skin, which lets heat escape from deep in the body. the skin’s surface layers.

Hair reclines to release Sweat droplets Hair stands up to trap Hair erector muscle
the heat around it evaporate, taking heat the heat around it contracts
away with them
Heat rises to the surface The skin rises into Sweat
of the skin from the a “goose bump” production
capillary network around the hair
stops

SWEAT GLAND CAPILLARY NETWORK Fat in the
lowest layer

of the skin
retains heat

Muscle in the capillary network relaxes, BLOODSTREAM Capillary muscle contracts, reducing the
shunting blood to the outer layers of the skin Hair erector muscle relaxes, flow of blood to the outer layers of the skin
allowing the hair to flatten

30 31HOLDING IT TOGETHER
Skin deep

Defensive barriers THE SKIN OF AN AVERAGE
ADULT MEASURES 20 SQ FT
The skin is made up of three layers, each of which (1.9 SQ M) IN AREA
plays a vital role in our survival. The upper layer, called
the epidermis, is an ever-regenerating defense system
(see pp.32–33) and has its roots in the middle layer,
called the dermis. The inside layer is the hypodermis—
a cushion of fat that keeps us warm, protects our bones,
and keeps us supplied with energy (see pp.158–59).

Microbe Sebum Ultraviolet light DO GOOSE
BUMPS REALLY HELP?

Antibacterial oil Ultraviolet light protection Goose bumps do help us retain
Glands secrete an oil The skin uses ultraviolet light heat in cold weather. However,
called sebum into hair to synthesize vitamin D— they were much more effective
follicles to condition but too much ultraviolet millions of years ago, when we
the hair and waterproof light can cause skin cancer. were covered in thick hair. The
the skin. Sebum also A skin pigment called thicker the hair, the more heat
suppresses the growth melanin helps maintain
of bacteria and fungi. a balance between the is trapped when the hair
two (see pp.32–33). stands on end.
Sebaceous
gland secretes Constantly
regenerating
sebum epidermal cells

NICOTINE PATCH

EPIDERMIS

HAIR SHAFT DERMIS

Nicotine reaches One of the skin’s
the bloodstream many types of
nerve endings
(see pp.74–75)

HAIR Letting things pass
BULB Although skin is a barrier, it is selectively
permeable, letting through drugs, such
as nicotine and morphine, from patches
applied to the skin’s surface. Various
creams, such as sunblock, moisturizer, and
antiseptic cream, can also cross the barrier.

The epidermis stretches all HYPODERMIS
the way under the hair bulb

Outer defenses ARE FINGERPRINTS
REALLY UNIQUE?
The skin is the frontier between us and the outside
world—a boundary at which enemies are fought The curls, loops, and swirls
and friends let in. Key features of its defenses are of each finger are unique,
a self-renewing outer layer and a pigment that and each grows back the
shields us from ultraviolet light. same way after injury—

The self-renewing layer a handy fact for police
detective work.
The epidermis is a conveyor belt of cells, which are constantly
forming at its base—the basal layer—and traveling upward EPIDERMIS
to the surface. As they move, they lose their nucleus, flatten,
and fill with a tough protein called keratin, and so form a DERMIS
protective, outer layer. This layer is constantly being HYPODERMIS
worn away and replaced by new,
upthrusting cells. Each cell
dies by the time it reaches the
surface. The dead cells fall
off and contribute to the
dust in our houses.

Dead cell flakes off Tattoo
Cells travel up through

the epidermis

Transparent defense Basal layer New cells form
Because the epidermis sheds its in the basal layer
cells, tattoos have to be inscribed
beneath it, on the dermis. The
epidermis is transparent, so tattoos
can be seen through it.

Scaffolding Collagen fiber Elastin fiber Wrinkle Weakened fibers

Beneath the epidermis lies Young skin Ageing skin
the dermis, a thick layer that The collagen and elastin fibers of The collagen and elastin fibers of
gives the skin its strength youthful skin are strong, keeping aging skin are weak, causing wrinkles
and flexibility. It contains the skin smooth and firm. Proper to form on the surface. Smoking,
the skin’s nerve endings, hydration and a healthy diet keeps sunlight, and poor diet accelerate
sweat glands, oil glands, the skin youthful. the aging process.
hair roots, and blood vessels.
It is made primarily of
collagen and elastin fibers,
which form a kind of
scaffolding that enables the
skin to stretch and contract
in response to pressure.

32 33HOLDING IT TOGETHER
Outer defenses

Skin color FRECKLES ARE
CAUSED BY
One of the skin’s many functions is to make vitamin D, MELANOCYTES
which it does by harnessing ultraviolet (UV) light from CLUMPING
the Sun. However, UV light is also very dangerous (it can TOGETHER
cause skin cancer), so we also need protection against it.
As protection, the skin produces melanin—a pigment that Intense rays of UV light
serves as a Sun shield, and so determines skin color.

Dark skin 5 UV shield
At the equator, the sun’s rays strike the Melanosomes break
Earth almost vertically, and with great apart, spreading melanin
intensity. This means that people born across the skin. This forms
near the equator have a great need of
UV protection. To provide this, the skin
produces large amounts melanin—
which results in dark skin.

2 Dendrites a shield against UV rays.
Melanocytes have
fingerlike extensions 4 Absorption
called dendrites. Each Melanosomes are
absorbed by neighboring
of these touches around skin cells.
35 neighboring cells.

1 Melanocytes 3 Melanosomes
Melanin is Melanin moves along
produced by special cells the dendrites in packets
called melanocytes. called melanosomes.
These are embedded in
the base of the epidermis.
Melanosome

Dendrite

Melanocyte Basal layer

Pale skin Mild rays of UV light
North and south of the equator, the
sun’s rays hit the Earth at increasingly
shallow angles. The shallower the
angle, the less intense the light, and less
need for UV protection. In response,
the skin produces smaller amounts of
melanin—which results in pale skin.

3 Weaker shield
The weaker melanin
shield is sufficient against
weaker UV rays.
Dendrite

1 Melanocytes 2 Paler melanosomes
In pale skin, the Melanosomes are
melanocytes are less paler and taken up by fewer
active, and have fewer surrounding cells.
dendrites.

Melanocyte Melanosome

The extremities

Hair and nails are both made of a tough, Thick, straight, and red
fibrous protein called keratin. Nails A mixture of pale and dark melanin
strengthen and protect the tips of your produces hair that is gold, auburn,
fingers and toes, while hair reduces heat or red. Large, round follicles
loss from the body to help keep you warm. produce thick hair. Thickness
also depends on the number
of active follicles present.
Redheads tend to have
relatively few follicles.

Hair color, thickness, and curliness A large proportion
of pheomelanin
Each hair has a spongy core (medulla) and a middle
layer (cortex) of flexible protein chains that give
it wave and bounce. An outer layer (cuticle) of scales
reflects light so hair looks shiny, but if these are
damaged, hair looks dull. The color, curliness,
thickness, and length of your hair are determined
by the size and shape of your follicles (in which
they grow), and the pigments they produce.

WHY DOES HAIR Fine, straight, A little
LENGTH VARY? and blonde eumelanin
Cells at the base of Medulla
Scalp hair can grow for years, each follicle feed melanin
but hair found elsewhere on pigments through to the root. Scales A little dark melanin,
the body only grows for weeks Blonde hair contains a pale or eumelanin
or months. That’s why body melanin pigment that is
hair is usually short—it falls only present in the
middle of the shaft
out before it can grow (medulla). Small,
very long. round follicles
produce straight,
fine hair.

Cuticle

Pale melanin pigment,
pheomelanin
Cortex

Hair growth Hair shaft Elongated shaft

Each hair follicle goes through around Hair root
25 cycles of hair growth during its
lifespan. Each cycle has a growth Blood Hair bulb
stage when it lengthens, followed vessel
by a resting phase in which the hair
remains the same length, starts to 1 Early growth 2 Late growth 3 Resting
loosen, and falls out. After the resting The follicle The shaft elongates The follicle
phase, the follicle reactivates and activates, producing new over a period of 2–6 years. shrinks and the hair stops
starts to produce a new hair.

cells within the hair root. A longer growth period growing as the bulb pulls
These die and are pushed (more common in women) away from the root. This
upward to form the shaft. produces longer hair. takes 3–6 weeks.

34 35HOLDING IT TOGETHER
The extremities

Thick, black, and curly Nails
Dark hair contains black
melanin pigment in both Nails are transparent plates of keratin. They
the cortex and the act as splints to stabilize the soft flesh of your
medulla, producing fingertips, and improve your grip on small
more depth of color. objects. Nails also contribute to the overall
Wavy hair grows from sensitivity of your fingertips. However,
oval-shaped follicles. As because they project from the body,
follicles become flatter, nails are easily damaged.
hair curliness increases.
Matrix, or growth area

NAIL CUTICLE
NAIL BED

BONE
FAT

Dense How nails grow
eumelanin Growing areas at the base and sides of each nail are
protected by folds of skin called cuticles. Cells in the
Air space nail beds are among the most active in the body.
They are constantly dividing, and nails grow up to
1⁄5 in (5 mm) per month.

Curly and gray Impoverished Poor diet 4 months ago caused amonths Splinter hemorrages
Hair turns gray due to reduced eumelanin pale patch, called leukonychia, from tiny bleeding
activity of an enzyme that due to a lack of protein blood vessels 5–6
produces melanin pigment. Hair months ago may
without melanin is snow white; hair 6 be due to a heart
with a little pigment appears gray. 5 infection
4
Old hair 3 Serious illness
2 2 months
1 ago caused
0 a horizontal
trench known
New hair as a Beau’s line
An injury
1 month ago
caused bleeding
under the nail

Bulb detached Diary of a nail
from blood Because nails are nonessential structures, blood
vessel and nutrients are diverted away from the nail beds
in times of deficiency. Nails are therefore a good
4 Detatchment 5 New growth indicator of your general health and diet. A doctor
The loose hair is shed The follicle starts its glances quickly at a patient’s hands because the nails
naturally, or dislodged by next cycle. With age, fewer can indicate a number of illnesses.
brushing or combing. follicles reactivate, so hair
Sometimes it’s pushed out becomes thinner, recedes,
by a new hair growing. and bald areas may appear.

Blood vessels thread MORE THAN HALF
throughout all the OF YOUR BONES
bone’s tissues ARE IN YOUR

Dense, compact HANDS AND FEET
bone makes up
80 percent of a Osteons are cylindrical structures
formed by concentric layers of
bone’s weight compact bone tissue

MARROW

HOW STRONG Periosteum is a surface COMPACT BONE
IS BONE? layer functioning as
the bone’s “skin”
Bone is five times stronger Bone marrow
than a steel bar of the same
weight, but it is brittle and can Small arteriole supplying
fracture on impact. Low levels blood to bone cells
of calcium and/or vitamin D
can lead to the brittle bone

disease, osteoporosis.

Pillars of support THE SMALLEST BONES

Your skeleton is rather like a coat hanger on which The stapes in the middle ear is
your flesh is draped. As well as giving your body the smallest named bone. You
support and shape, your bones provide protection also have small sesamoid bones
and, through their interaction with muscles, allow (named after the sesame seeds
your body to move and adopt different poses. they resemble) in long tendons
at sites of pressure to prevent
Living tissue the tendons from wearing away.

Bone is a living tissue made up of collagen protein fibers filled with LIFE SIZE
minerals—calcium and phosphate—which give them rigidity. Bones contain
99 percent of all the calcium in your body. Bone cells constantly replace old, STIRRUP (OR STAPES)
worn-out bone with new bone tissue. Blood vessels supply these cells with EAR BONE
oxygen and nutrients. A surface layer of skinlike periosteum covers a shell
of compact bone, which provides strength. Beneath this is a spongelike
network of struts that reduces the overall weight. Bone marrow in certain
bones, including the ribs, breast bone, shoulder blades, and pelvis, has
a special job—it produces new blood cells.

36 37HOLDING IT TOGETHER
Pillars of support

How the skeleton fits together Cranium SKULL
protects brain MANDIBLE

The skeleton can be divided into two main parts. The axial

skeleton consists of the skull, vertebral column (spine), and

ribcage and protects the internal organs and the central nervous

system. The appendicular skeleton includes the upper and lower

limbs, plus the shoulder and pelvic girdles that attach them to SHOBULLADDEER

the axial skeleton. It anchors the muscles muscles that

bring about conscious movement. HUMERUS

Inside a living bone VERTEBRAL COLUMN (SPINE) RIB RADIUS
Dense, compact bone is made up of ULNA
tiny tubes of bone (osteons). Spongy
bone has a honeycomb-like structure
that provides strength yet remains
relatively lightweight.

SPONGY BONE Elbow—called the funny
bone because knocking it
SACRUM PELVIS Femur—or thigh
traps the ulnar nerve, bone—the longest
which creates an electric bone, averaging one

shock sensation quarter of your
height as an adult
LIGAMENTS OF THE FOOT FEMUR

Fibula—helps
stabilize the ankle

Lightweight Strong, Bone FIBULA
spongy bone stretchy TIBIA
ligament

Natural foot strapping Heel bone anchors Tibia (shin
Bones are held together by bands the Achilles tendon bone)
of tough tissue called ligaments.
Nowhere are they more abundant HEEL BONE
than in the foot, which consists of
26 bones. Over 100 strong, elastic Skeleton in action
ligaments bind the bones The arms join to the vertebral column via the shoulder
together, allowing some flexibility girdle, which contains the collar bones and shoulder
and absorbing shock. They are blades. The legs connect to the vertebral column
resilient enough to limit the range through the pelvic girdle. The pelvis is made up of
of movement within each joint. three bones on either side, which are fused together.

Growing bones

A healthy baby measures 18–22 in NEWBORN BABY WEIGHT
(46–56 cm) in length at birth. Growth
is rapid during infancy as the long An average newborn baby
bones elongate. Bone growth slows weighs 51⁄2–91⁄2 lb (2.5–4.3 kg).
during childhood, but then speeds up Babies normally lose weight
again at puberty. Bones stop growing in the first days after birth,
at around 18 years of age, when final but by 10 days, most have
adult height is reached. regained their birth weight
and start to put on around
1 oz (28 g) per day.

How bones grow Articular cartilage
Cartilage growth
Growth in height occurs at special growth plates at the plate (epiphysis)
ends of the long bones. Bone growth is controlled by
growth hormone, with an additional growth spurt occurring
in response to sex hormones at puberty (see pp.223–23).
The cartilage growth plates fuse by adulthood, after
which no further increases in height are possible.

New bone formation (secondary
ossification center)

Cartilage Articular cartilage Cartilage growth Compact
plate (epiphysis) bone
Developing Medullary cavity
periosteum Medullary
Developing spongy (marrow cavity
formation)
bone (primary Compact bone containing
ossification center) bone marrow
Cartilage not yet Spongy bone
converted into bone

1 Embryo 2 Newborn baby 3 Child 4 Teenager
Bones initially form At birth, bones still In childhood, most of At puberty, a surge in
from soft cartilage that acts as consist mostly of cartilage, but the shaft consists of hardened
a scaffold on which minerals there are active sites of bone compact and spongy bone. sex hormones causes a rapid
growth spurt. Increases in
are laid down. Hardened formation (ossification). The Growth plates (epiphyses) at the height occur when new bone
bone starts forming when the first to develop is the primary ends allow lengthening. Bone is
fetus reaches 2–3 months of ossification centre in the shaft, still soft and can bend on impact is laid down at the cartilage
growth plates (epiphyses) to
development in the womb. followed by those at the ends. to form a greenstick fracture. lengthen the bone shaft.

38 39HOLDING IT TOGETHER
Growing bones

79 Bone growth
(200) Male average
Male upper 5% and lower 5%
Articular cartilage Female average
Fused growth plate Female upper 5% and lower 5%

(epiphyseal line)

Length/height / in (cm) Boys’ average adult
Proportion changeheight is greater
from birth to adultGirls reach their adult
height earlier than boys

39
(100)

02 8 12 18

0 5 10 15 20

Age (years)

Growth patterns

A baby’s head is one-quarter of his or her total body length. Changes in
relative growth means that by age two that ratio is down to one-sixth. An
adult’s head is only one-eighth of body length. Girls enter puberty earlier
than boys and reach their adult height around 16–17 years of age. Males
only reach their final height between the ages of 19 and 21.

HOW TO CALCULATE YOUR FINAL HEIGHT

Assuming both parents are of normal stature, a child’s potential adult height can
be calculated as follows. Add father’s height to mother’s height. For a boy, add
5 in (13 cm) and for a girl deduct 5 in (13 cm). Then divide the total by two. Most
children will have a final adult height within 4 in (10 cm) of this estimate.

+ 5 in ÷ 2=
(13 cm)
SON’S HEIGHT
Adult +
After puberty, the cartilage 2=
5
DAUGHTER’S HEIGHT
growth plates are converted into FATHER’S MOTHER’S - 5 in ÷
bone (calcified) and fuse. This HEIGHT HEIGHT (13 cm)
leaves a hardened area called the
epiphyseal line. Bones can still
increase in diameter, but can
no longer increase in length.

Flexibility

Your joints allow you to move your body and manipulate
objects. Movements can be small and controlled,
such as when writing your name,
or large and powerful, such as
when throwing a ball.

Joint structure Ellipsoidal
These complex joints involve a
A joint forms where two bones bone with a rounded, convex
come into close contact. Some end fitting into a bone with a
joints are fixed, with the bones hollow or concave shape. This
locked together, such as the allows a variety of movements,
suture joints in an adult skull. including sideways tilting, but
Some joints have a limited not rotation.
range of movement, such as
the elbow, while others can
move more freely, such as
the shoulder.

BONE Ball and socket
Found in the shoulders and hips, this
LIGAMENT Cartilage type of joint allows the widest range
Sinovial of movement, including rotation.
fluidBONE The shoulder joint is the most
mobile joint in the body.
SINOVIAL Gliding
JOINT These allow one bone
to slide over another
Inside a joint in any direction within
The bone ends within a one plane. Gliding joints allow
mobile joint are coated with the vertebrae to slide over each other
slippery cartilage and oiled when you flex your back. They are
with synovial fluid to reduce also found in your feet and hands.
friction. These synovial joints
are held together by bands
of connective tissue, called
ligaments. Some joints, such
as the knee, also have
internal stabilizing ligaments
to stop the bones from
sliding apart while bending.

40 41HOLDING IT TOGETHER
Flexibility

Saddle Types of joints
This is only found at the base of
the thumb and allows a similar Although your body as a whole moves in
but wider range of movement to complex ways, each individual joint has only
ellipsoidal joints, including a circular a limited range of movement. A few joints have
motion, but without rotation. a very limited amount of movement so that
Pivot they can absorb shock, such as where the
This allows one bone to rotate two long bones in your lower leg (tibia and
around another, for example fibula) meet or some of the joints in the feet.
when you move your forearm The temporomandibular joints (see pp.44–45)
to twist your palm to face up between your jawbone and each side of the skull
or down. A pivot joint in your are unusual in that they each contain a disk of
neck allows your head to turn cartilage that allows the jaw to glide from side to
from side to side. side and protrude forward and backward during
chewing and grinding your food.

THE SMALLEST JOINTS ARE
FOUND BETWEEN THE
THREE TINY BONES OF
THE MIDDLE EAR THAT
HELP TRANSMIT SOUND
WAVES TO THE INNER EAR

DOUBLE-JOINTED PEOPLE

People who are said to be double-jointed have the
same number of joints as everyone else, but their
joints have a wider than normal range of movement.
This trait is usually due to inheriting unusually elastic
ligaments or a gene that codes for
the production of a
weaker type of collagen
(a protein found in
ligaments and other
connective tissues).

Hinge
This type of joint mainly allows
movement in one plane, rather
like a door opening and closing.
Good examples are found in the
elbow and knee.

Biting and chewing SEINCCOINSODR
CANINE
Humans struggle to swallow large pieces of food so your PREMFPIORRSLSEEATMCROONLADR MFIORLSATR
teeth break down food as part of the first stage of digestion.
Teeth also play a role in speech—it would be difficult to
make the sound “tutt” without any teeth, for example.

From baby to adult

Your teeth are all present at birth as tiny buds deep within each
jawbone. The first “milk” teeth need to be small to fit within an
infant’s mouth. These teeth are shed during childhood as the
mouth enlarges, leaving more room for adult-sized teeth.

6–12 months SECOND
10–19 MOLAR
16–23
Third molar, or
9–18 “wisdom tooth”

23–33 Crown
Pulp cavity
BABY TEETH

Eruption of milk teeth ENAMEL
The 20 milk teeth usually start to appear DENTINE
between the ages of 6 months and 3 years,
although some infants have to wait a year.

6–8 years TOOTH ROOT GUM
7–9
9–12 Cementum,
a natural glue
10–12 holding the
11–12 tooth in its
6–7 socket
Periodontal
11–13 ligaments help
anchor tooth
17–21 ADULT TEETH in jawbone

Eruption of adult teeth Root canal
The 32 adult teeth appear between the ages Blood vessel
of 6 and 20 years and should last for the
rest of your life—even if you live to be 100. Tooth structure
Each tooth has a crown, above the gum, which is
MUCH LIKE A coated in hard enamel. This protects the softer
FINGERPRINT, dentine forming the tooth root. The central pulp
EACH PERSON cavity contains blood vessels and nerves.
HAS A UNIQUE
BITE IMPRESSION

42 43HOLDING IT TOGETHER
Biting and chewing

WHAT ARE
WISDOM TEETH?

INFCIIRSSOTR The last set of molars Infection
C IANNSICEINSCEOORND usually appear between the
ages of 17 and 25. It is thought Tooth enamel is the hardest
that they are called wisdom substance in the body, but readily
teeth because they appear dissolves in acid, exposing the underlying
parts of the tooth to bacteria and infection.
after childhood. Acid can come from some foods, juices, and
sodas, or from bacterial plaque, which
breaks down sugar to form lactic acid.

Cavity Region of tooth to Filling of
be drilled, to amalgam
remove decay

Different types DECAYING TOOTH TOOTH WITH
FILLING
Your teeth differ in shape
and size depending on Decay and filling
their use. Sharp-edged When the hard enamel dissolves, it allows infection
incisors cut and bite, to rot the softer dentine beneath. A cavity forms as
canines tear, and molars the weakened enamel overhead collapses.
and premolars have
flattened, ridged surfaces
that chew and grind food
into tiny pieces.

Bacteria and pus fill Crack lets
pulp cavity and in bacteria
root canals

ARE YOU A GRINDER? FLATTENED TEETH TOOTH WITH AN ABSCESS
AFTER TREATMENT Abscess
One in twelve people grind their If bacteria reach the pulp cavity, they may set up an
teeth while asleep, and as many as infection in a place that is difficult for the immune
one in five clench their jaws while system to tackle and lead to an abscess that can
awake. Known as bruxism, this spread to the jawbone.
weakens your teeth. You could be
a grinder if your teeth look worn
down, flattened or chipped, if your
teeth are increasingly sensitive, or if
you wake with jaw pain, a tightness
in your jaw muscles, earache, or a
dull headache—especially if you
also chewed the inside of your
cheeks. Worn-down teeth may
be reshaped with crowns.

The grinder

Your jaws are powered by strong muscles that produce
considerable pressure as you cut and grind food with your
teeth. The lower jaw can withstand these forces because
it is the hardest bone in your body.

How we chew From side Forward and
to side backward
Chewing is a complex motion in JAW MOVEMENT
which the temporalis and masseter Up and down
muscles control movement of the
jaw back and forth, up and down,
and side to side. This grinds food
between the back molars like
a pestle and mortar. The flexibility
of the joints in our jaws allow us to
slide effortlessly between chewing
movements, depending on what
we are eating.

WHEN WE ATE LEAVES How the jaw works

Once, our primitive ancestors had smaller The two temporomandibular joints
skulls and a chewier diet, rather like today’s between the lower jawbone and the
gorilla, pictured. Their powerful jaw skull each contain a disk of cartilage that
muscles were anchored by a tall, sagittal provides a wider range of movement than
crest along the top of the skull. This acted is possible in other hinged joints, such
in a similar way to the breastbone of a as the elbow and knee. This disk is what
bird, which anchors its giant flight muscles. allows the jaw to glide from side to side and
move forward and backward when talking,
Sagittal crest chewing, or yawning.

GORILLA SKULL WHAT CAUSES A
CLICKING JAW?

If the protective disk of
cartilage is displaced forward,
you may have a clicking jaw.

The lower jawbone clicks
against the zygomatic
arch as you chew.

44 45HOLDING IT TOGETHER
The grinder

Temporalis tendon attaches to the cranium 975
with hundreds of extensions of the tendon’s
collagen fibers, which perforate the bone (442 KG) THE POUNDS
and anchor the muscle

Temporalis muscle
forms a thin sheet over
the side of the skull

CRANIUM OF FORCE THAT THE
MASSETER MUSCLE CAN
TEMPORALIS TENDON Chewing muscles EXERT DURING A BITE

TEMPORALIS MUSCLE attach to the front
and back of the
cheek bone
Cartilage disk in C

temporomandibular joint LOSED

Condyloid
process of lower

jawbone sits in
its socket

Cartilage ZYGOMATIC ARCH Mouth shut
disk The cartilage disk within the temporomandibular
joint sits in a socket in the skull and wraps
MASSETER MUSCLE around a knob on the lower jawbone called the
condyloid process. The disk cushions the joint
and prevents the jawbone from grinding against
the skull bones when you chew.

Cartilage disk slides forward

Pterygoid UPMPEARXJIALLWAB, OONRE OPEN
muscle pulls

hinge joint
open when

using jaw

Masseter muscle can LOMWAENRDJAIBWLBE,OONRE Condyloid process
close the jaw with rocks forward
great force out of socket

Jaw muscles Mouth gaping
The chewing muscles are attached Both the lower jaw and the cushioning disk of cartilage can rock
to the skull. The strong temporalis forward out of their socket, allowing your lower jawbone to hang
and masseter muscles control the open. Three fingers should fit between your upper and lower teeth.
jaw as it grinds, snaps, and closes.

Skin damage WHY DO
SCABS ITCH?
Damaged skin, whether it is a superficial scrape
or a cut that penetrates deeper into the skin, During healing, when the
lets infection enter the body. It is therefore cells move around the base of
important for healing to occur quickly, to the wound, they begin to contract,
prevent infections from spreading. which helps stitch the skin back
together. As the tissues shrink,
Wound healing they stimulate specialized itch-

When the skin is breached, the first important step for the sensitive nerve endings.
circulatory system is to stem bleeding from a cut, or weeping Try not to scratch the
fluid loss from a burn or blister. Some wounds need medical scab off, though!
attention to seal them more firmly with stitches, butterfly
bandages, or tissue glue. Covering the wound with a dressing Dry scab of fibrin,
will aid healing and reduce the chance of infection. platelets, and dead
blood cells
Fibrin thread Itch-receptor
Platelet WOUND Red blood cell caught nerve cell is
by fibrin threads stimulated

Histamine triggers Damaged Collagen fiber
inflammation cell produced by
and swelling fibroblast
Widened Granulocyte Skin cells
blood vessel regrowing
Immune
protein

Fibroblast Blood vessel

1 Clotting and inflammation 2 Skin cells proliferate
Platelets, which are fragments of blood cells, clump together to Proteins called growth factors attract fiber-producing cells
form a clot. Clotting factors form fibrin threads, which hold the clot in (fibroblasts), which move into the wound. They make granulation
place. Inflammation floods the area with granulocytes and other cells tissue, which is rich in tiny new blood vessels that grow into the area.
and proteins of the immune system, which attack invading microbes. Skin cells multiply to heal the wound from the base and sides.

46 47HOLDING IT TOGETHER
Skin damage

WET AND DRY HEALING Burns

When exposed to the air, a scab hardens so new skin If skin is heated above 120°F (49°C), its
cells have to burrow underneath and dissolve it away. cells are damaged to cause a burn. Burns
Modern dressings help keep a wound moist so skin cells can also result from contact with chemicals
can leapfrog across the moist wound surface. This helps and electricity.
wounds heal more quickly, with less pain, less risk of
infection, and less scarring. EPIDERMIS 1st degree burn
Only the top layer of skin is
DRY HEALING Dry dermis DERMIS injured, causing reddening
Scab HYPODERMIS and pain. Dead cells may
Epidermis peel after a few days.
WET HEALING Skin growing
Dressing deep beneath 2nd degree burn
dry wound Cells in the deeper layers are
destroyed and large blisters
Body fluid seeps out and form. Enough live cells may
keeps wound moist remain to prevent scarring.
3rd degree burn
Skin cells The full skin thickness is
taking shortcut burned and skin grafts may
straight across be needed. There is a risk
Wound bed of scarring.

Scar tissue Blisters

A combination of heat, moisture, and Blister
friction may cause layers of skin to
Repaired tissue separate from each other and form a
fluid-filled bubble, which protects the
Granulation tissue damaged skin. Covering them with a
is new connective hydrocolloid gel blister bandage will soak
tissue that forms to up the fluid and form a cushioning,
antiseptic environment
fill in the wound so that the blister
can heal faster.

Acne Sebaceous Blackhead
gland
Sebaceous glands release Sebum
Hair root
oil (sebum) onto the skin

and hair. When the glands

produce an excessive amount

of sebum, the hair follicle can

become clogged with sebum

and dead skin cells to form a

3 Remodeling blackhead. Skin bacteria can
The surface skin cells have completed their job of growing
over the damaged area and converting the scab into scar tissue. infect the plug to cause a

The scar shrinks to leave a red area that slowly becomes paler. pimple or cyst, which can

Granulation tissue remains for a while. leave a scar when it heals.

Breaking and mending

A fracture is a break in a bone, which commonly results from an accident
such as a fall, a traffic accident, or a sports injury. Some fractures are
relatively minor dents or hairline cracks that heal quickly, while severe
impacts can shatter a bone into more than three pieces.

OPEN CLOSED Immature bones are not fully mineralized
FRACTURE FRACTURE and their bone may split on one side when

bent, rather than breaking in two. This is
known as a greenstick fracture and is often

seen when a child falls out of a tree!

Also known as a In a closed fracture, GREENSTICK
compound fracture, an the skin remains intact. FRACTURE
open fracture is a nasty
injury in which the skin It is also known as a Spiral Fracture
is punctured either by simple fracture. The A spiral fracture
injury is more likely to winds around the
the broken bone or remain relatively sterile shaft of a long bone
by the impact that and avoid infection. rather than breaking
caused the injury. This Often, all that’s needed across. It results
means that infection is a cast to keep the from a twisting force
can get in, so antibiotics bone still in the correct such as when a
position for healing toddler lands on an
are usually given outstretched leg
when jumping.
A comminuted fracture occurs
when a bone shatters into three or
more pieces. This may need surgery
to insert a plate and screws to hold

loose bone fragments in position
for stable healing

A compression injury may cause the THE BONE IN YOUR NOSE
fractured ends of bone to collapse into
COMMINUTED Pinch your nose with your fingers
one another and shorten the bone. FRACTURE and you will feel where the bone in
The fracture must be stretched by the bridge of the nose is connected
SPIRAL to cartilage at the tip. When you
traction—a gentle, steady action FRACTURE break your nose, it’s the bone at
to pull the bones apart the top that gets fractured.

Types of fractures

Bones may be broken by impacts and

crushing, but also by repeated stress, Bone in the
bridge can
such as marathon running. In young
fracture
people, the most common broken bones BONE

are elbows and upper arms, which are Cartilage is
flexible and
often broken during play, or lower leg COMPRESSION bends with
FRACTURE
bones, often injured in sports and other impacts

activities. Older people with brittle bones CARTILAGE

affected by osteoporosis (see p.50) are

more likely to fracture hips and wrists.