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Human Body^149

FIND OUT MORE. Hormones 147 • Skeleton 130–131 • Skin 133 • Teeth 143

It takes nine months for an embryo to grow into

a baby inside its mother’s uterus (womb). After birth, the

baby keeps growing for another 20 years, changing all

the time. Growth is quickest in the first few years of life.

WHAT HAPPENS WHEN PEOPLE GROW OLD?

After early adulthood, the body gradually begins

to decline. The skin loses its elasticity, muscles get

weaker, and internal organs become less efficient.

Certain diseases become more common as we get

older, including heart disease and cancer. Aging is

a very slow process, and most people lead active

lives well into their 70s and 80s.

HOW DOES A BABY GROW?

An embryo begins life as one cell. It divides

repeatedly, changing shape to form a ball of cells.

After four weeks it has the early stages of a brain

and spinal cord. At six weeks it has the flap-like

beginnings of limbs and its heart starts beating.

At 12 weeks it looks like a miniature baby.

WHAT HAPPENS AS CHILDREN GROW?

Children change shape as they grow because

parts of the body grow at different speeds. The

brain grows quickest at first, which is why babies

have such large heads. Muscles and bones grow later

on. The rate of growth slows down during childhood,

but shoots up again at puberty.

HOW DO GIRLS CHANGE AT PUBERTY?

The period when a child changes into an adult is

called puberty. For girls, puberty usually begins

between 10 and 12 years. They grow taller, their

breasts develop, and their hips widen. Hair grows

under the arms and around the groin. Girls’ ovaries

start to release ova each month and they have periods.

HOW DO BOYS CHANGE AT PUBERTY?

Boys go through puberty later than girls — between

12 and 14 years. They shoot up in height, and their

shoulders and chests get broader. Hair starts to grow

on the face, under the arms, around the groin, and

sometimes on the chest. The voice gets deeper and the

testes start to make sperm.

AT BIRTH

A baby’s brain has a full set of brain cells, but

there are few connections between them. The

brain grows quickly in the first two years.

AT SIX YEARS

The brain is nearly adult size, but continues

to change and learn by growing new

connections between the brain cells.

AT 18 YEARS

The brain is fully formed and brain cells have

formed billions of complex connections, but

we still continue to learn.

AGING SKIN 3

These hands of a child and an elderly person show the effects

of aging on the skin. The outer layer of skin becomes thinner

and loses the protein fibres that make it elastic, causing it

to wrinkle. Harmless dark spots also often appear, the

result of the overproduction of melanin.

MILK TEETH 1

A human has two sets of teeth in

their lifetime. At about six

months old, the first set, called

milk teeth, begin to appear. Milk

teeth start to fall out when we

are about six years old as

permanent teeth, shown here in

green, grow up beneath them. In

adult life, most people have a full

set of 32 permanent teeth.

ACNE BACTERIA 3

During puberty, many people get

a rash of reddish spots, called

acne, especially around the face.

Acne develops because the sex

hormones released during

puberty make the skin oilier.

Bacteria get trapped in plugs of

oil at the base of hairs, making

the skin become inflamed.

1 OSTEOPOROSIS
A natural part of aging is a
condition called osteoporosis,
which makes bones become
weaker and more brittle. Some
people with osteoporosis shrink in
height or become hunched
because the bones of their spine
lose shape and crumble.

This vertebra

(coloured red) has shrunk

and become wedge-shaped

Gaps between the

bones of a baby’s

skull allow the

head to grow

BRAIN AND SKULL DEVELOPMENT

This bacterium

is multiplying

by dividing

in two

The facial bones

are now mature

Skull plates

are now fixed

at the sutures

NEURAL NETWORK

NEURAL NETWORK NEURAL NETWORK

Neuron

Growth

growth