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134 Human Body

WHY DO WE HAVE WHITE BLOOD CELLS?

White blood cells destroy germs and

damaged tissue. A drop of blood contains

about 7,000 white cells, and there are many

different types. Some patrol the body like

soldiers, swallowing germs. Others produce

chemicals, called antibodies, which stick

to germs and so make them easier to kill.

A liquid tissue, blood consists of trillions of cells suspended in a

watery liquid called plasma. Blood is the body’s transport system,

keeping all tissues and organs supplied with the chemicals needed

for life and removing waste. Plasma makes up 55 per cent of

blood, and red cells around 44 per cent. White cells account for

less than 1 per cent of blood.

WHAT DO RED BLOOD CELLS DO?

Red blood cells pick up the life-giving gas oxygen in the

lungs and release it throughout the body. A single drop

of blood contains about five million of these tiny

cells. Each cell is packed with the bright red

protein haemoglobin, which binds with oxygen

and then releases it where it is needed.

WHAT ARE CAPILLARIES?

About 98 per cent of blood vessels

are microscopically thin vessels, called

capillaries, that form a network between

arteries and veins. Their walls are only one

cell thick, so that chemicals can pass between

the blood and the body’s tissues.

The heart,. BLOOD, and blood

vessels make up the circulatory

system, which supplies the body with

oxygen and nutrients, removes waste,

distributes heat, and fights disease.

ARE ARTERIES AND VEINS DIFFERENT?

Arteries carry blood from the heart,

and veins bring it back. Arteries

have thicker walls than veins, to

withstand the force of the blood

pumping directly out of the heart.

FIND OUT MORE. Respiration 136 • Skeleton 130–131

4 BLOOD CLOT

When the skin is cut, a tough protein called fibrin

(shown in grey) forms a tangle of solid fibres that

traps blood cells and seals the wound.

BLOOD

BLOOD CELLS 3

An adult has about 5 litres (11 pints) of

blood. Blood cells are made inside bones,

and every second, 2 million red blood cells

are made and the same number are destroyed.

Circulation

White blood cells

are round with tiny

projections on the

surface to help

them stick to germs

Red blood cells

are flexible so they

can squeeze through

capillaries

The vena

cava is the

main vein

taking

blood to

the heart

Arteries

are shown

in red

Heart

The aorta is

the body’s

biggest artery

4 CIRCULATORY SYSTEM

The heart pumps blood continually

around the circulatory system. Arteries

and veins divide into ever finer branches

as they reach throughout the body.

Veins

are shown

in blue

circulation