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Flowing all around your body, blood delivers
food, oxygen, and other essentials to trillions
of cells and removes their wastes. Blood also
distributes heat around your body and defends
it against infection. Blood is made up of a yellow
liquid called plasma in which blood cells float.
Red blood cells pick up oxygen in the lungs
and deliver it to your body’s cells. White blood
cells defend the body against disease-causing
germs. Blood also contains platelets, which
enable it to clot to seal damaged blood vessels.
The heart pumps blood around the body
along three types of blood vessels. Arteries
carry blood away from the heart, while
veins return blood to the heart. Microscopic
capillaries link arteries and veins
and supply blood to cells.Blood
Circulating around the
average body are 5 litres
(8.8 pints) of blood.Oxygen-rich blood in
arteries is bright red
in colour. Oxygen-poor
blood in veins is dark
purple-red.Plasma is 90 per cent
water and contains
more than 100 different
dissolved substances
including food, waste,
hormones, and salts.White blood cells
called neutrophils
and macrophages eat
germs. Lymphocyte
white blood cells
disable germs by
releasing chemicals
called antibodies.Unravelled and
stretched out, one
adult’s blood vessels
would encircle Earth
twice. Capillaries
would make up
98 per cent of the
total length.Blood is 55 per cent
plasma and 45 per cent
blood cells.One drop of blood
contains 250 million
red blood cells, 375,000
white blood cells, and
16 million platelets.Each as broad as a
thumb, the largest
artery and vein (the
aorta and vena cava)
are 2,500 times wider
than a capillary, which
is just one-tenth the
width of a hair.072_073_Blood.indd 72 03/01/19 12:09 PM