Arteries, capillaries and veins
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ARTERIES, CAPILLARIES AND VEINS
Your
heart and blood vessels make up your blood system.
The
rest of the blood system is made up of tubes called blood vessels.
Arteries
carry blood away from the heart. The heart pumps blood at high pressure
into the arteries. The artery walls are made of muscle and elastic tissue.
They stretch when blood is pumped in, then contract, squirting it along.
When arteries get to an organ in your body, they branch into smaller tubes
called arterioles, and then arterioles branch many times into tiny tubes
called capillaries.
Capillaries
are the smallest branches. They have thin walls, to allow chemicals like
oxygen and food pass from the blood to the body cells, while carbon
dioxide and waste chemicals pass from the body cells to the blood.
Capillaries then join up to form larger tubes called venules, and venules
join up to form veins.
Veins carry blood back to the heart. They have thinner walls than arteries because the blood is at a lower pressure. Veins have valves to keep blood going in the right direction.
Text
adapted from:
Gareth
Williams, "Biology for you" - Nelson Thornes
http://www.arrowvale.worcs.sch.uk/sportscollege/blood.htm
http://www.bbc.co.uk/schools/gcsebitesize/biology/humans/circulationrev3.shtml