The role of blood vessels is to carry
blood and its constituents around the body. There are 3 types of blood vessel:
- Arteries - Are muscular tubes with thick walls. They are under considerable pressure from the force of the pumping action of the heart. There are three components to artery walls:
- the inner lining, or endothelium
- the muscular, elastic middle section of fibres
- the inelastic outer covering
The muscles contract to squeeze blood along the arterial passageways away from the heart. Blood is prevented from travelling back to the heart by the pulmonary and aortic valves in the ventricles. The arteries themselves contain no valves, as the blood travels under high pressure.
The pulmonary artery carries de-oxygenated blood away from the heart towards the lungs. The aorta carries oxygenated blood away from the heart. It divides and sub-divides like the branches of a tree, so that the arteries become smaller and smaller as they reach every part of the body.
- Veins - In contrast to arteries, veins have thin walls and little ability to contract. Their job is to carry the de-oxygenated blood back to the heart (apart from the pulmonary vein, which carries re-oxygenated blood from the lungs to the heart). Like arteries, they branch and divide-or rather, since the flow is in the opposite direction, the smallest veins (called venules) unite into larger ones, which in turn form into still larger veins, just like a river. Veins contain one-way valves thoughout their length, which prevent blood from flowing in the wrong direction.
- Capillaries - Are thin-walled networks of tiny blood vessels at the ends of the arterioles and venules. They are the smallest of the blood vessels, and do not have muscles or valves.
In most tissues, capillaires allow:
The Capillaires in the lungs do the opposite, they:
- transfer oxygen fron the lungs to the blood, and
- pass carbon dioxide to the lungs to be exhaled.
The walls of capillaries are so thin that the constituents of blood (water, oxygen, carbon dioxide, dissolved foods, and other substances) are able to diffuse between the cells of body tissues. Because there is a dense capillary network throughout the body, no single cell is far from a supply of food and oxygen.