2.65 describe the structures of arteries, veins and capillaries and understand their roles

Arteries

Arteries carry blood away from the heart. Their walls are strong, elastic and thick as the heart pumps blood out at high pressure. The lumen (hole in the middle) is quite small. Arteries also contain thick muscle layers to make the arteries strong and retain their shape.

NOTE: the largest artery is the aorta (taking oxygenated blood from the left ventricle to the rest of the body)

Capillaries

Capillaries branch off arteries and are involved in the exchange of materials (such as food, oxygen and waste) at tissue cells etc. They carry blood really really close to every cell in order for the cell to exchange substances with the blood (by diffusion). In order to make substance exchange quick and easy, capillaries are only 1 cell think and have permeable walls. Permeable walls are needed so substances can diffuse in and out, and having walls 1 cell think is useful as it will increase the rate of diffusion by decreasing the distance over which diffusion occurs.

Capillaries supply cells with food and oxygen and they remove away all waste from cells, such as carbon dioxide.

Veins

Arteries branch into capillaries, and capillaries eventually join together forming veins. Veins take blood to the heart, this means the blood is at not as high pressure than in arteries, therefore the walls of veins do not have to be as thick. Despite the low pressure the blood is under, veins have larger lumen (than arteries) to help encourage blood flow. Veins also have valves to stop the blood flowing backwards (as the blood is under low pressure it is not really forces through the veins, it kind of needs a little encouragement).

NOTE: the largest vein is the vena cava (takes deoxygenated blood from the body into the right atrium)