supported by a thin layer of loose CT (basement membrane)
Blood pumped out of the heart is at a very high pressure, so the structure of the arteries must be adapted to this. They can withstand high pressure by having very thick walls made up of elastic fibres and smooth muscle. The elastic fibres increase its elastic strength. The smooth muscle can change the diameter of the lumen, especially in the smaller arteries. (higher students The smaller arteries and arterioles are more muscular and can contract ( vasoconstriction ) to close off the capillary beds to which they lead; or relax ( vasodilation ) to open up the capillary bed. ) . Lumen is relatively narrow. The tunica media layer is thicker in arteries than in veins tunica intima In arteries this layer is composed of an elastic membrane lining and smooth endothelium that is covered by elastic tissues.
carry blood from every tissue in the body to the heart.
A blood vessel is a tube with a space in the centre, called the lumen.
So blood leaves the heart through arteries, which in turn give way to arterioles and then capilliaries where the oxygen is delivered and exchanged for carbon dioxide. The capillaries then become larger venules and then veins which then deliver the blood back to the heart.
Now lets have a look at the structure of these vessels There are three layers of tissue in both arteries and veins 1 - tunica intima 2 - tunica media 3 - tunica externa (or adventitia) The capillaries are made of a single layer of endothelial cells.
Capillaries are where the transported substances actually enter and leave the blood No exchange of materials takes place in the arteries and veins, whose walls are too thick and impermeable. Capillaries are very narrow and thin-walled, but there are a vast number of them (10 8 m in one adult!), so they have a huge surface area : volume ratio, helping rapid diffusion of substances between blood and cells. Capillaries are arranged in networks called capillary beds feeding a group of cells, and no cell in the body is more than 2 cells away from a capillary.
Capillary diameter is very small. A typical small artery or vein may be 5mm (in diameter in a human) But a capillaries are approx 10 micrometer about the size of a red blood cell (erythrocyte)
A realistic representation of the major blood vessels of the systemic circuit shows how the systemic arteries and veins are arranged in the body.
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