A level Biology exam revision resources written by A level Examiners


The blood vessels are the tubes through which blood is transported round the body. They are often split into four or five groups for classification, though in reality from the aorta and pulmonary artery blood passes through successively smaller and smaller blood vessels until it enters the capillary beds. From here it then passes through vessels with successively larger lumens until it enters the venae cava or the pulmonary vein.

The five main classes of blood vessel are; arteries, arterioles, capillaries, venules and veins.


Are vessels with thick walls containing many smooth muscle & elastic fibres, and a small lumen (bore). Arteries distribute blood after the heart has pumped it. The blood flow is pulsile through arteries and the elastic recoil of their walls when the heart is in diastole helps maintain blood pressure.


Are the smallest arteries that can control blood flow to the capillary beds they supply. Blood flow changes from pulsile to smooth in the arterioles.


These vessels have walls that are one cell thick. Exchange between blood and tissue fluid occurs here. Blood flow is at its slowest through the capillaries, because they are the narrowest vessels which means they have the greatest friction which restricts blood flow. This slow velocity aids the exchange of materials.


Are the smallest veins and their structure is similar to veins


Veins are vessels with thin walls and a large lumen, which means there is less resistance to blood flow than in capillaries and venules, and therefore the velocity of blood in veins increases. Veins contain pocket valves, which prevent the backflow of blood (which could happen in veins because the pressure of blood is very low here).

Figure 6 : Veins structure
Blood Vessels

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