Arteries, Arterioles & Veins
- The body contains several different types of blood vessel:
- Arteries: transport blood away from the heart (usually at high pressure)
- Veins: transport blood to the heart (usually at low pressure)
- Arterioles: arteries branch into narrower blood vessels called arterioles which transport blood into capillaries
- The walls of each type of blood vessel have a structure that relates to the function of the vessel
- Blood flows through the lumen of a blood vessel; the size of the lumen varies depending on the type of blood vessel (with arteries having a narrow lumen, and the veins a wider one)
The blood vessels form a continuous network; the structure of each allows it to carry out its function.
How structure relates to function
- Arteries must be able to withstand high pressures generated by the contracting heart, and maintain these pressures when the heart is relaxed
- The wall of the artery is relatively thick with layers of collagen, smooth muscle and elastic fibres
- The elastic fibres allow the artery wall to expand around blood surging through at high pressure when the heart contracts, these fibres then recoil when the heart relaxes – this alongside a narrow lumen maintains high blood pressure
- In contrast, veins receive blood that has passed through capillary networks; blood is at very low pressure and must be returned to the heart
- The wall of the vein is relatively thin with thinner layers of collagen, smooth muscle and elastic fibres
- The lumen of the vein is much larger than that of an artery
- Veins contain valves that prevent the backflow of blood, helping return blood to the heart
- Arterioles can contract and partially cut off blood flow to specific organs
- Eg. During exercise blood flow to the stomach and intestine is reduced which allows for more blood to reach the muscles
- Unlike arteries, arterioles have a lower proportion of elastic fibres and a large number of muscle cells
- The presence of muscle cells allows them to contract and close their lumen to stop blood flow
Examiner Tip
For “Explain” questions, remember to pair a description of a structural feature to an explanation of how it helps the blood vessel to function. For example, “Capillaries are one-cell thick, which enables quick and efficient diffusion of substances.”
