AQA AS Biology

Revision Notes

3.5.6 Blood Vessels

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)

Comparing arteries and veins, IGCSE & GCSE Biology revision notes

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

Exam 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.”

Author: Lára

Lára graduated from Oxford University in Biological Sciences and has now been a science tutor working in the UK for several years. Lára has a particular interest in the area of infectious disease and epidemiology, and enjoys creating original educational materials that develop confidence and facilitate learning.
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