IB Biology SL

Revision Notes

6.2.2 The Blood System: Vessels

Arteries, Veins & Capillaries: Structures & Functions

  • Arteries, veins and capillaries all have varying structural features

Blood vessels structure & function table

4. Blood vessels structure & function table, downloadable AS & A Level Biology revision notes

Worked example: Blood vessel features

Blood vessels structure & function WE table, downloadable AS & A Level Biology revision notes

The correct answer is B.

This is because elastic arteries do not contain many smooth muscle fibres and are not able to perform vasoconstriction or vasodilation. In contrast, muscular arteries contain much more smooth muscle. Veins have large amounts of collagen for strength and keeping their shape, while capillaries are only one-cell thick and so do not contain any elastic, smooth muscle or collagen.

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.”

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.”

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