Explain how the structure of the blood vessels is related to their function

The idea

Blood vessels are tubes whose wall structure matches their job. Arteries carry blood away from the heart at high pressure, so they have thick muscular and elastic walls; the elastic recoil helps maintain pressure between heartbeats and the muscle adjusts flow. Arterioles are narrower arteries that control flow into capillary beds. Capillaries are one cell thick, giving a short diffusion distance for exchange of oxygen, glucose and carbon dioxide between blood and tissue. Veins carry blood back to the heart at low pressure, so they have thinner walls and wider lumens, and valves prevent backflow.

What earns the marks

• Arteries carry blood away from the heart at high pressure and have thick muscular and elastic walls; elastic recoil helps maintain pressure between heartbeats.
• Arterioles are narrow arteries that control the flow of blood into capillary beds.
• Capillaries have walls one cell thick, giving a short diffusion distance for exchange of substances between blood and tissues.
• Veins carry blood back to the heart at low pressure, have thinner walls and wider lumens, and contain valves that prevent backflow.

Common mistakes and how to put them right

• Thinking arteries always carry oxygenated blood.

  Put it right: That arteries carry blood away from the heart, which is not the same as oxygenated.

• Assuming veins have thick walls to withstand pressure.

  Put it right: Veins have thinner walls and wider lumens because blood returns at low pressure.

• Saying capillaries have thick walls for strength.

  Put it right: Their walls are one cell thick to shorten the diffusion distance for exchange.

Exam tips

• Read the question to see whether it asks about structure, function or the link between them, then choose the option that states both correctly.
• Eliminate options that swap artery and vein features, such as thick walls on veins or valves in arteries.
• For capillary questions, look for the phrase about a short diffusion distance or one-cell-thick walls.