(c) the structure and functions of arteries, arterioles, capillaries,…

The idea

Blood vessels are adapted to their position in the circulatory system. Arteries carry blood away from the heart at high pressure: thick muscular walls with elastic fibres stretch during systole and recoil during diastole, smoothing flow, while a narrow lumen maintains pressure. Arterioles are small arteries whose smooth muscle controls lumen diameter; vasoconstriction and vasodilation redistribute blood to capillary beds. Capillaries have a single endothelial cell layer and narrow lumen, giving a short diffusion distance and large surface area for exchange; their walls are permeable to plasma and tissue fluid. Venules collect blood from capillaries and merge into veins. Veins return blood at low pressure: thin walls, a wide lumen and valves prevent backflow, aided by skeletal muscle action during movement.

What earns the marks

• Arteries: thick muscular and elastic walls withstand and smooth high-pressure flow from the heart; narrow lumen helps maintain pressure.
• Arterioles: smooth muscle in the wall changes lumen diameter, so vasoconstriction and vasodilation direct blood to different capillary beds.
• Capillaries: one-cell-thick endothelium and narrow lumen give a short diffusion distance and large surface area for exchange between blood and tissues.
• Venules: small vessels that collect blood from capillary beds and carry it towards veins at low pressure.
• Veins: thin walls, wide lumen and valves prevent backflow of low-pressure blood, assisted by skeletal muscle contraction during movement.

Common mistakes and how to put them right

• Assuming arteries always carry oxygenated blood.

  Put it right: The pulmonary artery carries deoxygenated blood, so identify vessels by direction of flow relative to the heart, not by oxygen content.

• Confusing arterioles with capillaries.

  Put it right: Arterioles are muscular resistance vessels controlling flow, whereas capillaries are thin-walled exchange vessels.

• Stating that veins have no muscle or that valves push blood.

  Put it right: Valves only prevent backflow, and return depends on skeletal muscle action and pressure changes.

Exam tips

• For each vessel, pair a named structural feature with the function it performs rather than listing features alone.
• Use comparative language (thicker, narrower, wider, thinner) when distinguishing artery, capillary and vein structure.
• Check whether a question asks about direction of blood flow or oxygen content before choosing an answer.