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Transport in Animals: Question 9

Syllabus 9.3

Structured Extended 6 marks

During an operation, a surgeon needs to identify whether a blood vessel of a certain diameter is an artery or a vein, without being able to observe the direction of blood flow directly. The surgeon instead examines the thickness and composition of the vessel wall and the width of the space inside it.

(a) State two differences between the wall of an artery and the wall of a vein of similar overall diameter. [2]

(b) Explain how the thick wall of muscle and elastic fibres in an artery relates to its function. [2]

(c) Explain how the relatively wide lumen of a vein relates to its function. [2]

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Worked solution

Part (a): Structural differences

Compared with a vein of similar overall diameter, an artery has a thicker wall, containing more muscle and elastic fibres. Because much of the artery’s overall width is taken up by this thick wall, the space inside it (the lumen) is narrower than that of the vein, whereas the vein has a wider lumen.

Part (b): Why arteries have a thick, elastic muscular wall

Every time the heart’s ventricles contract, blood is forced into the arteries in a surge at high pressure. The thick wall of muscle and elastic fibres allows the artery to withstand these repeated high-pressure surges without bursting. The elastic fibres also stretch as each surge of blood passes and then recoil afterwards, which helps to smooth out the pressure between heartbeats and keeps the blood moving onwards even while the heart is relaxed.

Part (c): Why veins have a wide lumen

By the time blood has passed through the capillaries and reached the veins, it has lost most of its pressure, so it no longer surges and does not need a thick wall to contain it. Instead, the vein’s relatively wide lumen offers little resistance to the flow of this low-pressure blood, making it easier for blood to keep moving through the vein and back towards the heart.

Final answers

  • (a) Any two of: artery wall thicker; more muscle/elastic fibres in artery wall; artery has a narrower lumen, vein has a wider lumen.
  • (b) The thick muscular, elastic wall withstands high-pressure surges from the heart, and elastic recoil smooths the pressure between heartbeats.
  • (c) The wide lumen offers little resistance to the already low-pressure blood, helping it continue flowing back to the heart.