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Transport in Mammals: Question 7

Syllabus 8.1

Multiple choice AS 1 mark

A vein returning blood from the leg to the heart has a much wider lumen relative to its overall diameter than an artery of similar overall diameter, a relatively thin wall containing little smooth muscle or elastic tissue, and a series of semilunar (pocket) valves at intervals along its length.

Which statement best explains why veins have a wide lumen and valves?

Choose an answer to check it, then compare with the worked solution below.
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Worked solution

Step 1: Recall the conditions inside a vein

By the time blood has passed through a capillary bed and entered a vein, it has lost most of the pressure it had when it left the heart: pressure in veins is low and fairly steady, rather than high and pulsatile as in arteries. Blood in the veins of the limbs must often flow upward, against gravity, to get back to the heart.

Step 2: Recall what a wide lumen and valves actually do

A wide lumen offers relatively little resistance to flow, which helps compensate for the low pressure driving blood through veins. Since this low, steady pressure alone is not always enough to keep blood moving forward, particularly against gravity in the legs, veins also contain semilunar valves at intervals along their length. These valves are pushed open by blood flowing toward the heart but close if blood begins to flow backward, so they prevent backflow and allow the intermittent squeezing action of nearby skeletal muscles (the skeletal muscle pump) to push blood steadily toward the heart in one direction only.

Step 3: Evaluate each option

  • A: correct. This is exactly the low-resistance, backflow-prevention role of the wide lumen and valves described above.
  • B: incorrect. Veins do not experience high, pulsatile pressure surges (that is a feature of arteries close to the heart), and valves do not regulate blood entering capillaries.
  • C: incorrect. Veins are not a site of exchange with surrounding tissues; that role belongs to capillaries, whose thin walls and large total surface area suit them for exchange.
  • D: incorrect. Vein walls contain little smooth muscle and do not contract rhythmically to pump blood; venous return instead relies mainly on external compression by contracting skeletal muscles, with the valves only preventing backflow.

Final answer

  • The wide lumen reduces resistance to low-pressure venous flow, and the valves prevent backflow so blood continues moving toward the heart, option A.