Studyledge

Transport in Mammals: Question 1

Syllabus 8.1

Multiple choice AS 1 mark

A cross-section of the aorta, close to where it leaves the heart, shows a thick wall containing a large proportion of elastic tissue and smooth muscle in its middle layer.

Which statement best explains the function of this elastic tissue in the aorta wall?

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

Worked solution

Step 1: Recall the conditions inside an artery close to the heart

Blood leaving the left ventricle is under very high, but pulsatile, pressure: pressure surges upward during ventricular systole and falls (though it does not reach zero) during ventricular diastole. A vessel carrying this blood therefore needs a wall that can cope with large, repeated changes in pressure rather than a constant, low pressure.

Step 2: Recall what elastic tissue actually does

Elastic tissue (elastic fibres) in the tunica media of an artery wall can stretch when pressure inside the vessel rises, and then recoil (spring back) as pressure falls. During ventricular systole, the surge of blood entering the artery stretches the elastic tissue, temporarily storing some of the energy of the heartbeat as elastic energy in the stretched wall. During ventricular diastole, the wall recoils inward, continuing to push blood onward even while the heart itself is relaxed and not actively pumping. The overall effect is to smooth out the pulsatile output of the heart into a less variable, more continuous flow by the time blood reaches the smaller arteries and arterioles.

Step 3: Evaluate each option

  • A: correct. This is exactly the stretch-and-recoil role of elastic tissue described above.
  • B: incorrect. Arteries do not contain valves along their length (valves are a feature of veins and of the heart itself), and elastic tissue does not contract rhythmically; that description instead resembles how contracting skeletal muscles around veins help push blood back to the heart.
  • C: incorrect. Reduced friction along the lumen is a function of the smooth endothelium lining the vessel, not of the elastic tissue in the middle layer of the wall.
  • D: incorrect. Arteries are not normally a site of exchange with surrounding tissues (that role belongs to capillaries), so preventing diffusion is not the function of the arterial wall; the thick, elastic wall exists to withstand and smooth out high pressure.

Final answer

  • The elastic tissue in the artery wall stretches and recoils with each heartbeat, smoothing the pulsatile flow of blood from the heart, option A.