Question

During heavy exercise the cardiac output (CO) increases up to five fold while pulmonary arterial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by:

• A. Increase in the number of open capillaries
• B. Sympathetically mediated greater distensibility of pulmonary vessels
• C. Large amount of smooth muscle in pulmonary arteries
• D. Smaller surface area of pulmonary circulation

Hint:

Think recruitment of resting capillaries that lowers pulmonary vascular resistance.

Answer 1

The Correct Option is A

Step 1: The pulmonary circulation is a low pressure, low resistance, high compliance system. It must accept the same output as the systemic side, yet it can take a large rise in flow without much rise in pressure.
Step 2: At rest many lung capillaries, especially at the apices, are collapsed or carry little flow because pressure there is low. There is a large reserve of unused capillary bed.
Step 3: When cardiac output rises in exercise, the pulmonary pressure increases slightly, and this small rise opens previously closed capillaries. This is called recruitment. Already open capillaries also widen, called distension. Both lower pulmonary vascular resistance.
Step 4: Because recruitment and distension drop resistance just as flow increases, pulmonary arterial pressure barely changes despite a five fold rise in output. Among the options, increase in the number of open capillaries (recruitment) is the best explanation.
Step 5: The wrong options: pulmonary vessels actually have little smooth muscle and are not the main reason here (C is wrong), the protective fall in resistance is passive and not chiefly sympathetic distensibility (B is wrong), and the pulmonary capillary surface area is huge, not small (D is wrong).
Conclusion: Answer is option A.