Question:medium
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:
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:
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Think recruitment of resting capillaries that lowers pulmonary vascular resistance.
Updated On: Jun 24, 2026
- Increase in the number of open capillaries
- Sympathetically mediated greater distensibility of pulmonary vessels
- Large amount of smooth muscle in pulmonary arteries
- Smaller surface area of pulmonary circulation
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The Correct Option is A
Solution and Explanation
The lung circulation is built to be soft and low in pressure. Its key trick is that flow can jump dramatically while pressure stays almost flat, which is exactly what happens when exercise pushes cardiac output up several fold.
The reason lies in spare capacity. At rest, a large portion of pulmonary capillaries, particularly toward the lung apex, are shut or nearly so because local pressure is too low to keep them open. They form a reserve waiting to be used.
During exercise the modest bump in pulmonary pressure pops these closed vessels open, a process termed recruitment, and it also widens vessels that were already carrying flow, termed distension. Adding more parallel channels lowers the overall pulmonary vascular resistance.
So as flow climbs, resistance falls in step, and the product, which determines pressure, hardly moves. The single best phrase for this among the choices is an increase in the number of open capillaries.
The distractors fail on basic anatomy and physiology: pulmonary arteries are thin walled with scant smooth muscle, the adaptation is mainly passive rather than a sympathetic distensibility effect, and the pulmonary capillary surface area is enormous rather than small.
\[\boxed{\text{Increase in the number of open capillaries}}\]
The reason lies in spare capacity. At rest, a large portion of pulmonary capillaries, particularly toward the lung apex, are shut or nearly so because local pressure is too low to keep them open. They form a reserve waiting to be used.
During exercise the modest bump in pulmonary pressure pops these closed vessels open, a process termed recruitment, and it also widens vessels that were already carrying flow, termed distension. Adding more parallel channels lowers the overall pulmonary vascular resistance.
So as flow climbs, resistance falls in step, and the product, which determines pressure, hardly moves. The single best phrase for this among the choices is an increase in the number of open capillaries.
The distractors fail on basic anatomy and physiology: pulmonary arteries are thin walled with scant smooth muscle, the adaptation is mainly passive rather than a sympathetic distensibility effect, and the pulmonary capillary surface area is enormous rather than small.
\[\boxed{\text{Increase in the number of open capillaries}}\]
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