During heavy exercise the cardiac output (CO) increases upto five fold while pulmonary arterial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by
In lungs, there are inactive capillaries that open up during increased blood flow, as in case of rigorous exercise. Along with that, the flow rate through each capillary is also increased. So when there is an increased systemic BP, the pressure in pulmonary circulation is raised only by a small level.
Increase in the number of open capillaries
Sympathetically mediated greater distensibility of pulmonary vessels
Large amount of smooth muscle in pulmonary arterioles
Smaller surface area of pulmonary circulation