During heavy exercise the cardiac output (CO) increases upto five fold while pulmonary aerial pressure rises very little. This physiological ability of the pulmonary circulation is best explained by:
Increase in the number of open capillaries
Sympathetically mediated greater distensibility of pulmonary vessels
Large amount of smooth muscle in pulmonary aerioles
Smaller surface area of pulmonary circulation
A i.e. Increase in the number of open capillaries