<prompt> 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 byIncrease in the number of open capillariesSympathetically mediated greater distensibility of pulmonary vesselsLarge amount of smooth muscle in pulmonary arteriolesSmaller surface area of pulmonary circulation  <response>: 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. 