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.