During Exercise the cardiac output rises upto 5 times, but pulmonary vascular resistance only few mm hg. Why?

Ans. B. Opening of parallel channelsDuring exercise there is an increase in CO due to increased HR and SV.a. Peripheral resistance and DBP also increase due to sympathetic vasoconstriction. Sympathetic cholinergic system does cause Vasodilatation but only in few organs like Skeletal muscle, Uterus and Sweat glands.b. In Lungs there is not much rise in pulmonary vascular resistance because of opening of precapillary sphincters. They cause opening of more blood vessels leading to increasing in pulmonary perfusion.c. But the vessels are arranged in parallel to each other, their resistances are added asRT 1 =R1 1 +R2 1 +R3 1 +R4 1 ...d. So, more is the number of resistances (vessels) in parallel lower is the Total resistance (RT ).e. During exercise this mechanism counters the effect of sympathetic vasoconstriction in lungs and there is only small rise in pulmonary vascular resistance.