During exercise there is an increase in CO due to increased HR and SV.
I. Peripheral resistance and DBP also increase due to sympathetic vasoconstriction. Sympathetic cholinergic system does cause Vasodilatation but only in few organs like SkeletalmuscleQ, Uterus and Sweat glands.
II. 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.
III. But the vessels are arranged in parallel to each other, their resistances are added as
IV. So, more is the number of resistances (vessels) in parallel lower is the Total resistance (RT).
V. During exercise this mechanism counters the effect of sympathetic vasoconstriction in lungs and there is only small rise in pulmonary vascular resistance.