During Exercise the Pulmonary Circulation Must Accommodate a Large Increase in Blood Flow
To transport the extra oxygen required for muscular effort, cardiac output increases sixfold to eightfold during strenuous exercise. This increase in blood flow must pass through the pulmonary circulation, where it collects oxygen.
To accommodate the increase in blood flow during exercise, the pulmonary blood vessels dilate; that is, PVR decreases. This dilation is in part passive as a result of the increase in intravascular pressure, which is a result of the increased blood flow. In addition, flow-induced release of NO from the endothelium also causes relaxation of smooth muscle and vessel dilation.In most species, pulmonary arterial pressure during exercise is about 35 mm Hg, but in the horse it increases to more than 90 mm Hg. The latter increase is attributable in large part to a very high left atrial pressure (50 mm Hg), which is probably necessary for rapid left ventricular filling when the heart rate exceeds 200 beats/min. These high exercise-associated intravascular pressures cause leakage of erythrocytes from the pulmonary capillaries when horses exercise strenuously, a phenomenon known as exercise-induced pulmonary hemorrhage.