Control of skin blood flow during exercise by thermal reflexes and baroreflexes.

Four male subjects exercised on cycle ergometers at 40-51% of maximal aerobic power in the upright and supine positions at air temperatures of 15, 25, and 40 degrees C. Esophageal temperature (Tes) was measured at heart level, and mean skin temperature was computed from a weighted average of eight skin temperature measurements. Forearm blood flow (ABF) was measured by venous occlusion plethysmography, and cardiac output was measured by a CO2 rebreathing technique. At air temperatures of 15 and 25 degrees C, cardiac stroke volume was slightly lower during upright exercise than it was during supine exercise, as was ABF at a given Tes. At 40 degrees C, however, stroke volume was much lower and ABF at a given Tes was considerably lower during upright as opposed to supine exercise. The reduced stroke volume during upright exercise in the heat shows that gravity compounds the effect of cutaneous venodilation by allowing blood to pool in dependent limbs and thus impairing cardiac filling. The proportionality between reduced stroke volume and reduced forearm blood flow suggests that the reduction in blood flow may be mediated by cardiopulmonary baroreflexes that are activated by reduced cardiac filling pressure.