Journal of Applied Physiology, Vol 48, Issue 4 717-723, Copyright © 1980 by American Physiological Society
Control of skin blood flow during exercise by thermal reflexes and baroreflexes
M. F. Roberts and C. B. Wenger
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.
