Venous return from distal regions affects heat loss from the arms and legs during exercise-induced thermal loads

Summary

To study the role of venous return from distal parts of the extremities in influencing heat loss from the more proximal parts, changes in mean skin temperature (¯T _sk) of the non-exercising extremities were measured by color thermography during leg and arm exercise in eight healthy subjects. Thirty minutes of either leg or arm exercise at an ambient temperature (T _a) of 20° C or 30° C produced a greatly increased blood flow in the hand or foot and a great increase in venous return through the superficial skin veins of the extremities. During the first 10 min of recovery from the exercise, blood flow to and venous return from the hand or foot on the tested side was occluded with a wrist or ankle cuff at a pressure of 33.3 kPa (250 mm Hg), while blood flow to the control hand or foot remained undisturbed. During the 10-min wrist occlusion, ¯T _sk increased significantly from 28.3°±0.41° C to 30.1°±0.29° C in the control forearm, but remained at nearly the same level (28.0°±0.34° C to 28.2°±0.25° C) in the occluded forearm. In the legs, although ¯T _sk on both sides was virtually identical (32.0°±0.31° C, control vs 32.0°±0.36° C, tested) before occlusion, ¯T _sk on the control side (32.6°±0.27° C) was significantly higher than that on the tested side (32.2°±0.21° C) after ankle occlusion. As monitored by a laser-Doppler flowmeter, skin blood flow in both forearms and legs did not increase but rather decreased during the 30-min recovery. Thus, the increase in ¯T _skin the control forearm or leg cannot be explained by the change in forearmor leg-skin blood flow. We estimated the contribution of the heat transferred from the venous blood of the hand to be 89% of the increase in heat loss from the forearm during occlusion atT _a of 20° C, and to be 67% from the occluded leg atT _a of 30° C. The present results suggest that heat loss from proximal parts of the extremities is greatly affected by the change in blood flow through the distal parts of the extremities, and such venous blood flow plays an important role in the regulation of heat dissipation during exercise in cool (20° C) to warm (30° C) environments.