Altered control of skin blood flow during exercise at high internal temperatures.

We have investigated further the behavior of skin blood flow (SkBF) as internal temperature (measured as esophageal temperature (Tes)) rises in a heated exercising man. A previous study showed that when skin temperature (Tsk) is driven up in exercising men, the increase in SkBF is less than that found for the same Tes and Tsk in a resting man. In this study, we extended our observations into the range of higher Tes to see if SkBF (measured plethysmographically as changes in forearm blood flow) could be driven to higher levels despite competition with skeletal muscle for cardiac output. After Tsk was elevated to 38 degrees C by means of water-perfused suits, subjects exercised at 525-900 kp.m/min (86-147 W) for 17-30 min while Tsk was held at 38 degrees C. We found that SkBF increase per unit increase in Tes is attenuated at higher Tes. In seven men, the average slope obtained from linear regression of FBF on Tes below 38 degrees C was 7.6 flow units/degrees C. Above 38 degrees C, the average was 2.12 flow units/degrees C. In some subjects, the FBF response appeared virtually saturated at a plateau despite a steady rate of increase in Tes with time. Clearly, SkBF does not increase in direct proportion to Tes without bound. Other stimuli (presumably related to blood pressure regulation) interact either to reduce the rate of SkBF increase or to prevent any further increase after Tes exceeds 38 degrees C.