Altered control of skin blood flow at high skin and core temperatures

Five subjects were studied during periods of controlled increases and decreases in skin temperature (Ts) over the Ts range of 34-40 degrees C. One protocol was designed to observe changes in forearm blood flow (FBF) and heart rate (HR) with changes in core temperature (Tc; right atrial blood temperature and esophageal temperature were measured) with Ts held constant at two levels. FBF and HR changed linearly with Tc in the Tc range of 37-38 degrees C with Ts constant at 38 degrees C. A second protocol imposed Ts changes at two levels of Ts and Tc; this protocol also included a prolonged cooling period. The influence of Ts on FBF and HR was reduced when Ts changes occurred at an elevated Ts and Tc, and FBF showed considerable hysteresis during cooling. We conclude that a linear model for the control of FBF or HR is inadequate as a tool for predicting the control of these variables.