Control of skin blood flow in the neutral zone of human body temperature regulation.

In humans, matching of heat loss and heat production in the "neutral" zone, defined operationally in terms of a range of skin temperatures (Tsk), is accomplished by regulation of skin blood flow (SkBF). Our studies were designed to reveal the characteristics of control of SkBF [from measurements of forearm blood flow (FBF)] in this zone. We controlled the temperature of water sprayed on most of the body of supine men and women at 33 or 35 degrees C in a square-wave pattern (15 min at each temperature) or a step pattern (60 min at 33 degrees C separated by short periods at 35 degrees C). FBF followed Tsk (0.5 ml.min-1.degrees C-1). Esophageal temperature changed approximately 0.11 degrees C with each 2 degrees C change in Tsk, falling with Tsk increase and vice versa. Little influence on FBF, < 0.1 ml.min-1.100 ml-1. degrees C-1, was observed when only the forearm was sprayed with 33 and 35 degrees C water. We conclude that SkBF control in the 33-35 degree C range of Tsk is dominated by the feedforward reflex influence of Tsk on SkBF. The reflex response overcompensates for the effect of Tsk on thermal balance in the neutral zone, so that equilibrium core temperature has an inverse relationship to Tsk.