Age, fitness, and regional blood flow during exercise in the heat.

During dynamic exercise in warm environments, the requisite increase in skin blood flow (SkBF) is supported by an increase in cardiac output (Qc) and decreases in splanchnic (SBF) and renal blood flows (RBF). To examine interactions between age and fitness in determining this integrated response, 24 men, i.e., 6 younger fit (YF), 6 younger sedentary (YS), 6 older fit (OF), and 6 older sedentary (OS) rested for 50 min, then exercised at 35 and 60% maximal O2 consumption (VO2max) at 36 degrees C ambient temperature. YF had a significantly higher Qc and SkBF than any other group during exercise, but fitness level had no significant effect on any measured variable in the older men. At 60% VO2max, younger subjects had significantly greater decreases in SBF and RBF than the older men, regardless of fitness level. Total flow redirected from these two vascular beds (deltaSBF + deltaRBF) followed YF >> YS > OF > OS. A rigorous 4-wk endurance training program increased exercise SkBF in OS, but deltaSBF and deltaRBF were unchanged. Under these conditions, older men distribute Qc differently to regional circulations, i.e., smaller increases in SkBF and smaller decreases in SBF and RBF. In younger subjects, the higher SkBF associated with a higher fitness level is a function of both a higher Qc and a greater redistribution of flow from splanchnic and renal circulations, but the attenuated splanchnic and renal vasoconstriction in older men does not appear to change with enhanced aerobic fitness.