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 ((Q) over dot c) 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 O-2 consumption ((V) over dot O-2max) at 36 degrees C ambient temperature. YF had a significantly higher (Q) over dot c 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% (V) over dot O-2max, 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 (Delta SBF + Delta RBF) followed YF much greater than YS > OF > OS. A rigorous 4-wk endurance training program increased exercise SkBF in OS, but Delta SBF and Delta RBF were unchanged. Under these conditions, older men distribute (Q) over dot c 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 (Q) over dot c 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.