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Human thermoregulation and the cardiovascular system

J Gonzlez‐Alonso - Experimental Physiology, 2012 - Wiley Online Library A key but little understood function of the cardiovascular system is to exchange heat between the internal body tissues, organs and the skin to maintain internal temperature within a narrow range in a variety of conditions that produce vast changes in external ( ...

The literature directly comparing the effects of environmental heat and cold stress on the haemodynamic responses to small muscle mass exercise is sparse (e.g. Savard et al. 1988), yet evidence in resting limbs is extensive. Studies in resting limbs generally show that heat stress increases blood flow to the arms and legs, whereas cold stress reduces limb perfusion (e.g. Barcroft & Edholm, 1943). A highly controversial issue, however, is whether these differences in limb perfusion reflect changes only in skin blood flow or in both muscle and skin blood flow. With respect to heat stress, early investigations into the partition of limb perfusion between skin and skeletal muscle in the human forearm led to conflicting results, with some studies suggesting an elevation in muscle blood flow (Barcroft & Edholm, 1946; Barcroft et al. 1947), but others not (e.g. Rodie et al. 1956; Detry et al. 1972; Johnson et al. 1976). The negative findings together with the estimate of maximal skin blood flow of 6 8 l min 1, based on indirect measures of cardiac output and visceral blood flow during whole body heat stress, promoted the idea that increases in skin blood flow with heat stress accounted fully for the rise in systemic hyperaemia and blood flow redistribution (Detry et al. 1972; Rowell 1974; Minson et al. 1998). Recent evidence in the human leg, however, challenges this widely held dogma. Using 133 Xe washout or positron emission tomography techniques, Keller et al. (2010) and Heinomen et al. (2011) recently showed that passive leg heating increases calf blood flow by approximately 60 65%. In parallel, we have shown significant increases in leg tissue blood flow, deep femoral venous O2 content and muscle oxygenation and a parallel significant decline in leg arterial deep venous O2 differences during whole body heat stress, both at rest and during mild knee extensor exercise, with a small effect or no effect on aerobic metabolism (Pearson et al. 2011). The enhanced muscle blood flow was closely associated with increases in arterial plasma ATP concentration and muscle temperature (Pearson et al. 2011), which is in turn coupled to a temperature mediated release of ATP from erythrocytes (Kalsi & Gonz lez Alonso, 2012). On the other hand, cold stress has repeatedly been shown to reduce limb blood flow in resting humans (e.g. Barcroft & Edholm, 1943; Gregson et al. 2011). For instance, the classic work of Barcroft & Edholm (1943) clearly showed lower forearm blood flows and deep muscle temperatures when the forearm was immersed in a water bath at 13, 20 and 25 C compared with higher water temperatures. Likewise, Gregson and co workers recently reported a 35 40% decline in femoral artery blood flow and conductance following 10 min of cold and temperate water immersion (8 versus 22 C), which evoked drastic decreases in muscle and skin temperatures, but less cutaneous vasoconstriction at 8 than at 22 C water temperature, possibly reflecting a lower muscle blood flow (Gregson et al. 2011). Taken together, growing evidence from the human leg suggests that heat and cold stress not only alters blood flow to the skin, but also to the skeletal muscle. These circulatory adjustments might have important implications for heat transfer in resting and exercising human limbs.