Physiological Responses to Acute Exercise-Heat Stress

Abstract

Human body temperature is regulated by a proportionate control system. It is nuclear however, which internal body temperature is regulated by the control of the thermoregulatory effector responses of skin blood flow and sweating. The core temperature increases during exercise as a result of a 'load error' and not a change in the regulated set-point temperature. During exercise the magnitude of core temperature elevation at steady-state is proportional to the metabolic rate and is largely independent of the environmental condition. However, dependent upon the environmental conditions the relative contributions of sensible (radiative and convective) and insensible (evaporative) heat exchange to the total heat loss will vary. The hotter the environment the greater the dependence on insensible heat loss. During exercise in the heat, the primary problem is to simultaneously provide the cardiovascular support to maintain the metabolism for skeletal muscle contraction and to dissipate the associated heat release. In hot environments, the core to skin temperature gradient is reduced to skin blood flow needs to be relatively high (compared to cooler environments) to achieve heat transfer sufficient for thermal balance. In addition, sweat secretion can result in a reduced plasma (by dehydration) and thus blood volume. Both high skin blood flow and reduced plasma volume can reduce cardiac filling and perhaps cardiac output during exercise in the heat. As a result syncope or reduced exercise performance will occur.