Fluid shifts and hydration state: effects of long-term exercise.

During exercise, increased capillary hydrostatic pressure caused by elevation of arterial pressure produces plasma volume shifts from the vascular space to the interstitial fluids. Following a rapid efflux of vascular fluid within minutes of exercise, there is very little further reduction in plasma volume during long-term exercise, suggesting protective mechanisms against loss of circulating blood volume. These mechanisms probably include increasing plasma protein oncotic pressure, differences in peripheral vasoconstriction in active muscles and inactive tissues, and elevated lymph flow. The interaction of these factors provides optimal thermoregulatory and cardiovascular stability. The dynamics of fluids shifts during long-term exercise are altered by hydration state. The hypovolemia caused by dehydration acts to conserve blood volume by reducing the amount of plasma shift and sweat loss during exercise. The consequence is less heat dissipation and greater cardiovascular stability. In contrast, the hypervolemia produced by hyperhydration promotes greater shifts of fluid and sweat loss, resulting in lower body temperature and heart rate during prolonged work. The beneficial effects of hyperdydration and subsequent hypervolemia are manifest in the adaptation of body fluids and electrolytes to exercise training. Thus, with regard to fluid shifts during long-term exercise, training is an effective way to become hyperhydrated and to reduce the limiting effects of working in 'hostile' environments.