Salt and water control at altitude.

The physiological effect of altitude hypoxia, in the absence of exercise, is a sodium and water diuresis with decrease in plasma and extra-cellular volumes. Plasma aldosterone concentrations (PAC) are reduced but plasma atrial natriuretic peptide (ANP) levels are modestly increased. Day-long exercise at low altitude has almost opposite effects on fluid balance. There is an anti-diuresis, sodium retention, expansion of the plasma and extra-cellular compartments, elevation of PAC and ANP. Subjects who develop acute mountain sickness (AMS) show a pathological response to hypoxia even before the development of symptoms. There is an anti-diuresis, sodium retention, increased plasma and extra-cellular volumes and increased PAC compared with subjects resistant to AMS. Plasma ANP tends to be elevated compared with sea level values but the relation of ANP levels to AMS is variable. In general therefore, the pathological response to altitude hypoxia parallels that of exercise at low altitude and is opposite to the physiological response. Both exercise and the pathological response predispose the subject to edema and are probably important in the genesis of AMS.