Abstract
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 retension, 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.
Key words
Hypoxia - acute mountain sickness - renin - aldosterone - atrial natriuretic peptide
