As reviewed by Anand and Chandrashekhar (see Chapter 18 in this volume), data from more than 50 studies on fluid shifts at high altitudes are conflicting and controversial. Many studies show that diuresis occurs upon acute exposure to high altitudes (for example, Phillips et al., 1969), but an equal number show anti-diuresis (for example, Singh et al., 1990). Few studies have had adequate experimental controls for the insufficient dietary intake observed. This dietary insufficiency could result in diuresis from the excretion of breakdown products of lean and fat tissues. Conversely, water retention is most common at extremely high altitudes, and contributes to AMS as well as to both HAPE and HACE (Anand and Chandrashekhar, 1992; Anand et al., 1990). Exercise also stimulates an accumulation of body electrolytes and water, and may cause the plasma volume to increase (Withey et al., 1983).
Changes in water excretion are influenced by a number of regulatory hormones. Hypoxic subjects, at rest, generally show decreased plasma concentration of aldosterone, which results in a decrease in sodium retention and thus in water retention (Bouissou et al., 1989; Colice and Ramirez, 1985, 1986; Heyes et al., 1982; Hogan et al., 1973; Maher et al., 1975; Maresh et al., 1985; Milledge et al., 1983; Olsen et al., 1992; Raff et al., 1986; Ramirez et al., 1992; Shigeoka et al., 1985; Slater et al., 1969a, b; Sutton et al., 1977). Hypoxia also blunts the normal increase in aldosterone associated with exercise even when the same amount of exercise is done (Maher et al., 1975; Milledge et al., 1983; Olsen et al., 1993; Shigeoka et al., 1985). A decrease in renin activity seems to follow similar patterns. The function of renin is to convert the prohormone angiotensinogen to the active hormone angiotensin. Angiotensin is a potent peripheral vasoconstrictor that promotes renal sodium retention and stimulates secretion of aldosterone by the adrenals. Thus, with hypoxia the renin-angiotensin-aldosterone relationship becomes uncoupled. Responses of antidiuretic hormone (ADH) also differ with the degrees of hypoxia and exercise. ADH secretion decreases when hypoxia is moderate and exercise is light, thus resulting in decreased fluid retention, and increases with extremely high altitudes and/or severe exercise stress, which would increase fluid retention (Bärtsch et al., 1991).
Increases in the stress-related hormones, cortisol and norepinephrine, are associated with both high altitude and exercise and may affect fluid balance. Subjects who are prone to develop AMS have an atypical ADH and aldosterone response, which increases fluid retention (i.e., both hormones tend to increase in concentration in the blood at high altitudes, rather than decrease, with the increases being exaggerated by exercise).
Relatively few data exist on renal function at high altitudes; the small amount of information available suggests that high altitudes lead to a decrease in effective renal plasma flow (ERPF), although glomerular filtration rates are