Peripheral venous constriction on ascent to altitude causes an increase in central blood volume, which triggers baroreceptors to suppress antidiuretic hormone (ADH) and aldosterone and induce a diuresis. Combined with the bicarbonate diuresis from the respiratory alkalosis, this can result in decreased plasma volume and hyperosmolality (serum osmolality of 290 to 300 mOsm/L), which the body appears to permit by a reset of the osmol center of the brain. Clinically, diuresis and hemoconcentration is considered a healthy response. Antidiuresis is a hallmark of acute mountain sickness.
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