1. Several phenomena observed in man during acute exposure to high altitude could be explained in part by a decrease in compliance of systemic capacitance vessels.

2. Serial measurements of forearm venous compliance were made with a water-filled plethysmograph in healthy men during a 24 h stay at 4300 m.

3. A decrease in venous compliance (increase in venous tone) occurred in all subjects and averaged 17·0 ± 6·2% (SEM).

4. In seven normal men during 1 week at 4300 m, a decrease in venous compliance again occurred in all subjects and was sustained throughout the 1 week period. There was also a rise in forearm blood flow and a fall in forearm vascular resistance. Increases in urinary excretion of noradrenaline and vanilmandelic acid suggested that these responses are mediated by the sympathetic nervous system.

5. Acute laboratory studies suggest that the response is primarily triggered by hypocapnia and potentiated by hypoxia; it cannot be duplicated by hyperventilation or by mild hypoxia in the absence of hypocapnia.

6. This decrease in systemic venous compliance may play an important role in the increased central blood volume and cardiac output which are seen acutely following ascent to high altitude.

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