Fluid shifts in vascular and extravascular spaces during and after simulated weightlessness.

To simulate weightlessness in a normal-gravity environment, eight male subjects were tilted 5 degrees head-down for 8 h to determine vascular and extravascular shifts of fluid. Most of the initial loss of leg volume during head-down tilt represented a passive shift of venous blood toward the head. Facial edema, headache, nasal congestion, and a pronounced diuresis were associated with this redistribution of blood volume. As measured by the wick-catheter technique during head-down tilt, interstitial fluid pressure in lower-leg muscle and overlying subcutaneous tissues decreased by 7.4 and 4.4 mmHg, respectively. Interstitial fluid was shifted from the lower legs at a rate of 12 ml X h-1. Dehydration of lower-leg tissues probably resulted from decreased capillary blood pressure within these tissues during tilt. Other transcapillary pressures were unchanged. The abrupt alterations in local blood pressure upon changes in body posture were probably sufficient to explain all shifts of vascular and extravascular fluid. In this regard, countermeasures may be necessary to maintain precapillary-muscle tone during long space flights in order to prevent swelling of lower-leg tissues upon readjustment to Earth's gravity.