Validity of microgravity simulation models on earth

Many studies have used water immersion and head-down bed rest as experiment al models to simulate responses to microgravity. However, some data collect ed during space missions are at variance or in contrast with observations c ollected from experimental models. These discrepancies could reflect incomp lete knowledge of the characteristics inherent to each model. During water immersion, the hydrostatic pressure lowers the peripheral vascular capacity and causes increased thoracic blood volume and high vascular perfusion. In turn, these changes lead to high urinary flow, low vasomotor tone, and a h igh rate of water exchange between interstitium and plasma. In contrast, th e increase in thoracic blood volume during a space mission is combined with stimulated orthosympathetic tone and lowered urine flow. During bed rest, body tissues are compressed by pressure from gravity, whereas microgravity causes a negative pressure around the body. The differences in renal functi on between space and experimental models appear to be explained by the phys ical forces affecting tissues and hemodynamics as well as by the changes se condary to these forces. These differences may help in selecting experiment al models to study possible effects of microgravity. (C) 2001 by the Nation al Kidney Foundation, Inc.