Fluid Regulation in a Microgravity Environment
The extraterrestrial environment presents human beings with innumerable physiologic challenges. Astronauts engaged in space flight may experience alteration of their bodily fluids and electrolyte balance. Weightlessness generally causes a reduction in total fluid volume. In addition, there is typically a cephalad redistribution of the blood and other body fluids. This cephalad shift initiates a series of compensatory mechanisms, many of which involve hormonal fluctuations. Unfortunately, data collected from human subjects during actual space flight is rather scarce. This paucity has led to the use of various ground-based weightlessness simulations. Two important experimental techniques include the water immersion and bed rest methods. Unfortunately though, despite the considerable research that has been performed on the effects of weightlessness, the data currently available remain inconsistent.
Over three decades ago, the initiation of manned space flight programs in the United States and the former Soviet Union focused considerable attention upon the physiologic effects of different gravitational forces. On the Earth's surface, all objects are continuously subjected to a gravitational stress induced by the ground's reactionary force (10:505-521). Within the Earth's gravitational field, weightlessness may be defined as "a condition where no other forces but gravity affects a body" (10:505-521). Hence, weightlessness may be characterized as a condition of static equilibrium whereby there are no counteracting reactional forces. Such circumstances exist, for example, when an object is either in a free fall or engaged in a circular or elliptical trajectory around the planet.
The state of weightlessness can perhaps be best explained by considering a person standing on a scale in an elevator. When the elevator is motionless, the scale reads the person's body weight. Thi...