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

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, on one particular mission, an astronaut's extracellular fluid was found to be decreased 15 percent after just 24 hours of flight. After eight days of flight, however, that particular astronaut's extracellular fluid compartment had normalized somewhat. One particularly important body fluid compartment which is usually altered by spaceflight includes the blood plasma. Plasma volume normally consists of about one-fifth of the extracellular fluid volume. This compartment has been found to be reduced practically every time that it has been measured following a United States space mission (5:351). The decreases typically occur soon after the onset of microgravity, but revert to preflight levels within 2 weeks of astronauts' return to Earth. The relatively large changes in plasma volume relative to the extracellular fluid may indicate that the interstitial fluid volume (i.e., the other four-fifths of the extracellular fluid) is conserved during weightlessness. Observed losses of body water during weightlessness could result from either decreased intake, increased excretion, or both. Data from Skylab missions indicate that fluid losses are caused by a "decrease in fluid intake and partly to an augmented renal water excretion (10
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Some common words found in the essay are:
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Approximate Word count = 2574
Approximate Pages = 10 (250 words per page)