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Chapter: Medical Physiology: Aviation, High-Altitude, and Space Physiology

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Weightlessness in Space

A person in an orbiting satellite or a nonpropelled spacecraft experiences weightlessness, or a state of near-zero G force, which is sometimes called micro-gravity.

Weightlessness in Space

A person in an orbiting satellite or a nonpropelled spacecraft experiences weightlessness, or a state of near-zero G force, which is sometimes called micro-gravity. That is, the person is not drawn toward thebottom, sides, or top of the spacecraft but simply floats inside its chambers. The cause of this is not failure of gravity to pull on the body, because gravity from any nearby heavenly body is still active. However, the gravity acts on both the spacecraft and the person at the same time, so that both are pulled with exactly the same acceleratory forces and in the same direction. For this reason, the person simply is not attracted toward any specific wall of the spacecraft.

Physiologic Problems of Weightlessness (Microgravity). Thephysiologic problems of weightlessness have not proved to be of much significance, as long as the period of weightlessness is not too long. Most of the problems that do occur are related to three effects of the weight-lessness: (1) motion sickness during the first few days of travel, (2) translocation of fluids within the body because of failure of gravity to cause normal hydro-static pressures, and (3) diminished physical activity because no strength of muscle contraction is required to oppose the force of gravity.

Almost 50 per cent of astronauts experience motion sickness, with nausea and sometimes vomiting, during the first 2 to 5 days of space travel. This probably results from an unfamiliar pattern of motion signals arriving in the equilibrium centers of the brain, and at the same time lack of gravitational signals.

The observed effects of prolonged stay in space are the following: (1) decrease in blood volume, (2) decrease in red blood cell mass, (3) decrease in muscle strength and work capacity, (4) decrease in maximum cardiac output, and (5) loss of calcium and phosphate from the bones, as well as loss of bone mass. Most of these same effects also occur in people who lie in bed for an extended period of time. For this reason, exer-cise programs are carried out by astronauts during prolonged space missions.

In previous space laboratory expeditions in which the exercise program had been less vigorous, the astro-nauts had severely decreased work capacities for the first few days after returning to earth. They also had a tendency to faint (and still do, to some extent) when they stood up during the first day or so after return to gravity because of diminished blood volume and diminished responses of the arterial pressure control mechanisms.

Cardiovascular, Muscle, and Bone “Deconditioning” During Prolonged Exposure to Weightlessness. During very longspace flights and prolonged exposure to microgravity, gradual “deconditioning” effects occur on the cardio-vascular system, skeletal muscles, and bone despite rig-orous exercise during the flight. Studies of astronauts on space flights lasting several months have shown that they may lose as much 1.0 percent of their bone mass each month even though they continue to exercise. Substantial atrophy of cardiac and skeletal muscles also occurs during prolonged exposure to a micro-gravity environment.

One of the most serious effects is cardiovascular “deconditioning”, which includes decreased work capacity, reduced blood volume, impaired barorecep-tor reflexes, and reduced orthostatic tolerance. These changes greatly limit the astronauts’ ability to stand upright or perform normal daily activities after return-ing to the full gravity of Earth. Astronauts returning from space flights lasting 4 to 6 months are also sus-ceptible to bone fractures and may require several weeks before they return to pre-flight cardiovascular, bone, and muscle fitness. As space flights become longer in preparation for possible human exploration of other planets, such as Mars, the effects of prolonged microgravity could pose a very serious threat to astro-nauts after they land, especially in the event of an emergency landing. Therefore, considerable research effort has been directed toward developing counter-measures, in addition to exercise, that can prevent or more effectively attenuate these changes. One such countermeasure that is being tested is the application of intermittent “artificial gravity” caused by short periods (e.g., 1 hour each day) of centrifugal accelera-tion of the astronauts while they sit in specially designed short-arm centrifuges that create forces of up to 2 to 3 G.


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