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Physiological Effects of Weightlessness
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While humans have little difficulty surviving in space for short periods of time, long-term exposure to weightlessness can trigger detrimental physiological responses in the human body
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All life on Earth is accustomed to the presence of gravity. When that presence is removed or altered, biological processes can go awry. Initial ventures into space were conducted with great care to ensure that no damage would be incurred during the spaceflight. Animals were the first living organisms to be sent up into space, and when they returned unharmed (although scared), they were quite confident that humans could also withstand the journey. In April of 1961, a Russian cosmonaut named Yuri Gagarin became the first person in space when he orbited the Earth in 108 minutes.
Since that historical flight, mankind had ventured beyond low Earth orbit and the average duration of space missions has increased. While humans have little difficulty surviving in space for short periods of time (with the necessary equipment, oxygen and food of course), long-term exposure to microgravity can trigger detrimental physiological responses in the human body. There is a long list of such effects, ranging from serious medical conditions to less severe side effects.
First we will discuss the more serious adverse effects of weightlessness. Fluid redistribution is one of these effects. It occurs when bodily fluids shift from the lower body (where they normally abound due to the downward tug of gravity) to the head and upper body. This redistribution of fluids is coupled with fluid loss. When the brain senses the increased volume of fluid in the upper body, it interprets this as being an increase in the total volume of fluid in the body. The brain then responds by triggering the excretion of fluids, making astronauts prone to dehydration.
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This is a graphic showing the areas of the human body that are affected by weightlessness. Image courtesy Daniels & Daniels |
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Another negative consequence of weightlessness is the effect zero-gravity has on the cardiovascular system. On Earth, the heart must operate against gravitational pressure to sustain blood flow. Under zero-gravity conditions, that force is absent, causing the heart to lessen its pace according to the decreased demands.
Bone deterioration as a result of zero-gravity is extremely deleterious to an astronaut’s health. This deterioration occurs when the amount of physical stress on bones decreases. Studies have shown that stresses (like the stress of bearing a body in the presence of gravity) actually stimulate bone formation and strengthen bones by increasing bone density. Bone loss and dimineralization occur when the force of gravity is reduced or non-existent.
When a great deal of the crew's precious time in space is spent exercising rather than doing science, money and potential knowledge are being squandered for the sake of health. It is a sacrifice, but a necessary one
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Another effect of weightlessness on human physiology is muscle loss. Similar to bone deterioration, muscles atrophy as a result of disuse. In space, actions and movement require considerably less exertion because the force of gravity is practically non-existent. As a result, astronaut’s muscles become deconditioned.
The decreased production of red blood cells is another consequence of living in microgravity. Scientists are not sure why this occurs, but evidence has shown that decreased exertion and prolonged weightlessness result in mild cases of‘space’ anemia. While the condition has not shown to be life threatening, scientists are concerned about how anemia negatively affects crew performance.
Balance disorders can also result from extended exposure to zero-gravity. Receptors in the inner ear allow humans to sense direction and gravity while on Earth. In space, however, these receptors do not receive the same cues. Hand-eye coordination, posture and balance are all affected by the disorientation that occurs in microgravity. Upon returning to Earth, astronauts are often overwhelmed by dizziness and are unable to maintain their balance.
The immune system is also affected by weightlessness. Astronauts become quite susceptible to illness when in space. The human immune response lowers and the quantity of infection-fighting cells in the immune system decreases after prolonged exposure to microgravity. Space adaptation syndrome often results from the weakening of the immune system (and from the general stresses of a microgravity environment). Approximately half of all astronauts are affected by this unpleasant syndrome (ref. NASA) which is characterized by nausea, headache, lethargy and sweating. Fortunately the sickness only lasts a few days.
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Astronaut Mary Ellen Weber aboard the 'Vomit Comet' (aka KC-135) during a brief period of weightlessness. Photo courtesy NASA |
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In addition to the conditions mentioned above, there is a multitude of minor effects of weightlessness on the human body. Symptoms such as puffiness in the face, flatulence, weight loss, nasal congestion and sleep disturbance are usually only minor (yet common) annoyances.
Fortunately there are countermeasures to some of the detrimental effects of microgravity. The most obvious and simplest countermeasure is rigorous exercise. Exercise actually addresses two of the main adverse effects of weightlessness; both bone and muscle deterioration can be combatted with frequent, strenuous exercise. The downside of this countermeasure is the amount of time it consumes. The cost of sending a crew into space is very expensive. When a great deal of the crew’s precious time in space is spent exercising rather than doing science, money and potential knowledge are being squandered for the sake of health. It is a sacrifice, but a necessary one.
For the most part, the effects of zero-gravity are reversible. Astronauts gradually recover from voyages into space and their bodies return to their normal function. The length of the recovery period varies in accordance with the duration of the mission. Even when astronauts undergo strenuous exercise routines daily to try and maintain bone and muscle mass during a long space mission, some still have to carried on stretchers when they return to earth. Because of the impact weightlessness has on the human body, it is obvious to see why astronauts must be at the peak of fitness. Imagine what would happen if NASA chose your average couch potato as a space mission candidate. Since the couch potato would already have relatively poor health, the physiological effects of microgravity would be compounded tenfold.
It is interesting to note the striking similarities between aging and the effects of space flight on a person’s health. Like astronauts after a prolonged space mission, the elderly experience weakening muscles and bones, insomnia, difficulty balancing and depressed immune response. Studies are being undertaken to determine exactly how close the relationship is between aging and the effects of weightlessness. These studies could yield information that could help the elderly lead healthier lives. The key difference between the symptoms of aging and the health of astronauts is that the changes in astronauts are, for the most part, reversible.
Humans may not be perfectly suited to living in a weightless environment, but that won’t stop us from exploring space. The many benefits of space exploration (such as inspiration, technological impetus and knowledge) easily outweigh the negative aspects.
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