What type of physiological problems the astronauts undergo after spending a long period in the weightless condition?

Human body which has evolved under the condition of gravity can not endure prolonged weightlessness without adverse effects, the most serious one of which is depletion of calcium from the bones. For example, all the astronauts of one of the missions of NASA’s space laboratory named Skylab had lost 7% calcium through urine although they were in space for only 84 days. In another case, one Russian astronaut had lost quite significant 15% calcium after spending 237 days in the space. In 1987 when the Russian astronaut Vladimir Titov was sent to replace Yuri Romanenko who had spent the record period of 300 days aboard the space station Mir, he was categorically instructed not to embrace Romanenko or shake his hand vigorously because Romanenko’s bones might have become too brittle to withstand such exuberance. This apprehension of the Russian physiologists turned out to be true as the subsequent physical examination of Romanenko showed that his bones had lost as much as 18% calcium.
Another adverse impact of the prolonged weightlessness had been observed on the muscles of the astronauts. Since the legs do not have to bear the body weight, they run the risk of becoming atrophied, i.e. losing flesh, muscle and strength etc. due to the lack of their main function in the space, viz. bearing the weight of the body. The physiologists of NASA found that the astronauts returning to Earth after spending a long period in Skylab had not only lost the muscle power of their legs by 25% but their legs had actually shrunk by 11%. Similarly, the muscles of the heart’s left ventricle had lost elasticity by about 10% because the heart was not required to struggle against the gravity. The astronauts had even lost about 10% of the total quantity of blood during their prolonged stay in the space.
Biological reasons of these adverse side effects of prolonged stay in the space have not been fully revealed yet but one thing is quite clear: The astronaut’s body mechanism definitely slips into a low-gear and their body starts shedding or wearing away its own capacity which had become superfluous in the weightless state. And these physiological and anatomical changes manifest themselves in the form of the above mentioned adverse effects.

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