The Effects of Weightlessness on the Human Body

     While in space, the human body will feel weightless despite having the same amount of mass. But this is not because of the absence of gravity; it is instead the feeling of being in orbit. When in orbit, the centrifugal force is balanced by the force of gravity toward Earth. Therefore, an astronaut will feel weightless in space.
     The state of weightlessness is also called freefall, zero-gravity (zero-g), or microgravity. And in this state, walking becomes impossible; instead you or any other movable objects float from place to place. So large objects that would normally be too heavy to move would easily be handled in space.

The Body's Natural Position

     In the state of zero-gravity, the relaxed body naturally assumes a slightly curled-up position, with arms and legs floating freely in front of the body. This means that an astronaut will be unable to lie on a bed to sleep. Instead, he or she must zip him or herself up into a sleeping restraint similar to a sleeping bag. 

Space Sickness

     Also called "space adaptation syndrome" by NASA, space sickness often affects astronauts upon arrival in space. Like other forms of motion sickness (e.g. car sickness), the problem originates in the astronaut's inner ear, the mechanism that senses acceleration and orientation. The symptoms include nausea, headaches, cold sweating, and vomiting. These symptoms usually pass within a few days but could be dangerous to an astronaut in a spacesuit. 

Bodily Fluid Circulation

     The absence of gravity also affects the heart and circulation. Because they no work against gravity, blood and other fluids rise into the upper part of the body. Thinking that the body is over-supplied with blood and fluids, the heart secretes hormones, which reduces the level of bodily fluids. This lowers the amount of red blood cells in the body. And on landing, the blood flows back into the lower parts of the body, reducing the blood supply to the brain, which can cause dizziness and fainting.

Deterioration of Muscles

     In zero-gravity, astronauts do not have to use their muscles very much to move. So daily activities that maintain our muscle tones, such as walking or even standing, becomes non-existent in space. This will cause muscles to deteriorate, and in turn, their hearts and bones as well. The solution to this problem is regular exercise for one or two hours a day. For example, astronauts, held down by elastic rope, run on treadmills. Also, Soviet doctors have invented special spring-loaded suits that forces cosmonauts to work hard to stand upright.