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Effect of Gravity Variation on Human bodies

Human physiological adaptation to the conditions of space is a challenge faced in the development of human spaceflight. The fundamental engineering problems of escaping Earth's gravity well and developing systems for in-space propulsion have been examined for well over a century, and millions of man-hours of research have been spent on them. The environment of space is highly dangerous without appropriate protection.

The effects of space travel on human body are described as below: 
Gravity receptors 
Living on earth we constantly feel the gravitational pull and our bodies react automatically to maintain posture and locomotion in a downward pulling world. In microgravity environments, these constant signals the body is adapted to are absent. These changes can immediately result in visual-orientation illusions where the astronaut feels he has flipped 180 degrees. People returning to Earth after extended weightless periods initially have great difficulty in maintaining their balance but recover the ability very quickly, highlighting the remarkable ability of the human body to adapt. Over half of astronauts also experience symptoms of motion sickness for the first three days of travel due to the conflict between what the body expects and what the body actually perceives. 
Fluids 
The second effect of weightlessness takes place in human fluids. The body is made up of 60% water, much of it intra-vascular and inter-cellular. Within a few moments of entering a microgravity environment, fluid is immediately re-distributed to the upper body resulting in bulging neck veins, puffy face and sinus and nasal congestion which can last throughout the duration of the trip and is very much like the symptoms of the common cold. In space the autonomic reactions of the body to maintain blood pressure are not required and fluid is distributed more widely around the whole body. This results in a decrease in plasma volume of around 20%. These fluid shifts initiate a cascade of adaptive systemic effects that can be dangerous upon return to earth. 
Weight bearing structures 
The third and most worrying effect of long-term weightlessness involves bones and muscles. Without the effects of gravity, skeletal muscle is no longer required to maintain our posture and the muscle groups used in moving around in a weightless environment are very different to those required in terrestrial locomotion. Consequently some muscles atrophy rapidly. The Bone metabolism also changes. Normally bone is laid down in the direction of mechanical stress, however in a microgravity environment there is very little mechanical stress. This results in a loss of bone tissue approximately 1.5% per month especially from the lower vertebrae, hip and femur. Elevated blood calcium levels from the lost bone result in dangerous calcification of soft tissues and potential kidney stone formation. It is still unknown whether bone recovers completely. Loss of bone and muscle make it very difficult for humans to move and even breathe under the weight of Earth's pull upon their return. 
Sense of taste 
One effect of weightlessness on humans is that some astronauts report a change in their sense of taste in space. Some astronauts find that their food is bland, others find that their favorite foods no longer taste as good, some astronauts enjoy eating certain foods that they would not normally eat and some find no change whatsoever.