This document was written when I was in middle school. I have little or no authority on this topic. Make of this what you will.
There are many bad things that happen or can happen to astronauts in space. Fluids in the body shift from the lower half of the body to the upper half of the body. This redistribution of fluids results in other problems. The brain interprets this shifting of liquids as an overall increase in fluid volume and activates the excretory system. This action causes calcium loss and bone demineralization. Also blood loss may be up to 10%. Red blood cell loss is caused by a change in splenic function and that results in premature destruction of red blood cells. Muscle damage occurs when proteins are lost and the tissue shrinks. This loss may be accompanied by changing in muscle type. In a study in rats there was an increase in “fast-twitch” white fiber and a decrease in “slow-twitch” bulkier red fiber. Bone damage happens because mechanical demands on bones decrease in weightlessness and they essentially dissolve. They do this because bone tissue is deposited where needed and absorbed where it isn’t. Loss occurs mostly in the weight-bearing structures such as legs and the spine. However loss is not major in non-weight-bearers such as the skull or fingers. Fluid loss and demineralization of bones increase the amount of calcium in the blood stream and increases the risk of urinary stones. Surgery becomes harder because the organs float in the body and the blood doesn’t pool. The shifting of fluids changes how antibiotics are taken by the body and bacteria membranes become less permeable and reduces the effects of antibiotics on the body. With no main gravitational source the astronauts experience changes in their sense of vertically. In adoption to weightlessness the brain comes to rely on vision rather than motion or position. They also get his by nausea, vomiting, anorexia, headache, malaise, drowsiness, and sweating because they are used to gravity. Exercise capacity decreases and exercise becomes clumsier because of weightlessness. Senses of smell and taste decrease in weightlessness too. The increases of fluid in the head cause cold like stuffiness. Food smells the same and astronauts crave spices. Fluid loss, lack of exercise, and loss of appetite result in weight loss. There is an increase in flatulence because gases can’t rise to the mouth but go out the other end. The face becomes puffy and the voice pitch and tone are affected and speaking is more nasal. The posture approaches the fetal position and the spine lengthens. Some astronauts gained one or two inches and that affected their space suits fitting. The immune system’s efficiency decreases in space. It does because the zero gravity blocks a specific type of white blood cell from turning on the genes to mount an immune response. I also found out that astronauts loose 1% of their bone mass per month. Scientists are trying to figure out what gravity is needed to maintain the bone mass during space travel.