The biggest difference is weightlessness.
When astronauts are in weightlessness, objects will float in the air, and the droplets will be absolutely spherical, and bubbles will not float in the liquid. It is as comfortable for astronauts to sleep standing as lying down. If they are not careful when walking, they will be unable to get up to the sky or get down to the ground. Food should be made into a paste like a block or toothpaste to prevent food scraps from "floating" in the air and entering the eyes and nostrils of astronauts ... However, the balance of force and weightlessness are completely different. For example, people standing on the ground, sitting in a chair, lying in bed, flying at the same speed by plane, etc., are in a state of force balance, but they are not weightless. Because in these cases, there are mutual forces between various parts of the human body.
Real weightlessness should make the interaction between various parts of human body, especially internal organs and internal organs disappear. Otoliths in human vestibular organs no longer contact with surrounding nerve cells due to weightlessness, and transmit signals to the central nervous system, thus losing their directional function. Therefore, once the vestibular organs do not work, the normal interaction between the internal organs of the body disappears, which will cause dizziness, nausea, vomiting and other symptoms in space pilots.
In the past 3 years of space flight, scientists from Russia and the United States have collected some preliminary data. These data show that weightlessness has a certain influence on endocrine, the production of red and white blood cells, the balance organs of the inner ear and the osteoporosis of bone, but the most obvious physiological weightlessness is the water loss in space and some symptoms caused by it, such as space anemia, decreased endocrine and muscle atrophy in both legs.
weightlessness can also lead to bone decalcification, which is very similar to osteoporosis in the elderly.
Extended information
Weightlessness has a great adverse effect. In addition to bone loss, weightlessness can also lead to muscle relaxation, decreased immunity and aging of astronauts. Caused a variety of space motion sickness, space motion sickness occurred frequently in the history of manned space flight in recent 2 years.
The function of human immune system is mainly attributed to various immune cells, among which B lymphocytes and T lymphocytes are the most important. B lymphocytes can secrete antibodies, prevent the invasion of pathogens and mark pathogens, while T lymphocytes kill pathogens.
But in space, these two kinds of cells are not so diligent. For example, T lymphocytes can't proliferate well in space, and their numbers are much less than those on the earth. Moreover, the migration in the body and the communication signals between them are also abnormal. So that the ability to resist foreign pathogenic bacteria is greatly reduced.
Reference: Baidu Encyclopedia-Weightlessness