Space missions force astronauts to live and work in environments that the human body is unaccustomed to. With the use of man-made equipment and vehicles the body is able to tolerate the space environment, which includes elements such as microgravity, radiation, extreme temperatures, low pressure, isolation, and confinement.
For short-duration missions of one month or less, the physiological adaptation that takes place, although important, is relatively mild when compared to the changes that take place during long-duration missions, such as those on the International Space Station (ISS). Studies have shown that the longer an astronaut remains in space, the more significant are the changes that occur.
While in space, many of these changes tend not to be problematic. Upon return to Earth, however, the effects of living in space are often felt. In some instances, the effects of these changes are noticed immediately (eg. reduced blood volume and diminished reflexes), while others may not present themselves for months or years (eg. radiation-induced health problems).