Our body is made specifically to live on Earth, all the systems and tissues based on the particular conditions of pressure, temperature, etc .. The gravitational force adaptation study of our body in such situations "extreme" is fundamental and still totally in the research phase: until they are revealed any short- and long-term changes that the body adopts a result of more or less important environmental perturbations in which he lives, you can not schedule interplanetary travel.
An example is shown in the picture below, it relates to the cardiovascular system. In fact, on Earth the system is made to spray in a certain way the body and organs; cardiac pump and valves are used to push the blood in a certain way, such that it has the correct pressure and density depending on the area, and in its "intelligence" knows the most important areas to preserve both in terms of blood supply or temperature (such as the brain). In microgravity and in the absence of hydrostatic pressure, the main force that regulates the system fails: the blood does not precipitate more naturally down as on Earth but migrates to the upper part of the body. This phenomenon is responsible for Puffy Face (and chicken legs) of the astronauts and, deep behind the odd name, is a very important phenomenon because the different intracranial pressure or for example eyeball, can lead to problems, which are still being search; In fact, the pressures within the body are to stay on Earth, with certain characteristics and we do not know what can lead a more or less long-term pressure. A consequence of this is, especially the first few days in orbit, a balance which implements the body as a result of what he perceives as a pressure increase: so much pee and is a phenomenon known also to the more traditional medicine: in fact the same what happens to those who are bedridden for a long time
Photo credit: NASA