In answering this question, most science fiction films depict gory scenes in which the human body resembles a balloon that blows up in a carnival of blood, flying viscera and shattered bones.
Reality, luckily (or not so, depending on your perspective), is not as 'exciting'. Animal experiments, controlled human experiments and past accidents give us a pretty good estimate of what may happen to the human body under such harsh conditions.
The earliest documented experiments date back to the 1960s. NASA was constantly pursued by policy makers to send humans to more distant and challenging destinations (the pinnacle being of course the 1969 lunar landing). In order to maximize the safety of its astronauts, NASA installed hypobaric (below normal pressure) chambers at high altitudes in order to simulate as much as possible the vacuum conditions prevalent in outer space. Volunteers stayed in these chambers until the first signs of disease, while animals were kept inside for longer times, sometimes until death. Not all experiments were premeditated: in some instances technicians found themselves in extreme vacuum for several seconds or even minutes, and suffered from medium wounds, unconsciousness and hypoxia (lack of oxygen).
In 1971, a Soviet space capsule was on its way back to Earth when as a result of a technical failure it opened while still in outer space. The three passenger cosmonauts (Soviet equivalent to the American astronaut) lost their consciousness within 30-60 seconds due to lack of oxygen and died several minutes later. Upon landing, they were still seated without any massive trauma to their bodies.
With all this information at hand, scientists can now draw quite an accurate picture of the damages sustained by the human body when exposed to vacuum. On Earth, the pressure inside our body is similar to the atmospheric pressure of 1 atm. Upon exposure to vacuum, air will begin to rapidly flow out of the body due to pressure equilibration. The air will flow out of the body cavities: mouth, nose, ears, eyes genitals and anus. In the absence of air, the average person will experience motor and mental impairment within seconds. This will soon be followed by loss of consciousness, contractions and gradual drop in body temperature due to evaporation of water and air. This stage (about 30-90 seconds from the beginning of the process) is still reversible, as the heart still beats and the brain is yet to suffer critical damage. Should the astronaut be brought back into a normal environment at this point, there is a good chance that he will survive and recover from the physical trauma to his body.
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