When the human body is suddenly exposed to the vacuum of space, a number of injuries begin to occur immediately. Though they are relatively minor at first, they accumulate rapidly into a life-threatening combination. The first effect is the expansion of gases within the lungs and digestive tract due to the reduction of external pressure. A victim of explosive decompression greatly increases their chances of survival simply by exhaling within the first few seconds, otherwise death is likely to occur once the lungs rupture and spill bubbles of air into the circulatory system. Such a life-saving exhalation might be due to a shout of surprise, though it would naturally go unheard where there is no air to carry it.
In the absence of atmospheric pressure water will spontaneously convert into vapor, which would cause the moisture in a victim's mouth and eyes to quickly boil away. The same effect would cause water in the muscles and soft tissues of the body to evaporate, prompting some parts of the body to swell to twice their usual size after a few moments. This bloating may result in some superficial bruising due to broken capillaries, but it would not be sufficient to break the skin.
Within seconds the reduced pressure would cause the nitrogen which is dissolved in the blood to form gaseous bubbles, a painful condition known to divers as "the bends." Direct exposure to the sun's ultraviolet radiation would also cause a severe sunburn to any unprotected skin. Heat does not transfer out of the body very rapidly in the absence of a medium such as air or water, so freezing to death is not an immediate risk in outer space despite the extreme cold.
For about ten full seconds– a long time to be loitering in space without protection– an average human would be rather uncomfortable, but they would still have their wits about them. Depending on the nature of the decompression, this may give a victim sufficient time to take measures to save their own life. But this period of "useful consciousness" would wane as the effects of brain asphyxiation begin to set in. In the absence of air pressure the gas exchange of the lungs works in reverse, dumping oxygen out of the blood and accelerating the oxygen-starved state known as hypoxia. After about ten seconds a victim will experience loss of vision and impaired judgement, and the cooling effect of evaporation will lower the temperature in the victim's mouth and nose to near-freezing. Unconsciousness and convulsions would follow several seconds later, and a blue discoloration of the skin called cyanosis would become evident.
At this point the victim would be floating in a blue, bloated, unresponsive stupor, but their brain would remain undamaged and their heart would continue to beat. If pressurized oxygen is administered within about one and a half minutes, a person in such a state is likely make a complete recovery with only minor injuries, though the hypoxia-induced blindness may not pass for some time. Without intervention in those first ninety seconds, the blood pressure would fall sufficiently that the blood itself would begin to boil, and the heart would stop beating. There are no recorded instances of successful resuscitation beyond that threshold.