That is a good question, for which we have only theoretical (not empirical) answers! Here's my understanding of what would happen and why:
First of all, your body is pressurized internally to match the altitude at which you live. Suppose you normally live at sea level, where the normal atmospheric pressure is one atmosphere, or 14.7 pounds per square inch. In order for you not to implode at sea level, the inside of your body is also at that pressure, which is why when you go up in an airplane, or drive up a steep mountain road, you feel pressure inside your ears and you have to yawn or swallow hard to make them "pop". Now: imagine putting a marshmallow in a vacuum chamber. A marshmallow is mostly air inside of it, and under normal conditions the air pressure inside the marshmallow balances the air pressure outside - 14.7 pounds per square inch. What happens when you evacuate the air in the vacuum chamber? The air inside the marshmallow has less and less air pressure outside the marshmallow to balance the internal pressure, so the marshmallow starts to expand. It will keep on expanding until there is no more air inside the vacuum chamber, and then it reaches a maximum size. So, your body would expand like that, very quickly, as you would suddenly be thrust into a situation with no air. Your skin has a certain amount of elasticity - it can stretch - so your skin would expand as much as it could to accommodate the pressure on the inside of your body pushing outward with no outside air pressure to balance it. Now, what happens to a stretchy balloon when you blow it up too much? Right, it pops. That would happen to your skin, too.
And, of course, with no air you would choke because you could not breathe.
But that's not the end! What happens to water when you put it in a vacuum chamber? Ask your physics teacher to do this for you, as it is quite amazing. The water boils when the air pressure drops below a certain level (I forgot just what that is.) You see, boiling is a relationship between temperature and vapor pressure - that is, the air pressure on the topsurface of the water. Even at room temperature, the water will start to bubble and yet when you remove it from the vacuum chamber, you will feel that it is actually cooler.
But there's more to the story: water freezes at 0 degrees Celsius, but space is at -270 degrees Celsius!! So, all your body fluids would freeze. Have you ever put a banana into liquid nitrogen? Ask your teacher to call the Physics Circus at UCSB and have them do this in one of their demonstrations. The banana freezes so hard that you could hammer a nail with it. And liquid nitrogen is quite warm compared to space!
So, put it all together, and here's what we can predict would happen to a naked human body in space, without a space suit:
First, you would choke because there's no air to breathe. At the same time, your body would expand because of the internal air pressure of 14.7 pounds per square inch inside that is not balanced by any air pressure at all outside. When your skin was stretched to the max, it would burst like a balloon, and all the water inside your body (blood is mostly water) would come boiling out because there is no air pressure, but space is so cold that it would instantly freeze. And what is left - any arms, legs, intestines and whatever you ate that is still inside your intestines, your brain, heart, liver, etc, - would all be instantly frozen rock-hard!
And if you were gently ejected from a spacecraft that had some orbital velocity, then the center of mass of the explosion that was you would continue at the same velocity, while all the body parts and frozen droplets of blood and other fluids followed their individual trajectories, having been ejected in the explosion, like the exploding fireworks in the physics problems in your text book on conservation of momentum.
It would not be pretty.
I hope you enjoyed this answer! Keep on asking questions!
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