As a space medicine expert, I can tell you that the human body is not designed to survive in the vacuum of space without proper protection. If an astronaut were to be exposed to the vacuum of space without a spacesuit, the lack of oxygen would be just one of several serious threats to their life. The body would indeed use up the available oxygen in the bloodstream relatively quickly, leading to unconsciousness in about 15 seconds. However, the lack of atmospheric pressure would also cause a number of other immediate and severe problems, such as ebullism, where body fluids would start to boil due to the low pressure, and decompression sickness as nitrogen in the body's tissues comes out of solution and forms bubbles. Additionally, without the protection of a spacesuit, the body would be exposed to extreme temperatures, radiation, and micrometeoroids, all of which could be fatal. In summary, while the lack of oxygen is a critical factor, it is the combination of all these factors that makes survival in space without oxygen and a spacesuit extremely unlikely for more than a few seconds to a couple of minutes at most. read more >>

At most, an astronaut without a suit would last about 15 seconds before losing conciousness from lack of oxygen. (That's how long it would take the body to use up the oxygen left in the blood.) Of course, on Earth, you could hold your breath for several minutes without passing out.read more >>