As I understand it, this in fact will not happen. Our circulatory system is a closed loop and strong enough to not expand into vacuum. The water inside our body is kept close to usual pressure by our skin, blood vessels etc.

The main immediate danger in space without a spacesuit would be holding your breath. Your lungs are not built to withstand a whole atmosphere of pressure from the inside, and will get damaged if you hold your breath.

Of course if you don't, you'll have another problem.