Discover has an excellent piece that dissects the new Star Trek movie by its major scenes and examines the scientific validity behind the admitted pseudoscience. There are SPOILERS, but passages like this make it great:
Kirk and McCoy are on a shuttle about to head up to orbit. McCoy, true to form, gripes about space travel, saying that if there's a hull breach, "… our blood will boil in 12 seconds."
I swear, every movie ever gets this all wrong...The temperature at which a liquid boils depends on the atmospheric pressure; at lower pressures liquids boil at lower temperatures. That's why there are high-altitude variations for some recipes...
This effect is so strong that in a vacuum, water boils at room temperature! Blood has things dissolved in it, which raises its boiling point, but even with that, at a body temperature of 37 Celsius blood would boil in a vacuum.
But if you're blown into space, your blood's not exposed to a vacuum! It's in a nice air-tight system, your circulatory system. The pressure inside your arteries and veins is kept relatively constant (unless I watch the news or Oprah, and then it skyrockets), so your blood won't boil.