Your blood would be at a higher pressure than the outside environment. For example: a typical person's blood pressure might be 75/120. The "75" part is the important number here, as this means that between beats the blood is at a pressure of 75 Torr (about 100 mbars) above the external pressure.

If you happen to be unfortunate enough to fall out of an airlock into a zero-pressure environment, at a blood pressure of 75 Torr the boiling point of water is 46 degrees Celsius (115 F). This is well above your body temperature of 37 degrees Celsius (98.6 Fahrenheit). Your blood won't boil due to the elasticity of your blood vessels. They will keep your blood pressure high enough that your body temperature is below the boiling point.

That is, at least until your heart stops beating—at which point you have other things to worry about.

Info gleaned from "Explosive Decompression and Vacuum Exposure" by Geoffrey A. Landis:

Editor's note: As outlined by Lucy-S at SFF Net, the site referenced above has gone offline. As of March 2017, you can find the referenced article at

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