When they say blood pressure is 100 mmHg, that really means 100 mmHg higher than atmospheric pressure. It's a [gauge pressure](http://en.wikipedia.org/wiki/Gauge_pressure#Absolute.2C_gauge_and_differential_pressures_-_zero_reference), not an absolute pressure. The corresponding absolute pressure would be about (760 + 100) mmHg.

<sup>Originally posted by [Georg](http://physics.stackexchange.com/users/1370/georg) in a comment</sup>