Atmospherical pressure is around 760 mmHg, while blood pressure is on average 100 mmHg. Then why do you bleed from cuts, and why does a cut aorta spray blood? The atmosphere should press the blood back in, or at least stop it from spraying.
When they say blood pressure is 100 mmHg, that really means 100 mmHg higher than atmospheric pressure. It's a gauge pressure, not an absolute pressure. The corresponding absolute pressure would be about (760 + 100) mmHg.
Originally posted by Georg in a comment