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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.

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Correct answer: because those 100 mmHg are measured relative to atmospheric pressure. The absolute arterial pressures would be 860 mmHg if atmospheric is 760 mmHg. – Georg Sep 23 '11 at 19:37
The distinction the Georg is (correctly) making is that between "gauge" (measured with respect to something (usually the surrounding atmosphere)) and "absolute" pressure (measured with respect to vacuum). – dmckee♦ Sep 23 '11 at 20:36
Thanks, @georg! – Karin Sep 24 '11 at 8:09
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@georg: This is an answer-- can you place it in the answers so this question can be removed from the "unanswered" list? – Ron Maimon Oct 10 '11 at 5:37

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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

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