|MadSci Network: Earth Sciences|
We measure atmosphere in pressure - pounds per square inch, for a reason. Our bodies do not support the entire weight of the atmosphere, but only the pressure on the surface area of our body. Because our bodies have evolved in this atmosphere, the pressure generated by our heart beating and pushing blood through arteries and veins (which is our blood pressure) just balances the external pressure on every square inch of our body. So the atmosphere pushes from the outside but our blood, pressurized, balances that pressure as long as the "system (our circulatory system)" is intact. If someone is cut in an artery, where the blood pressure is being sustained by the heart, blood will spurt from that wound until the pressure goes down due to blood loss.
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