Another key part of the dive reflex is massive peripheral vasoconstriction: the blood vessels in your arms and legs squeeze nearly shut, sending blood flooding back to your core, where it maintains the crucial oxygen supply to your heart and brain for as long as possible. This shift of blood volume to your torso also helps your lungs resist collapse under the pressure of deep dives, since fluids (unlike air) are nearly incompressible. All it takes to trigger these changes is dunking your face in cool water; in fact, the sensors appear to be primarily around the nose, lending credence to the
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