Submerging the face in water triggers the mammalian diving reflex. This is found in all mammals, and especially in marine mammals such as whales and seals. This reflex is designed to protect the body by putting it into energy saving mode to maximize the time it can stay under water. The effect of this reflex is greater in cold water than in warm water and has three principal effects:
* Bradycardia, a slowing of the heart rate of up to 50% in humans. * Peripheral Vasoconstriction, the restriction of the blood flow to the extremities to increase the blood and oxygen supply to the vital organs, especially the brain. * Blood Shift, the shifting of blood to the thoracic cavity, the region of the chest between the diaphragm and the neck, to avoid the collapse of the lungs under higher pressure during deeper dives.
The reflex action is automatic and allows both a conscious and an unconscious person to survive longer without oxygen under water than in a comparable situation on dry land.