Mammalian diving reflex

Submerging the face into water causes the mammalian diving reflex, which is found in all known mammals (including humans, although it is less pronounced), but especially in marine mammals (as, for example, whales and seals.) This reflex puts the body into oxygen saving mode to maximize the time that can be spent under water, and includes three factors:
 * Bradycardia, a reduction in the heart rate (of about 20% in humans).
 * Peripheral vasoconstriction, a decrease in blood flow to the extremities, in order to increase the supply of blood and oxygen to the vital organs, especially the brain.
 * Blood shift, the shifting of blood plasma to the thoracic cavity, i.e. the chest between the diaphragm and the neck to avoid the collapse of the lungs under higher pressure during deeper dives.

Thus, both a conscious and an unconscious person can survive longer without oxygen under water than in a comparable situation on dry land. Children tend to survive longer than adults when deprived of oxygen underwater.

When the face is submerged, receptors that are sensitive to water within the nasal cavity and other areas of the face supplied by cranial nerve V (trigeminal) relay the information to the brain and then innervate cranial nerve X, which is part of the autonomic nervous system. This causes bradycardia and peripheral vasoconstriction of blood vessels. Blood is removed from the limbs and all organs but the heart and the brain, creating a heart-brain circuit and allowing the mammal to conserve oxygen.

In humans, the mammalian diving reflex is not induced when limbs are introduced to cold water. Mild bradycardia is caused by the subject holding his breath without submerging the face within water. When breathing with face submerged this causes a diving reflex which increases proportionally to decreasing water temperature. However the greatest bradycardia effect is induced when the subject is holding breath with face submerged.

Criticism
"diving r. a reflex involving cardiovascular and metabolic adaptations to conserve oxygen while diving into water; seen in reptiles, birds, and mammals, including humans." Merck Medical Dictionary - Diving reflex is not just mammals but rather all vertebrates.