Mammalian diving reflex

The diving reflex, also known as the diving response and mammalian diving reflex is a response to immersion that overrides the basic homeostatic reflexes, which is found in all air-breathing vertebrates. It optimizes respiration by preferentially distributing oxygen stores to the heart and brain which allows staying underwater for extended periods of time. It is exhibited strongly in aquatic mammals (seals,otters, dolphins, muskrats), but exists in other mammals, including humans, in particular babies up to 6 months old (see Infant swimming). Diving birds, such as penguins, have a similar diving reflex. It may be the evolutionary development of a more primitive response to hypoxia exhibited by fishes. The diving reflex is triggered specifically by chilling the face and breath-hold. The most noticeable effects are on the cardiovascular system, which displays peripheral vasoconstriction, slowed pulse rate, redirection of blood to the vital organs to conserve oxygen, release of red blood cells stored in the spleen, and, in humans, heart rhythm irregularities.

Aquatic mammals have evolved physiological adaptations to conserve oxygen during submersion, but the apnea, bradycardia, and vasoconstriction are shared with terrestrial mammals as a neural response.

When the face is submerged, receptors that are sensitive to cold within the nasal cavity and other areas of the face supplied by the fifth (V) cranial nerve (the trigeminal nerve) relay the information to the brain and then innervate the tenth (X) cranial nerve, (the vagus nerve), which is part of the autonomic nervous system. This causes bradycardia and peripheral vasoconstriction. Blood is diverted from the limbs and all organs but the heart and the brain, concentrating flow in a heart–brain circuit and allowing the animal to conserve oxygen.

In humans, the diving reflex is not induced when limbs are introduced to cold water. Mild bradycardia is caused by subjects holding their breath without submerging the face in water. When breathing with face submerged the reflex strength increases proportionally to decreasing water temperature. However the greatest bradycardia effect is induced when the subject is holding breath with face submerged. Both apnea and facial cooling are triggers of this reflex, but actual water contact with the face appears to be unimportant.

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