The mammalian dive reflex (MDR) is a series of processes which reduce the use of O2 and conserve it for use by the vital organs. It is exhibited strongly in aquatic mammals (seals, otters, dolphins, etc.), but exists in a weaker version in other mammals, including humans. Diving birds, such as penguins also have a similar diving reflex. In humans the diving reflex can be triggered by cold water on the face, around the nose and sinuses. Water that is warmer than 21 °C (70 °F) does not cause the reflex (unless the person dives to depth) and neither does submersion of body parts other than the face. The reflex is more pronounced in younger individuals.
MDR consists of three main processes in this order:
(1) Bradycardia is the first response to facial submersion in cold water. Immediately upon facial contact, the human heart rate slows down ten to twenty-five percent of its normal rate. Seals experience changes that are even more dramatic, going from about 125 beats per minute to as low as ten on an extended dive. The reduction of the heart rate lessens the need for bloodstream oxygen, leaving more O2 to be used by other organs.
(2) Next, peripheral vasoconstriction sets in, this occurs when high pressure in deep diving causes the capillaries in the extremities to start closing off, stopping blood circulation to those areas. Normally the term vasoconstriction applies to the arterioles, but in this case it is completely an effect originating in the capillaries. The capillaries contract and restrict the circulation of blood first in the toes and fingers, then hands and feet, and lastly arms and legs, leaving more blood and thus more O2 for the heart and brain.
(The body’s muscles may suffer cramping in the later stages of this phase as human musculature accounts for only 12% of the body’s total oxygen storage. This means that muscles can only continue to function for a short time after the blood stops supplying fresh O2. Aquatic mammals store up to 25 to 30% of their O2 in muscle, and thus they can keep working long after capillary blood supply is stopped.)
(3) The last process is the blood shift, which occurs only during very deep dives or dives performed in exhale. When this happens, organ and circulatory walls allow plasma/water to pass freely throughout the thoracic cavity so its pressure stays constant and the organs are not crushed. In this stage, the lungs’ alveoli fill up with blood plasma, which is reabsorbed when the diver leaves the pressurized environment. This stage of the diving reflex has also been observed in free-divers. The spleen has been also observed to compress, releasing fresh O2 rich blood.
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. The exact mechanism for this effect has been debated but in cold water may be a result of brain cooling similar to the protective effects seen in patients treated for deep hypothermia. When the face is submerged, receptors that are sensitive to water within the nasal cavity and other areas of the face supplied by the cranial nerve V (trigeminal) relay the information to the brain and innervate cranial nerve X, which is part of the autonomic nervous system. This results in bradycardia and peripheral vasoconstriction. Blood flow to the limbs and all organs but the heart and the brain is reduced, creating a heart-brain circuit, thus allowing the mammal to conserve oxygen.
In humans, the mammalian diving reflex is not triggered when limbs are immersed in cold water. But mild bradycardia can be induced by the subject holding their breath even without submerging the face in water. Keeping the face submerged in cold water can cause a diving reflex even without holding the breath. The effects of MDR induced by facial immersion increase with lower water temperature. The greatest bradycardia effect is induced when the subject holds their breath with the face submerged in water that is significantly colder than ambient temperature.
Most importantly for free-divers the MDR is a reflex that can be developed with repetitive practice. The blood shift has even been observed in free-divers performing static apnea at the surface, something that would only be expected at depth.