The diving reflex, also known as the diving response and mammalian diving reflex, is a set of physiological responses to immersion that overrides the basic homeostatic reflexes, and is found in all air-breathing vertebrates studied to date. It optimizes respiration by preferentially distributing oxygen stores to the heart and brain, enabling submersion for an extended time. The diving reflex is exhibited strongly in aquatic mammals, such as seals, otters, dolphins, and muskrats, and exists as a lesser response in other animals, including human babies up to 6 months old (see infant swimming), and diving birds, such as ducks and penguins. Adult humans generally exhibit a mild response, although the dive-hunting Sama-Bajau people and the Haenyeo divers in the South Korean province of Jeju are notable outliers. The diving reflex is triggered specifically by chilling and wetting the nostrils and face while breath-holding, and is sustained via neural processing originating in the carotid chemoreceptors. The most noticeable effects are on the cardiovascular system, which displays peripheral vasoconstriction, slowed heart 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. Although aquatic animals have evolved profound physiological adaptations to conserve oxygen during submersion, the apnea and its duration, bradycardia, vasoconstriction, and redistribution of cardiac output occur also in terrestrial animals as a neural response, but the effects are more profound in natural divers.
Physiological response When the face is submerged and water fills the nostrils, sensory receptors sensitive to wetness 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. The tenth (X) cranial nerve (the vagus nerve) – part of the autonomic nervous system – then produces bradycardia and other neural pathways elicit peripheral vasoconstriction, restricting blood from limbs and all organs to preserve blood and oxygen for the heart, brain, and lungs, 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 the face submerged, the diving response increases proportionally to decreasing water temperature. However, the greatest bradycardia effect is induced when the subject is breath-holding with the face wetted. Apnea with nostril and facial cooling are triggers of this reflex. Children tend to survive longer than adults when deprived of oxygen underwater. The exact mechanism for this effect has been debated and may be a result of brain cooling similar to the protective effects seen in people treated with deep hypothermia. The diving response in animals, such as the dolphin, varies considerably depending on level of exertion during foraging.
Exceptions in human divers In humans whose historic way of life involves foraging for food underwater by breath-hold diving, there is evidence for more extensive physiological and genetic adaptations of the diving reflex than in typical humans. Having harvested underwater seafood over centuries, the nomadic Sama-Bajau people of Southeast Asia have enlarged spleens and more intense peripheral vasoconstriction during breath-hold diving – giving advantages for prolonged underwater hunting – and display natural selection for the genes controlling these adaptations. Similarly, the Haenyeo women divers of South Korea have pronounced bradycardia and exceptional cold tolerance during breath-hold diving, with evidence of adaptive genetic variation contributing to these advantages.
Carotid body chemoreceptors During sustained breath-holding while submerged, blood oxygen levels decline while carbon dioxide and acidity levels rise, stimuli that collectively act upon chemoreceptors located in the bilateral carotid bodies. As sensory organs, the carotid bodies convey the chemical status of the circulating blood to brain centers regulating neural outputs to the heart and circulation. Preliminary evidence in ducks and humans indicates that the carotid bodies are essential for these integrated cardiovascular responses of the diving response, establishing a "chemoreflex" characterized by parasympathetic (slowing) effects on the heart and sympathetic (vasoconstrictor) effects on the vascular system.
Circulatory responses Plasma fluid losses due to immersion diuresis occur within a short period of immersion. Head-out immersion causes a blood shift from the limbs and into the thorax. The fluid shift is largely from the extravascular tissues and the increased atrial volume results in a compensatory diuresis. Plasma volume, stroke volume, and cardiac output remain higher than normal during immersion. The increased respiratory and cardiac workload causes increased blood flow to the cardiac and respiratory muscles. Stroke volume is not greatly affected by immersion or variation in ambient pressure, but bradycardia reduces the overall cardiac output, particularly due to the diving reflex in breath-hold diving.
Bradycardia and cardiac output Bradycardia is the response to facial contact with cold water: the human heart rate slows down ten to twenty-five percent. Seals experience changes that are even more dramatic, going from about 125 beats per minute to as low as 10 on an extended dive. During breath-holding, humans also display reduced left ventricular contractility and diminished cardiac output, effects that may be more severe during submersion due to hydrostatic pressure. Slowing the heart rate reduces the cardiac oxygen consumption, and compensates for the hypertension due to vasoconstriction. However, breath-hold time is reduced when the whole body is exposed to cold water as the metabolic rate increases to compensate for accelerated heat loss even when the heart rate is significantly slowed.
Splenic contraction The spleen contracts in response to lowered levels of oxygen and increased levels of carbon dioxide, releasing red blood cells and increasing the oxygen capacity of the blood. This may start before the bradycardia.
Blood shift
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