Diving reflex

[1][2][3] It optimizes respiration by preferentially distributing oxygen stores to the heart and brain, enabling submersion for an extended time.

[1] Adult humans generally exhibit a mild response, the dive-hunting Sama-Bajau people being a notable outlier.

[7] The diving reflex is triggered specifically by chilling and wetting the nostrils and face while breath-holding,[2][8][9] and is sustained via neural processing originating in the carotid chemoreceptors.

[1] 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 and the brain (and lungs), concentrating flow in a heart–brain circuit and allowing the animal to conserve oxygen.

[17] Bradycardia is the response to facial contact with cold water: the human heart rate slows down ten to twenty-five percent.

[4][18] During breath-holding, humans also display reduced left ventricular contractility and diminished cardiac output,[10][19] effects that may be more severe during submersion due to hydrostatic pressure.

[19] Slowing the heart rate reduces the cardiac oxygen consumption, and compensates for the hypertension due to vasoconstriction.

[17] Snorkel breathing is limited to shallow depths just below the surface due to the effort required during inhalation to overcome the hydrostatic pressure on the chest.

[17] Facial immersion at the time of initiating breath-hold is a necessary factor for maximising the mammalian diving reflex in humans.

[3] Chronic physiological adaptations of blood include elevated hematocrit, hemoglobin, and myoglobin levels which enable greater oxygen storage and delivery to essential organs during a dive.

[3] Oxygen use is minimised during the diving reflex by energy-efficient swimming or gliding behaviour, and regulation of metabolism, heart rate, and peripheral vasoconstriction.

During diving, the hematocrit and hemoglobin are temporarily increased by reflex splenic contraction, which discharges a large additional amount of red blood cells.