Although we are strictly terrestrial animals, our body retains an ancestral survival mechanism shared with seals, whales, and dolphins: the mammalian dive reflex. This physiological circuit is triggered automatically and involuntarily when thermoreceptors of the trigeminal nerve, located on the forehead and around the eyes, detect water at a temperature lower than the ambient environment.
Within milliseconds of wetting your face, a triad of relentless responses is triggered:
- Reflex bradycardia: The vagus nerve sends a signal that drastically slows the heart rate, reducing it between 10% and 30% in average people, and up to 50% in trained freedivers, suddenly minimizing oxygen consumption.
- Peripheral vasoconstriction: Capillaries in extremities such as hands and feet close tightly, concentrating blood flow solely in the brain and heart.
- Thoracic blood shift: During deep dives, plasma fills the alveolar vessels to prevent hydrostatic pressure from crushing the lungs.
Discovered in humans in the mid-20th century by physiologist Per Scholander, this mechanism does not just protect us from hypoxia underwater. In modern medicine, emergency physicians take advantage of this reflex by submerging a patient's face in a basin of ice water to reverse paroxysmal supraventricular tachycardias in seconds, without needing medications.
Source: Journal of Applied Physiology (Scholander et al., 'Circulatory defense against asphyxia in diving man')
