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Mammalian diving reflex

An ancient automatic reflex present in every mammal, humans included, triggered when cold water touches the face and the breath is held. The trigeminal nerve carries the signal to the medulla, which answers with three things at once: the heart slows, vessels in skin and muscle clamp down to reserve blood for heart and brain, and peripheral blood is pushed by pressure into the chest. In humans the heart rate falls 15–40%; in seals and whales, by more than 85%. What matters is that the reflex is not learned — it is already in every body, and training only makes it arrive sooner and go deeper.