Every air breathing vertebrate studied has some version of the mammalian diving reflex, and infants up to about six months can often survive underwater longer than adults. This episode explains the trigger: cold water on the face around the nose and eyes while holding the breath. The trigeminal nerve signals the brainstem, the vagus nerve slows the heart, and rising carbon dioxide, which turns to carbonic acid in the blood, sets off the carotid bodies. In people the heart rate drops 10 to 25 percent while vessels in the limbs clamp shut to keep oxygenated blood flowing between heart and brain.

Seals take it much further, slowing from around 125 beats per minute to as low as 10, kept going by an elastic aortic bulb, a spleen that squeezes stored red blood cells into circulation, and muscles packed with myoglobin. The hosts then turn to the Sama Bajau sea nomads, whose enlarged spleens are linked to variants in the PDE10A gene, and Korea’s Haenyeo women divers of Jeju. The episode closes on the toll diving takes on the human body and why splashing cold water on your face can calm a racing heart.

• Immersion diuresis: blood pushed into the chest stretches the heart, which releases atrial natriuretic peptide and sends you looking for a bathroom
• Diving can trigger ectopic beats and ST depression as the vagus nerve and soaring blood pressure pull against each other
• Diving mammals also carry neuroglobin and cytoglobin to protect brain tissue
• The Haenyeo show unusually strong heart slowing and cold tolerance
• Physician Edmund Goodwyn described diving bradycardia in humans in 1786