Hypothermia Series – Part 4: The Mammalian Diving Reflex

In a prior post, I outlined the the “cold shock” response that the human body experiences during a sudden immersion into cold water. To recap, a person who has not properly prepared in advance for a cold water swim will experience a rather “rude awakening” upon entry into the water characterized by rapid, uncontrollable breathing (the “gasp reflex”) as well as an immediate increase in blood pressure and heart rate due to a sudden constriction of the capillaries under the skin. This is not a pleasant experience.

One way to minimize or avoid this response is to prepare the body in advance by using the “progressive cold shower” technique outlined in the above link. Another way of effectively managing the risk of cold water immersion shock is to take advantage of the “mammalian diving reflex.”

The mammalian diving reflex (MDR) is a universal physiological reflex found in all vertebrates, but it is more pronounced in marine mammals such as dolphins, seals, and whales. The MDR is essentially triggered by a combination of breath holding and of cold water exposure to the face. Once triggered, the MDR causes a bradycardia response (lowered heart rate), and it restricts one’s peripheral blood flow while preserving it for one’s brain and vital organs. In essence, the MDR is a protective mechanism for mammals (humans included) in cold water immersion situations.

The key takeaway here is that you can use the MDR to further minimize any potential cold water immersion shock. Instead of testing the water with your hands or feet, start out first by placing your face or head into the water while holding your breath. This will trigger the MDR and make it much easier when you decide to “take the plunge.”

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