How do world-class freedivers manage to break one apnea record after the next? Practice and skill have a lot to do with it, but so does a physical reflex that all of us have: The mammalian diving reflex. How does this fascinating process work? Let’s take a closer look.
Understanding the Mammalian Diving Reflex
If you are a scuba diver, you might remember how difficult it seemed to take those first few breaths from a regulator underwater. This might have been partly from nervousness, but part of your initial hesitation to breathe from your regulator is reflexive. The diving reflex is part of human nature: We instinctively want to hold our breath when underwater. A newborn baby or a person who has never learned to fear water automatically relaxes and swims while holding his or her breath underwater. These actions happen because of the diving reflex and the swimming reflex.
Every mammal has a diving reflex, and in marine mammals such as cetaceans, otters, and seals, this reflex is highly developed. Humans can learn to develop the diving reflex too, by practicing and by understanding how it works. There are two complicated physical reactions associated with the mammalian diving reflex; luckily, these can be simplified for easy understanding:
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Peripheral vasoconstriction – Peripheral vasoconstriction is accomplished via muscle contractions within the blood vessels themselves. This reduces the amount of blood flow as well as the amount of oxygen your muscles receive, and ensures that your vital organs receive the blood and oxygen that they require for your survival. All marine mammals and diving birds experience vasoconstriction while submerged in water.
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Reduced heart rate (Bradycardia) – The heart rate drops as soon as your face is submerged in water. While this might sound frightening, it benefits you by enabling you to conserve oxygen. The average person experiences a ten to thirty percent reduction in heart rate during freediving, but many experienced freedivers have trained their bodies to reduce heart rate by as much as fifty percent. The colder the water, the greater the reduction in heart rate.
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Blood shift – Blood shift occurs only during very deep dives. When this happens, the body’s circulatory walls and organs allow water and plasma to pass freely through the thoracic cavity, ensuring that pressure remains constant and vital organs are not crushed by water pressure. During blood shift, the tiny alveoli inside your lungs fill with plasma, which is reabsorbed when you leave the high-pressure environment. This stage of the diving reflex does not happen to freedivers enjoying descents to recreational depths. It has been observed only in humans who descend into very deep environments, typically over 90 meters.
Both vasoconstriction and bradycardia happen only with exposure to water. Dry apnea, which involves breath holding while out of water, is a good way to learn breath-up techniques and increase your lung capacity, however it doesn’t completely prepare your body for freediving. Submersion, even in shallow water, helps to trigger the mammalian dive reflex. Practice as frequently as you can, even in fairly shallow water, to help strengthen your own mammalian diving reflex and improve your freediving performance.