A couple of months back I finished the book Deep: Freediving, Renegade Science, and What the Ocean Tells us About Ourselves written by James Nestor. The author got into the idea and world of free diving after being assigned to cover a free diving competition with divers reaching depths of over 200 feet on just a single breath of air. The competition is gruesome and not for the faint of heart, but Nestor was captivated. He did not enjoy the competitive aspect of free diving, but the fact that humans have the ability to stay underwater for over three minutes on a single breath and can become super connected to the ocean while doing so amazed Nestor. Regarding this incredibly ability Nestor writes, “Scientists call it the mammalian dive reflex or, lyrically, the Master Switch of Life, and they’ve been researching it for the pat fifty years.”
What Nestor is discussing when he mentions the Master Switch of Life is the human body’s ability to change its physiological functions while under water unassisted by scuba gear. But, as Nestor explains, it is not just humans who accomplish this feat. The term mammalian dive reflex is the most accurate because there are many marine mammal species, whales, dolphins, sea lions, who have the same physiological reaction to the icy depths of the ocean that humans do. If a sea lion can live on the surface and dive hundreds of feet into the the ocean to hunt for food, then clearly life has found a way to adapt to these intense changes of environment. Restricting the mammalian dive reflex to marine mammals seems far more natural than extending the body’s reactions to humans, but Nestor explains what physiological changes take place.
During a dive the lungs are forced inwards by the incredibly pressure outside the body and the fact that there is not enough air in the body to hold the lungs out to their normal size. Around 30 feet down the lungs are half their normal size, and close to 300 feet down they shrink to the size of baseballs. As this happens our bodies force blood away from our extremities and back towards our chest and brain. The heart is slowed, but blood is brought back from our arms and legs to make it easier for our heart to pump. In this state, we can survive the massive pressures of the ocean on just a single breath of air.
Nestor also explains that during the dive our lungs change shape so dramatically that if we were to inhale oxygen in a tank filled at normal atmospheric pressures, that air would blast into our lungs so rapidly that the force would burst our lungs. While it sounds terrifying to dive so deeply and not be able to take a breath of air while we are under or as we ascend, it gives the body a unique advantage. A free diver with their single breath of air can dive incredibly deep and shoot back to the surface as quickly as they would like. Their body adjusts naturally for the changes in pressure both inside and outside the body, and as soon as the diver breaks the surface they can take a deep breath. Scuba divers on the other hand must stop to decompress as regular intervals as they ascent from the ocean depths. This is because the compressed air breathed in under water by divers cannot adjust to the changes of pressure as a diver surfaces. Nitrogen will bubble out of the blood in a scuba diver if they ascend too quickly, because there will be less pressure on the body and the cardiovascular system to keep the nitrogen dissolved within the blood. Divers must stop at regular intervals to assure that their body is able to handle the process of pulling nitrogen out of the blood in a safe manner, without it rapidly bubbling up and building up in their joints. This symptom is called the bends, but it only affects scuba and assisted divers, not free divers. The mammalian dive reflex evolved millions of years ago, and solved the problem that scuba divers must be cautious to prevent.