Home Dive Training The Mammalian Dive Reflex and Freediving Safety

The Mammalian Dive Reflex and Freediving Safety

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Blue whale - shutterstock

Depth record for sperm whales: 7,380 feet

Depth record for elephant seals: 7,835 feet

Depth record for humans: 702 feet

Going seven hundred feet beneath the surface in a single breath-hold dive may pale in comparison to the abilities other diving mammals, but for the average human it is an unimaginable feat. Humans did not evolve to venture to extreme depths or remain underwater for prolonged periods like cetaceans and pinnipeds, but one remnant of our common ancestry resurfaces every time we hold our breaths to dive underwater: the mammalian dive reflex.

By Caitlyn Ruskell

Diving beneath the surface brings some people a feeling of unrivaled peace and tranquility, but just the thought of doing so can cause others to panic, and for good reason: Humans cannot breathe underwater. As terrestrial organisms we survive best on land where we can readily breathe air to fuel our cells with oxygen, but we still have some traits that support our ventures into aquatic environments. The mammalian diving response, or “dive reflex,” is one response our body uses to protect our vital organs, primarily the brain and heart, from becoming hypoxic, or oxygen deficient, when we hold our breath underwater.

The mammalian dive reflex can be triggered in humans simply by putting your face in cold water, but the effects of this reflex are even more pronounced if you hold your breath while immersing your face. Whether you’re holding your breath or not, the trigeminal nerve in your face signals the brain as soon as your face hits the water; the brain then processes this signal and sends a signal to the vagus nerve. The vagus nerve is part of your autonomic nervous system, which controls our heart rate and breathing among other things. In this response, the vagus nerve signals the heart to beat slower and stimulates constriction of blood vessels in the extremities. When the whole body is immersed in water, hydrostatic pressure shunts some blood away from the extremities toward the core, and vasoconstriction prompted by the vagus nerve’s signal sends even more blood toward the core where it can be more easily pumped to the heart and brain.

Although this reflex helps our bodies conserve oxygen when we freedive, our bodies are constantly burning oxygen to fuel our muscles during breath-hold dives. Since we cannot breathe underwater, this oxygen isn’t replaced until we come back to the surface and take a breath. At depth, the elevated pressure surrounding your body keeps your tissues saturated with the oxygen you inspired in your last breaths before the dive, but if you are pushing yourself to make a longer dive and are running low on oxygen when you make your ascent, the oxygen in your tissues will continue to decrease as the pressure surrounding your body decreases. Some tissues, such as brain tissue, are very sensitive to oxygen deprivation, and if you do not have enough oxygen left to fuel your brain, you may become unconscious just before you reach the surface. This is known as a blackout, and 99 percent of blackouts occur at the surface or within 16 feet of it.

Blackouts in freediving do not have to end in tragedy; you can take measures to avoid them and manage them when they occur. One way to decrease the likelihood of a blackout is to NEVER hyperventilate before a breath-hold dive. Historically, divers would hyperventilate before a dive because it would enable them to hold their breath longer. Hyperventilation eliminates a lot of carbon dioxide (CO2) with each exhalation, and because CO2 triggers the urge to breathe, dampening the trigger limits a diver’s urge to breathe. If you hyperventilate before a dive you may not feel the urge to breathe, stay underwater even longer than you normally would, deplete even more oxygen and increase your chances of blacking out as you ascend.

Two other precautions to take every time you freedive are to have a vigilant, trained buddy and to weight yourself properly. Every time you freedive, your buddy should maintain direct, one-on-one supervision and proximity (on the surface) in case anything goes wrong. Before you dive, make sure your buddy is not only aware of the issues that can occur but make sure they are trained in proper rescue techniques and can correctly manage a blackout at the surface. Additionally, make sure they can dive to the same depth as you. Once you have a buddy that knows his or her stuff, make sure you both are wearing the proper amount of weight. It can be tempting to load up on lead to “get down easier” but if you were to have an emergency underwater, over-weighting can create a situation that even the most competent buddy couldn’t manage. Freedivers weight themselves so that if a blackout occurs they will float to the surface.

The human body can put up with some amazing things, and traits of our distant, ocean-dwelling ancestors are still apparent within us today. These traits help us conserve oxygen when we hold our breath and dive underwater, but to the dismay of many, we still cannot breathe underwater without a life-support system. Therefore, every time we freedive we must take extra precautions to keep ourselves safe. If you are interested in becoming a better and safer freediver — or trying it out for the first time — take a freediving course. Taking a course gives you crucial knowledge about freediving physiology and the practical training you need to enjoy the underwater world safely and comfortably.