Facts About The Mammalian Diving Reflex

How do our bodies react to being submerged in water? It might be second nature to animals that live underwater, but everyone else has to compensate for temporarily being underwater. Mammals do so via the mammalian diving reflex. It's what allows divers to remain several feet underwater with no need to come up for air and allows seals to hunt for food without taking a breath before they catch their prey.

What Is the Mammalian Diving Reflex?

Homeostasis

Nature of the Reflex

What Happens to Your Body during the Mammalian Diving Reflex?

woman wearing black dress underwater

For the response to happen, there has to be an appropriate stimulus. The stimulus in this case is when a nerve (the anterior ethmoidal nerve), positioned within the entrance to our nostrils, comes in contact with cold water. This doesn't have to happen from a dive either; simply splashing cold water across your face or directly into your nose would suffice.

Once the response is triggered, three primary things happen. First, our bodies reduce our heart rate to minimize the use of oxygen stored inside our blood cells. This is called bradycardia and may differ depending on how trained the mammal is—seal or human. While aquatic animals can always maintain longer deep dives than even the most trained humans, humans still can improve their ability to induce bradycardia with practice. Most people will only manage to slow their heart rate to 10-30%, while elite divers can reduce their heart rate by 50% or more.

The downside is that this decrease in heart rate, by itself, would likely cause us to pass out from exceedingly low blood pressure. Cue the next step of the mammalian diving reflex: peripheral vasoconstriction. Blood flow is limited to the most important areas of the body. It leaves the muscles, skin, and other temporarily unnecessary body parts to send oxygenated blood to the most important internal organs necessary for living—the heart and brain.

One homeostatic response that should typically happen at this point is breathing. Anyone who has intentionally held their breath for as long as possible may know how difficult it is. Our body's demand for oxygen inevitably exceeds our intention to hold breath in—normally. The impulse to breathe is reduced during a dive, despite lactic acid buildup increasing the urge to inhale. Lactic acid is only released into the bloodstream once the dive ends.

Fascinating Facts about the Mammalian Diving Reflex

Aquatic Divers Differ 

whale swimming underwater

Diving mammals have 9.5 times more hemoglobin in their bodies than land-dwelling mammals. This higher amount of blood volume allows them to dive for extended periods of time compared to animals that normally live on land.

Despite this, seals and other aquatic mammals are not proficient divers the moment they are born. Young seals begin diving by going into shallow waters to acclimate themselves to the experience before delving deeper. Earless seals even store more oxygen in their blood and muscles than in their lungs.

Seals appear capable of controlling their heart rate to some extent. They are able to decrease their heart rates before diving into water, where the diving response would trigger bradycardia for them. As their bodies enter the water, their heart rates quicken until leveling out once they are fully submerged.

The diving response in aquatic mammals also induces hypothermia. Lowering internal body temperatures slows their metabolism and minimizes the amount of oxygen needed to function during their deep dives.

Possible Cause of Death 

The mammalian diving reflex could explain sudden infant death syndrome, SIDS. SIDS is defined as the death of someone younger than one year old without any explanation. Even after a thorough autopsy, inspection of the area they died in, and examination of any possible history of inherited conditions, the deaths remain unexplained. One possible reason it occurs is due to some issues with brain and nervous system development that would inhibit a baby's ability to breathe normally.

There are other cases of sudden death that might find an explanation originating in the mammalian diving reflex. In 1978, a scientist reported a case of an elite diver experiencing sudden cardiac death due to sinus arrest while they were getting ready to dive. No further cases have been reported, implying that this rarely happens to people.

Another case of cardiac arrest happened to a young recruit in the army. Someone threw cold water on his face while he was asleep. What seemed to be one of the most common pranks in the world triggered his diving response and ended his life.

Anxiety Solution

woman underwater

Since the diving reflex slows down the diver's heart rate, it is useful for calming down before a stressful situation. Performances, job interviews, and competitive events can all be anxiety-inducing scenarios. Even if we understand how stressful it will be beforehand, anxiety can still reduce our ability to perform. By throwing cold water over your face or holding in your breath, you can induce this involuntary response and help your body relax.

Conclusion

Mammals have adapted to the need to dive underwater for brief periods of time. The diving response is one of the most ingenious ways of optimizing the use of oxygen until they can return above water for another breath. Even if you never take up swimming, it is still useful to calm yourself before important events. A simple dash of cold water could mean the difference between success and failure.

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