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 is the process of maintaining a level of internal balance that our bodies prefer. Leaving our bodies' equilibrium means possible negative effects, so we have automatic built-in responses to counter it.
When our body goes past its natural temperature of around 98 degrees Fahrenheit (36.67 °C), we sweat to reduce heat. If our blood sugar levels drop below natural levels, insulin is released into the bloodstream to bring them back to normal. If we become too cold, nerves on our skin may make us shiver involuntarily. Touching a hot object should make our hands jolt back before they're burned.
A need for homeostasis is also what makes us hungry when we eat and tells our immune system to defend us against invasive bacteria or viruses. Without homeostasis, our bodies wouldn't function anywhere near as optimally as they normally do. The mammalian diving reflex is one of the most common forms of reflexes meant to maintain homeostasis. Several individual reactions happen at once the moment our bodies receive the stimulus to trigger it.
Nature of the Reflex
The mammalian diving reflex is what mammals use to handle being underwater. Even the mammals that live underwater aren't capable of taking in oxygen by inhaling water. As a result, they have to store whatever oxygen their bodies already have for as long as they're underwater.
The process is akin to holding in a deep breath, except that your entire body is working to hold in any oxygen stored within it. Feeling cold water(below 70 degrees Fahrenheit (21.11 °C)) triggers the response, stimulating nerves around the front of the nose. Alternatively, humans can trigger this response by holding our breath. When this happens, the body undergoes a process of autonomic shifts in order to conserve oxygen for as long as possible—ideally until they return above water.
Our bodies store oxygen in several ways. The most well-known means is with our lungs. When we breathe, oxygenated air travels from our mouth or nose down our windpipe to reach our lungs. Once there, air is sent to ballon-shaped sacs called alveoli, where it's transferred to blood vessels surrounding our lungs that are called capillaries. Once it reaches the bloodstream, oxygen travels through the body, and carbon dioxide goes back to the lungs. We breathe out the carbon dioxide and the process starts all over again.
With oxygen taken from the lungs to our bloodstream, hemoglobin transfers the oxygen to the tissues in our body. One type of tissue it delivers oxygen to is our muscles in order for them to function optimally. Here oxygen is stored in proteins called myoglobin.
The levels of these various proteins in our blood helps to determine how much oxygen we have available to use. Animals with higher levels of myoglobin or hemoglobin can store more oxygen in their bodies. Aquatic mammals such as whales and seals increase the amount of hemoglobin and myoglobin in their bodies as part of the mammalian diving reflex. Some animals, like muskrats, can even vary the volume of these oxygen-storing cells based on which season it is. Humans have to work with what oxygen is already available.
This overall limit of how much oxygen an animal can store is the aerobic dive limit. Lactic acid builds internally when these limits are surpassed. The effectiveness of the response depends on the level of oxygen stores within the blood. Humans and other mammals that aren't considered aquatic can only briefly maintain oxygen stores underwater due to only being able to store a certain amount of oxygen within their blood.
What Happens to Your Body during the Mammalian Diving Reflex?
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
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.
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.
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.