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What is the mammalian diving reflex?

Diving is one of the most enjoyable water sports everyone should try at least once in their lifetime. Of course, given its nature, there are some interesting facts about the sport. When you dive underwater, your body instinctively changes some parts of its physiology. Known as the mammalian dive reflex, this set of intuitive adjustments that occur in humans is also found in other mammals. What exactly does the diving reflex involve? And what purpose does it serve? Learning about this biologically important, innate response will also teach you a lot about human physiology in general.

What happens as a part of the mammalian diving reflex and why?

Man swimming underwater
Duet PandG/

At its most basic, the dive reflex shuts off some parts of the body in order to conserve oxygen while underwater. The mammalian diving reflex in humans particularly affects the cardiovascular system. Three main changes happen when the nostrils are submerged underwater: The body stops breathing (apnea), the heart rate slows down (bradycardia), and the blood flow to the extremities is decreased (peripheral vasoconstriction). These alterations are done so that what limited oxygen is left in the body is rationed most efficiently.

Other modifications are made to the body’s physiology as a part of the diving reflex, too. For instance, the spleen contracts, which releases oxygenated blood cells into the bloodstream. This aspect of the mammalian dive reflex is arguably the most important in allowing an organism to stay underwater for an extended period of time. In fact, the Austronesian Sama-Bajau peoples, who are known for their ability to go on long, deep dives, have spleens that are on average 50% larger than people who are not Sama-Bajau. A larger spleen means a larger volume of red blood cells being produced, and in turn higher oxygen availability.

Another common feature of the mammalian diving reflex in humans is cardiac arrhythmias (irregular heartbeats). All of the changes that occur during the dive reflex serve to prioritize the distribution of oxygen to the most important organs — the brain, heart, and lungs. The brain contains the only cells in the body that constantly require oxygen, as all other organs can survive without oxygen for a limited amount of time. But hypoxia (low oxygen levels) in the brain can lead to irreversible damage, so your body works its hardest to always provide oxygen to the brain.

A similar but more intense set of cardiovascular events happens when humans are suddenly immersed in cold water, known as the cold shock response. The cold shock response is very taxing on a person and can lead to death if a person inhales water from hyperventilation or if they go into cardiac arrest from extreme vasoconstriction.

Do all people have the dive reflex?

Yes, everyone has the diving reflex. As the term reflex suggests, the mammalian dive reflex is inherent to humans. The diving reflex even happens in babies, and is the reason they do not choke on milk. However, this reflex is not as strong in babies older than 6 months.

Note that the diving reflex does not take place when just the limbs are placed in water. The nostrils must be covered for the body to register that it is underwater and needs to distribute oxygen accordingly. In humans, the mammalian dive reflex is more of a survival mechanism than a way to go about our daily activities — unless you condition your body to do the latter, like the Sama-Bajau have. But the same cannot be said for the diving reflex in other animals.

What other animals have the mammalian dive reflex?

Weddell seal in water between ice

Although often referred to as the mammalian diving reflex, because it was first observed in mammals, this physiological reaction happens in all vertebrates — mammals, birds, reptiles, and amphibians. It is especially pronounced in aquatic mammals. The three main changes occur in all animals, but others including cardiac arrhythmias do not.

Marine mammals like the Weddell seal are exceptional divers. Weddell seals have been recorded diving for as long as 80 minutes at a time to depths of 2,300 feet. They would not be able to achieve such a feat if it weren’t for the dive reflex — though some additional adaptations are also at work! Weddell seals and other aquatic animals have larger blood volumes than humans, and their blood cells also carry more oxygen.

The mammalian diving reflex is an important survival mechanism in many animals, including humans. As we have learned, a number of changes happen in a body when it is submerged in water so that the limited oxygen that remains in the body is used most effectively.

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