Understanding the mammalian diving reflex

The animals that live and feed underwater have developed a method to conserve oxygen and defend against hypoxia when diving under the water. This ability is termed the diving response or mammalian diving reflex (MDR), which controls the constriction of blood vessels in certain parts of the body (vascular constriction) and decreases heartbeat (bradycardia). This was not known to be capable by humans until the 1950s, but was observed by researchers in whales, seals, sea lions, and penguins in the early 1900s.

The mammalian diving reflex begins when you hold your breath, which is called apnea. The MDR is then reinforced when your head is submerged. When you return to the surface and begin breathing again, the MDR stops.

A full understanding of humans and the mammalian diving reflex is still not known, however, the ability for humans to have this reflex is dependent on several key factors, such as vascular constriction, bradycardia, and blood shunting and shifting within the body. Vascular constriction acts in parts of the body, such as the hands, feet, arms, and legs that can tolerate a decrease in oxygen and can function with an anaerobic metabolism. The body slows and eventually stops blood supply to these areas by the constricting of the blood vessels. The body’s organs that demand a constant supply of oxygen are the brain and heart. The mammalian diving reflex does all it can so that these organs receive a constant supply of oxygen rich blood. Because of decreased circulation in many parts of the body, the heart does not have to work as hard, which reduces the amount of oxygen used throughout the body.

Bradycardia, the slowing of the heart, is a mammalian diving reflex response that decreases pulse rate in a normal individual by 10 to 30 percent and 50 percent or more in professionally trained individuals, like free divers, which is similar to many of the semi-aquatic animals. For humans, there are many factors that influence the onset of bradycardia, such as the temperature of the water, lung volume, physical conditioning, the body’s position in the water during immersion, how long you hold your breath, psychological state, and how deep you dive. Each of these can play a significant role in how the heart responds. Some of these factors you can control, which means you have some ability to control and enhance your mammalian diving reflex response.

Mammals have been diving deep into ocean waters for thousands of years for food and protection from predators. Their abilities to dive to depths of 1500 feet or more on a single breath out do humans by leaps and bounds. Humans have dived to over 550 feet on a single breath thanks to our ability to use the mammalian diving reflex, and free divers today are training to go even deeper. With our ability to use the mammalian diving reflex to our advantage through training and education, who knows how deep we’ll be diving in the years to come.


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