The Mammalian Dive Reflex

The Mammalian Dive Reflex

 All mammals have a stress response to being submerged in water. If you were to lower a unsuspecting land horse into a water pond, its heart rate would drop. This is an evolutionary survival reflex that gives the land horse more time to escape the deadly water pond, before he becomes a pond horse, sleeping with the frogs. Anyway, the point is, water does exist and animals can fall into it. But the mammalian dive reflex in humans is a bit more than that.

Our distant ancestors probably made use of food resources found in marine environments. They may have started by collecting shellfish at low tide and over time ventured further and deeper. Natural selection would have favoured those that could freedive better. So what was once a simple stress response starts evolving into a way to compete for food recourses. This is what happened to whales and dolphins. If a giraffe falls into water enough times, it’s feet turn into fins. Don’t quote me on that.

The Aquatic Ape Hypothesis by Elaine Morgan is worth a read. It goes into a lot of detail about why humans probably went through a period of semi-aquatic lifestyle over the course of their evolution. We then veered away from the ocean in favour of grasslands and fast cars.

The fact that human babies can swim instinctively suggests we gave birth in water at some stage. Perhaps it was safer from predators.

For whatever reason, human beings have a natural ability in water. However if we don’t practice, the mammalian dive reflex becomes weak. If we practice, we become sea creatures again. On the first dive of a session we probably feel like the land horse falling into the water pond; slightly uncomfortable. But if we persist and keep trying, it gets easier. This is because our diving reflex kicks in. The more we do it, the stronger it gets.

Out physiology actually goes through profound changes when we start swimming underwater. If our body experiences cold water, increased pressure and build up of carbon dioxide (from not breathing) the following X-Men tranformations kick in, as we rediscover our mammalian dive reflex:

Bradycardia:   The heart rate is slows down and oxygen consumption Is reduced. This is usually between 10 – 30% but can be 50% slower in elite freedivers.

Peripheral Vasoconstriction:   Blood vessels in the extremities of the body become narrower and less blood flows here. This means the brain, heart and lungs are the focus of blood flow and oxygen.

Spleen contractions:   The spleen releases more red blood cells into the circulatory system. This increases the oxygen carrying capacity of the blood.

Blood shift:   At around 30m the lungs have reached the limit of their elasticity and become compressed to their minimum size. If the diver wishes to go deeper without sustaining a barotrauma (lung squeeze) the volume of air in the lungs must be somehow reduced without further reducing the size of the lungs. This is achieved through blood plasma collecting in the lung and chest cavity. By entering the air space, the blood plasma (being a uncompressible liquid) reduces the volume of the lungs without them collapsing or suffering damage. It’s like an involuntary equalization of the lungs. As the diver re-surfaces and pressure drops, the blood plasma is reabsorbed into the blood vessels.