THE MAMMALIAN DIVING REFLEX

What is it?

It is a special set of evolutionary adaptations that occurs mainly and most powerfully in marine mammals such as whales, dolphins and otters. Thanks to this reflex they are able to remain submerged for long periods of time and survive without being affected by the excessive pressure of the depths.
In humans it appears in a much weaker form, but enough to cause changes at the organic level that optimize breathing and help us stay underwater for longer periods of time.

How do we activate it?

It is as simple as immersing your face in cold water. The colder the water, the greater the effect of the reflection. And it has to be at least as low as 21ºC. Otherwise, it will not activate. In addition, it must be the face that comes into contact with the water. If we submerge the whole body leaving the face out, the reflex will not activate.

This is due to the existence of cold-sensitive receptors located inside the nasal cavity, sinuses and other parts of the face connected to the trigeminal nerve, composed of nerve branches that are located only in the face. This is how the information that the face has come into contact with cold water is transmitted to the brain through the autonomic nervous system. At this moment, the airways are closed and physiological changes occur that favor the conservation of oxygen in the organism.

What physiological changes occur and why do they help us in the practice of apnea?

Bradycardia
The heart rate is reduced by 10-25%. This translates into a lower oxygen demand by the heart, which facilitates a more efficient use of oxygen by other parts of the body. For an freediver this means increased dive times.

Peripheral vasoconstriction
As we freedive and gain depth during dives, the pressure around us increases. This causes the capillaries in the extremities to constrict, narrowing, restricting blood flow to those areas we call peripheral (hands, feet, arms, legs). Blood will then be directed to the major organs: the heart, lungs and brain will be supplied with much greater amounts of oxygen than the peripheral areas.

Blood migration
This phenomenon is related to the previous one. As mentioned above, due to the effect of the pressure at depth, the capillaries of the extremities will narrow, causing the blood to be pushed towards the thorax and vital organs.
The interesting thing comes now. What happens in the thorax is that as the lungs compress with depth, a space is created, which in this case is filled by the pushed blood due to peripheral vasoconstriction.
The blood, behaving physically as a liquid, like the water that surrounds us, does not compress, regardless of the depth we reach while diving.
Because the blood occupies this space, the lungs will not collapse as they become compressed with increasing pressure.

Spleen compression
The spleen is on the left side of the body and behind the stomach. It acts primarily as a reservoir for white and red blood cells. As a consequence of blood migration and as the last phase of the dive reflex, the spleen will contract, releasing large quantities of red blood cells into the circulatory system, thus facilitating the transport of oxygen.

CONCLUSIONS
As freedivers, the dive reflex is key to being able to stay underwater for increasing lengths of time, and can be reinforced through training and repetition to improve diving performance.
It can be said that the dive reflex is another of the advantages associated with freediving. However, its benefits are not limited to the diving environment, but it can also help us out of the water, for example to reduce the effects of anxiety in a fast way. I'm sure you've seen videos of people taking cold showers. What they are doing is activating their mammalian dive reflex and taking advantage of both the physical and mental benefits associated with it.