What happens to the human body when we freedive?

Mammalian Diving Reflex

This is an adaptation found on all diving mammals, which comprises a set of changes that allow them to perform extended depth/duration dives under phenomenal pressures. This amazing phenomenon was first observed and documented on animals such as whales, dolphins, penguins and seals, but up until as recently as the 1950's it was thought that the human being did not posses any of these faculties. The era of deep breath-hold dives that started at that time proved science (or scientists rather) wrong, and studies on the human diving potential began in full. So far, all of the different manifestations that are part of the mammalian diving reflex have been observed in human beings, proving that this fantastic capacity lays dormant in all of us. Here is an explanation of the 3 most important ones:

- Bradycardia -
Bradycardia is the most common of all the manifestations of the mammalian diving reflex. As soon as the brain detects the signs of immersion in water, such as decreased temperatures and an increase in ambient pressure, it slows down the heart, thus the name brady (low) cardia (heart beat). The heart is the one muscle in the body that is always working and its energy and oxygen consumption is high, so by decreasing the frequency of its beats, a reduction in the use of oxygen is achieved. Also, as the heart slows down, less blood is sent to the tissues and organs and more oxygen is conserved for the vital functions like brain activity. As the immersion becomes longer and/or deeper, the heartbeat becomes slower and slower, allowing the body to survive under such extreme conditions. Amazingly, heart rates as low as 6 beats-per-minute (one beat every 10 seconds) have been observed in human freedivers during deep dives!

- Peripheral Vasoconstriction -
As the mammal, or in this case the human being, continues to be immersed, other adaptations come into play. At higher pressures, which means deeper depths, the blood from the entire periphery starts being taken away. All blood vessels and capillaries from toes, fingers, hands, feet, and ultimately arms and legs, constrict themselves reducing circulation in these areas to a minimum. This blood will be used to irrigate more important organs, which require a steady supply of oxygen such as the brain and heart. As the diver descends deeper, this adaptation becomes more pronounced, redirecting most of the blood to the body's core, where it is used not only to feed the vital organs but also to counteract external pressure, in what is known as Blood Shift.

- Blood Shift -
The most amazing, and profound in meaning, of all the adaptations associated with the Mammalian Diving Reflex, the blood shift is truly what saves a mammal's life during deep dives. In simple words, the blood shift means that all the blood pulled from the extremities will be forced into some of the organs of the thoracic cavity, to prevent them from collapsing under pressure. This applies namely to the lungs, where every alveolus (the prototypical cell that forms the lungs, in the shape of a tiny air sac) is engulfed with blood plasma from the surrounding tissue. So in fact, the lungs are now "full" of liquid which cannot be compressed from the pressure of the surrounding water, preventing their volume from dropping below what is known as Residual Capacity, the effective collapse point. As the diver ascends, the blood shift starts reverting itself and the plasma is directed back into the circulation.

- Blackout -
The word "blackout" means, literally, absence of light, or obscurity. This defines very well an unconscious state, which can result during freediving. Why and how does unconsciousness happen underwater? Most commonly, it will be the result of a brain reaction to the balance of the 2 most important gases in our body: oxygen (O2) and carbon dioxide (Co2). Oxygen is the body's fuel, needed for all tissues and cells to function, like the gasoline to run an engine, and Co2 is the waste product of this engine, the "exhaust fumes" that remain. A proper balance will have adequate levels of both, with O2 not dropping too low and C02 not climbing too high. Whenever this equilibrium is broken, and one or both of these gases goes off its normal values drastically, the brain can shut down all main activity (resulting in unconsciousness) and remain in an "energy conservation" mode until balance is restored. All sensory activity is decreased to a minimum, and only the essential functions are kept. If the brain receives an adequate supply of oxygen within an acceptable time limit, then all nervous activity will be "restarted" and consciousness will be restored, but this must happen quickly, the quicker the better. The longer it takes for the brain to receive oxygen, the more damage it will sustain and the more permanent this damage will be.

The most common cause for a blackout is lack of oxygen, which can be compounded by high levels of Co2, as happens during a freedive, when the body is working without a constant supply of O2, and creating more and more Co2 as the diver works underwater. This scenario affects the O2/Co2 balance both ways, decreasing the former and increasing the latter one. Typically, and most commonly, the blackout happens as the diver is ascending, close to the surface and near the end of the dive. When the diver descends, the pressure exerted by the surrounding water on his lungs and blood will effectively increase the partial pressure of oxygen in the blood (not the amount of O2, just its pressure) so this will be enough to maintain functionality at depth. However, as he ascends, the surrounding pressure decreases and so does the O2 pressure, while the Co2 amount rises as a result of the effort of ascending, which can potentially lead to a blackout.  However, the generalization that this will only happen upon ascend must not be accepted as the norm. A blackout can happen anywhere, to anybody and at any time during any dive.

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