Freediving Physiology

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, seals, etc., 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 the human diving potential began. So far, all of the different manifestations that form the mammalian diving reflex have been observed in human beings, proving that this fantastic capacity lays dormant in all of us.


Bradycardia is the most common of all the manifestations of the mammalian diving reflex. As soon as the brain detects the signs that come with 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 muscle in the body, which is always working, thus its energy and oxygen consumption is high, so by decreasing the frequency of its beats, a reduction in the use of oxygen is achieved. As the immersion becomes longer and/or deeper, the heartbeat becomes slower and slower, allowing the body to survive under such extreme conditions.

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 all the periphery starts being taken away. All blood vessels and capillaries and 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.

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 mammals life during deep dives. In simple words, the blood shift means that all the blood pulled from the extremities will be forced into the organs of the thoracic cavity, to prevent them from collapsing under pressure. This applies also to the lungs, where every alveoli is engulfed with blood plasma from the surrounding tissue. So that in fact the lung volume never goes below residual capacity as the diver dives deeper, this adaptation will become more acute, and will consequently revert itself upon ascent.


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.