Up until the late 1960s, physiologists believed that the maximum depth a person could descend to was determined by the depth at which their total lung capacity (TLC) was compressed to the same volume as their residual volume (RV) which is the smallest lung volume a person can breathe to. The TLC was predicted to be reduced to RV at around 30 meters; below this depth, it was believed that total lung collapse would occur. However, in 1968, French FD Jacques Mayol proved the physiologist wrong when he dove to 70.4 meters. Then, on June 9, 2007, Herbert Nitsch set the current world record for the deepest free dive at 214 meters. This astonishing feat challenged everything physiologists thought they knew about the limits of the human body. Freediving is an extreme sport in which the freediver (FD) descends and returns using a single breath. There are five main subcategories of free diving. Of these, the no-limits category presents FDs with the greatest challenge as they descend using a loaded sleigh and ascend using an air balloon. This allows FD’s to reach extreme depths and pressures. But how extreme is extreme? The normal pressure exerted on a person at sea level is 1 atmosphere absolute (ATA). Every 10 meters a diver descends, they experience a 1ATA increase in pressure. This means that by the time freedivers reach 30 meters, they are already experiencing 4 ATA of pressure. This is around double the pressure in your car tires! At 214 meters, Nitsch experienced 22.3 ATA at which point his lungs were compressed to 0.45 liters in size – that’s just under 2 glasses of water. In addition to the challenges posed by these high pressures, FDs have no access to oxygen (O2) as they are underwater in a hypoxic (no O2) environment. These harsh conditions make FDs a fascinating case study for a better understanding of the endurance of the human body. How is it that organisms made to live on dry land are able to descend 214 meters with one breath? There are several critical physiological responses that are theorized to permit FDs to reach these impressive depths. The first is the mammalian diving reflex. This is an autonomic reflex found in diving mammals that activates a series of responses that reduce O2 consumption following the cessation of breathing, known as apnea. Another critical mechanism is a centralized shift of blood known as the thoracic blood shift (TBS) in which there is movement of blood from the extremities of the body toward the central thoracic cavity. This increase in central blood volume allows for the reduction of TLC past the RV and prevents lung squeeze as the blood occupies the space of the shrunken lungs. Finally, the frog breathing technique permits FD to breathe past their TLC. During frog-breathing, the diver takes gulps air into their lungs after having already filled them. This is thought to increase the oxygen content in their lungs and prolong their time underwater. But as impressive as the physical adaptations of the human body are under these conditions, the mental adaptation might be even more astonishing. FDs have to learn to go against one of the most basic human urges, the urge to breathe. However, there can be serious consequences if this signal is ignored for too long. The drive to breathe arises from high carbon dioxide (CO2) levels in our blood. After we inhale, our cells use up the O2 in our blood and produce CO2. As the CO2 builds up, sensors detect the heightened levels and send a signal to the brain to let it know that it is time to exhale the CO2. However, since FDs often hyperventilate, they reduce the CO2 levels in their blood below the normal level. This causes a delay in the signal to breathe. During this delay, O2 is still being consumed and therefore, a FDs oxygen saturation continues to decrease; if FDs wait too long to breathe, they can lose consciousness. This is known as hypoxic loss of consciousness, or more commonly as shallow water blackout. Anyone who hyperventilates before swimming can be at risk of this! The most serious consequence of a diver losing consciousness is drowning. This is why several safety divers are present during a free dive and ready to pull the FD to the surface if necessary. Every time physiologists believe they have found the absolute limit of free diving, they are disproved by the newest world record. This begs the question – can we find the limit to freediving? Currently, it is believed that maximal diving depth is not restricted by breath-hold time, but rather by the degree of hypoxemia, that is lowest blood oxygen saturation, that can be withstood upon ascent. There is also a distinction to be made between the maximal diving depth without injury determined by the RV/TLC depth and the absolute maximal diving depth using advanced techniques. The latter predicts a maximum depth of 320 meters; however, whether the FD would survive the hypoxemia on ascent is yet to be determined. @DanielaPadres Daniela is a recent B.Sc. graduate from the program of Physiology at McGill. She is very passionate about understanding the human body and how we can all individually adapt our daily lifestyles to improve its functioning. Part of the OSS mandate is to foster science communication and critical thinking in our students and the public. We hope you enjoy these pieces from our Student Contributors and welcome any feedback you may have!