The Urge to Breathe
Breathing is a multistage biological process that is designed to move air in (inhalation) and out (exhalation) of the lungs. As we all know breathing is a function that is essential to life as the process brings delivers oxygen (O2) to the body and removes metabolic waste products such as carbon dioxide (CO2). Oxygen is required for the release of energy during cellular respiration and this gas exchange occurs primarily in the capillary system of the lungs where oxygen is exchanged for carbon dioxide. The atmospheric air that we breathe consists of approximately 21% oxygen, 78% nitrogen, and ~1% carbon dioxide and other trace gasses.
Contrary to popular belief the breathing cycle is primarily driven by an increase in CO2 in the blood stream, which triggers the cycle, and not by a decrease in the levels of O2 (in fact, the typical exhalation consists of ~16% oxygen)! Low levels of CO2 are naturally present in the circulatory system and the body regulates this through the intensity of the breathing cycle. Exercise for example increases the demand for oxygen due to increased energy requirements at the cellular level and higher levels of carbon dioxide are then released due to the metabolization of the oxygen — as a result, breathing rates increase to facilitate the exhalation of these greater concentrations of CO2.
In states of apnea, such as freediving, the body is unable to regulate the increase of CO2 by moderating the intensity and frequency of the breathing cycle, and thus the body signals the brain that there are increased levels of CO2 in the bloodstream (hypercapnia) — creating the urge to breathe. For freedivers, the urge to breathe often manifests itself as a burning sensation in upper portions of the chest or in the form of contractions/convulsions of the diaphragm directly under the ribcage. Despite these feelings, which can be overwhelming and unnerving for untrained divers, there is no need to panic as these are only signs that the levels of CO2 in the bloodstream are increasing and there is still plenty of usable oxygen remaining in the system. The ability to recognize your personal limits and tolerance to increased carbon dioxide levels is something that will be developed overtime and with practice — ultimately dive times can be increased by training the body to tolerate higher levels of CO2.
Never attempt to over-breathe or hyperventilate while freediving. Hyperventilation or increasing the rate of breathing does not aid in the absorption of oxygen to the system (which is limited) and only serves to lower the level of carbon dioxide in the blood. With a lower level of CO2 in the bloodstream, it will take longer for CO2 to build up to a level that will trigger the urge to breath. By delaying the urge to breathe, the freediver will not feel the urge to breath until the levels of CO2 are significantly high and by that time the level of O2 in the system will be dangerously low — increasing the likelihood of a blackout and loss of motor control (LMC/Samba). In the past, many untrained freedivers have used hyperventilation as a technique to extend their bottom time by prolonging their urge to breathe. Hyperventilation is extremely dangerous. Breath-hold activities such as freediving and spearfishing are inherently safe when done properly and with the appropriate safety protocols. However, obtaining the proper instruction from a certified instructor/agency, following the proper safety protocols, always diving with a trained and capable buddy, and knowing your personal limits are very important.
Sources: Albert, B., & Craig, J. R. (1961). Causes of loss of consciousness during underwater swimming. Journal of applied physiology, 16(4), 583-586. Craig, A. B. (1961). Underwater swimming and loss of consciousness. JAMA, 176(4), 255-258. Harty, H. R., Mummery, C. J., Adams, L., Banzett, R. B., Wright, I. G., Banner, N. R., ... & Guz, A. (1996). Ventilatory relief of the sensation of the urge to breathe in humans: are pulmonary receptors important?. The Journal of physiology, 490(3), 805-815.