Why can people hold their breath longer underwater?

It may seem strange that people can easily hold their breath longer when underwater because of the increased pressure on the chest. But this is a phenomenon called the diving reflex of mammals. When mammals are submerged underwater, a number of automatic physiological changes occur, most likely triggered by sensory information (when the face is wet) transmitted by the trigeminal nerve, according to IFL Science.

When the reflex is activated, you automatically stop breathing, your heart rate slows, and peripheral vascular resistance increases. With increased vascular resistance, the body can conserve oxygen reserves for essential internal organs, including the brain and heart, while draining blood from inactive muscle groups. In addition, a slow heart rate helps conserve oxygen reserves by reducing the heart’s workload, thereby using less oxygen.

The record for holding one’s breath underwater is 24 minutes and 37 seconds, set by free diver Budimir Šobat in 2021. He beat the second ranked person by only 34 seconds. However, to achieve such a breath-holding time, divers need to increase ventilation with pure oxygen before entering the water. The urge to breathe is primarily controlled by chemosensory receptors that seek to maintain appropriate levels of oxygen and carbon dioxide in the blood.

While holding your breath, the amount of CO2 in the blood increases and oxygen decreases. The initial urge to breathe increases after 30 seconds of holding your breath, mainly from increased CO2 levels. At a certain threshold, chemoreceptors also respond to reduced oxygen levels, significantly increasing the urge to breathe, according to Anthony Bain, an associate professor of body kinesiology at the University of Windsor.

Eventually, the desire to breathe is so intense that the diaphragm (the main respiratory muscle) contracts in an involuntary respiratory displacement. This is the mark that untrained breath-holders usually cannot overcome and start breathing again (after about 3 minutes of holding their breath in conditions of determination and without a supporting oxygen source).

Through pre-inhalation of pure oxygen, the breathing reflex can be delayed, allowing divers who have undergone many years of training) to stay underwater for up to 20 minutes. If we don’t do that, the human ability to hold our breath is still impressive with the current record being 11 minutes and 35 seconds. Although the diving reflex is a complex process, it is characterized by a simple goal, preserving life through physiological adjustments in response to the environment.