What happens in your body when you hold your breath underwater?
Holding your breath underwater triggers a series of physiological responses, some of which differ from when you hold your breath in the open air. These responses, collectively known as the mammalian dive reflex, are an evolutionary adaptation shared with marine mammals like seals and whales. This reflex helps conserve oxygen, regulate circulation, and extend breath-hold time, allowing the body to function efficiently under limited oxygen conditions.
The Oxygen Consumption Process
During apnea (breath-hold), your body primarily relies on the oxygen stored in your lungs, bloodstream, and muscles. However, since this oxygen supply is finite, your body also resorts to anaerobic energy production—short-term chemical processes that generate energy without requiring oxygen. This results in the gradual buildup of carbon dioxide (CO₂) in your tissues, bloodstream, and lungs.
The Role of Carbon Dioxide
Contrary to what many believe, the urge to breathe is not triggered by a lack of oxygen but rather by rising CO₂ levels. As CO₂ accumulates, your body reacts by sending strong signals to your brain, prompting you to take a breath. This physiological response can be felt through:
- Diaphragmatic contractions- involuntary spasms of the muscles involved in breathing.
- A burning or (bursting) sensation in the lungs and throat.
- An increased need to swallow, which is often mistaken for thirst.
The Limits of Breath-Hold and Its Risks
If breath-holding continues beyond this point, more severe effects begin to set in. The brain becomes deprived of oxygen, leading to symptoms such as:
- Dizziness and lightheadedness
- Visual disturbances, often described as (graying out) or seeing tunnel vision
- Reduced motor control, which may lead to poor decision-making underwater
At a critical threshold, hypoxia (oxygen deprivation) leads to blackout. This condition, known as shallow water blackout, is particularly dangerous because it often happens during ascent when oxygen levels drop rapidly. Without immediate rescue, unconsciousness underwater leads to drowning.
Understanding and Training the Body
Freedivers and breath-hold enthusiasts train extensively to increase CO₂ tolerance, improve oxygen efficiency, and recognize their limits. By understanding how the body reacts to apnea and implementing proper safety measures, breath-hold diving can be practiced safely and effectively. Techniques such as diaphragmatic breathing, CO₂ tables, and progressive training help divers push their limits while reducing risks.