What happens in the body during a breath-hold?
During a breath-hold, oxygen falls and CO2 rises. It is CO2 that creates the urge to breathe, not the lack of oxygen. At depth, the air in the lungs is compressed (5 times smaller at 40 m), then expands on the way up: oxygen then drops all at once, hence the risk of a samba or a blackout near the surface.
Choose a breath-hold, then watch the lungs, the heart and the oxygen and CO2 gauges. The slider lets you go back to any moment.
Estimated values for a trained freediver, based on the studies cited. Time runs 5 to 10 times faster. Lung volume taken as 5 liters at the surface.
Why do you want to breathe during a breath-hold?
Because of CO2. The body mainly watches the CO2 that builds up, much more than the oxygen that falls. When CO2 rises enough, the diaphragm contracts on its own: this is the struggle phase. It does not mean that oxygen is already running out.
Why do the lungs shrink at depth?
Because the water presses harder and harder: pressure rises by 1 bar every 10 meters, and the volume of air is divided accordingly (Boyle's law). For 5-liter lungs filled at the surface:
| Depth | Pressure (absolute) | Pressure (barg) | Air volume |
|---|---|---|---|
| Surface | 1 bar | 0 barg | 5 L (100 %) |
| 10 m | 2 bar | 1 barg | 2,5 L (50 %) |
| 20 m | 3 bar | 2 barg | 1,7 L (33 %) |
| 30 m | 4 bar | 3 barg | 1,25 L (25 %) |
| 40 m | 5 bar | 4 barg | 1 L (20 %) |
The biggest change happens between the surface and 10 m: the volume is halved there. It is also there, on the way up, that oxygen in the lungs drops fastest.
What is a samba in freediving?
It is a loss of muscle control when the brain lacks oxygen: shaking, head lolling, a blank stare. It often happens just after leaving the water, because oxygen-poor blood takes a few seconds to reach the brain. In competition, about 1 static breath-hold in 10 ended in a samba or a blackout (Lindholm 2007).
Frequently asked questions
Does hyperventilating before a breath-hold help?
No, it is dangerous. Breathing very fast lowers CO2 but adds almost no oxygen to the blood. The urge to breathe therefore comes later, when oxygen is already very low: a blackout can happen without warning (Lindholm 2006).
What is the diving reflex?
With the face in the water and the breath held, the heart slows down and the blood vessels of the arms and legs narrow. Oxygen is saved for the brain and heart. The spleen also contracts and releases a reserve of red blood cells (Baković 2003).
Can you train breath-holding alone?
No, never in water, even in a bathtub or a shallow pool. A blackout gives no warning. You need a trained buddy who watches the freediver throughout the breath-hold, then until the freediver has fully recovered. Clubs usually ask for at least thirty seconds or so: this is a safety rule, not a figure from a study.
Why is the danger greatest near the surface?
On the way up, pressure falls and the oxygen in the lungs falls with it. Between 10 m and the surface, pressure is halved: oxygen drops all at once while the body has already used a lot of it. After deep dives, blood oxygen was lower than after shallower dives of the same length (Mulder 2023).
See also
The vagus nerve A workout The stress of an oral exam
Sources
- Lindholm P, Lundgren CE. Alveolar gas composition before and after maximal breath-holds in competitive divers. Undersea Hyperb Med. 2006;33(6):463-7. PMID 17274316
- Lindholm P. Loss of motor control and/or loss of consciousness during breath-hold competitions. Int J Sports Med. 2007;28(4):295-9. doi:10.1055/s-2006-924361
- Baković D et al. Spleen volume and blood flow response to repeated breath-hold apneas. J Appl Physiol. 2003;95(4):1460-6. doi:10.1152/japplphysiol.00221.2003
- Dujic Z et al. Involuntary breathing movements improve cerebral oxygenation during apnea struggle phase in elite divers. J Appl Physiol. 2009;107(6):1840-6. doi:10.1152/japplphysiol.00334.2009
- Mulder E et al. Unlocking the depths: multiple factors contribute to risk for hypoxic blackout during deep freediving. Eur J Appl Physiol. 2023;123(11):2483-93. doi:10.1007/s00421-023-05250-z
- Bosco G et al. Arterial blood gas analysis in breath-hold divers at depth. Front Physiol. 2018;9:1558. doi:10.3389/fphys.2018.01558