How the Body Adapts to CO₂ in Freediving
What science tells us about CO₂ tolerance training
In freediving, the main challenge is not the lack of oxygen — it is the build-up of carbon dioxide (CO₂).
CO₂ is what creates the strong urge to breathe, the discomfort, and eventually panic in untrained people.
With proper training, however, the body learns to tolerate much higher CO₂ levels while staying calm and in control. This is not just mental adaptation. It is supported by real physiological changes, well documented in recent scientific research.
Here is a clear and simple overview of what actually changes in the body of a trained freediver.
The blood becomes better at buffering acidity
As CO₂ accumulates, it makes the blood more acidic.
In untrained individuals, this drop in pH happens quickly and feels extremely uncomfortable.
In trained freedivers, the blood is much more efficient at neutralizing this acidity.
The key player is bicarbonate, a natural buffer in the blood that absorbs excess acidity and slows the pH drop.
Studies show that during long or deep breath-hold dives:
CO₂ rises sharply,
yet blood pH remains surprisingly stable.
👉 In simple terms: more CO₂ can accumulate without disturbing internal balance.
With repeated CO₂ exposure (CO₂ tables, repeated apneas), the body likely increases its buffering reserves over time, similar to what is observed in people chronically exposed to elevated CO₂.
The brain delays the “breathe” alarm
The urge to breathe does not come from the lungs, but from the brain.
Specialized sensors detect rising CO₂ and activate the respiratory drive.
In trained freedivers:
these sensors become less sensitive,
the CO₂ threshold that triggers the alarm is pushed higher.
This means:
the same CO₂ level causes less distress,
contractions start later,
discomfort is easier to tolerate.
Research consistently shows that freedivers have a reduced ventilatory response to CO₂, sometimes similar to what is seen in diving mammals.
⚠️ Important note:
This adaptation does not remove danger. It makes the warning quieter — which is why proper training, progression, and safety protocols are essential.
Oxygen is delivered more efficiently when CO₂ rises
CO₂ is not only an ennemy it is also a friend !
As CO₂ increases:
hemoglobin releases oxygen more easily to the tissues.
This is known as the Bohr effect.
In freedivers:
this effect is fully exploited toward the end of the dive,
oxygen is delivered where it is most needed (brain and heart).
Long-term training also leads to:
higher hemoglobin concentration,
and a stronger spleen contraction, which releases extra red blood cells during apnea.
Which result in:
more oxygen transport,
greater CO₂ buffering capacity,
increased safety margins near the end of a dive.
Muscles and metabolism become more efficient
Freedivers’ muscles adapt over time:
higher myoglobin levels (local oxygen storage),
increased mitochondrial density,
improved ability to produce energy with very little oxygen.
This leads to:
less reliance on anaerobic metabolism,
less lactic acid production,
and therefore less additional CO₂ to manage.
At the same time, freedivers learn to:
relax deeply,
slow the heart rate,
minimize unnecessary muscle activity.
The diving reflex becomes stronger, placing the body in an energy-saving mode that dramatically reduces oxygen consumption and CO₂ production
The brain is better protected
The brain is highly sensitive to CO₂ — yet it adapts remarkably well in trained freedivers.
Brain imaging studies show that:
cerebral blood flow stays stable early in the breath-hold,
then increases strongly at the end, when CO₂ is very high.
👉 Blood flow rises exactly when it is needed most.
Even more striking during very long apneas:
the brain remains mostly in aerobic metabolism,
without significant lactate accumulation.
This means the brain:
continues to function efficiently,
tolerates high CO₂ levels,
and stays conscious and clear-headed longer.
Summary
CO₂ tolerance training in freediving leads to deep, interconnected adaptations:
🩸 Better blood buffering
🧠 Reduced sensitivity to CO₂ at the respiratory center
🩸 More efficient oxygen delivery
💪 Muscles that consume less and produce less acidity
🧠 A brain protected by optimized blood flow
All these mechanisms work together.
They allow trained freedivers to remain calm, controlled, and functional in conditions that would quickly overwhelm an untrained body.
Modern research confirms what freedivers have experienced for decades:
the human body is highly adaptable, and with progressive, well-structured exposure, it can safely expand its tolerance to elevated CO₂.
This knowledge is not only relevant for freediving performance, but also valuable for medical research into hypoxia, sleep apnea, and chronic respiratory conditions.
