CO2 Tolerance


The impulse to breathe is largely triggered by higher levels of CO2 in the blood stream. Improved CO2 tolerance reduces the urge to breathe and controlled breath holding will build CO2 endurance.

Building CO2 tolerance does have benefits but too much CO2 will upset the balance between O2 and CO2.

Mild hypoventilation is said to have some benefits, but of course uncontrolled or long-term hypoventilation is to be avoided.

Hypercapnia is a build-up of Co2 in the blood stream (paco2).   

Impact of PaCO2 and PaO2

“Arterial carbon dioxide tension is a clinically important regulator of CBF; it crosses the blood-brain barrier readily and changes extravascular pH. Increasing PaCO2 (hypercapnia) precipitates vasodilatation.”


If mild hypercapnia is a vascular vasodilator, then we must suppose it has beneficial effects on such conditions as dementia? Vasodilation will also reduce blood pressure.  


C02 has a bad reputation, but it is essential for good health   

Low C02 content causes airway passages and blood vessels to constrict and narrow. Higher C02 content causes these passages to relax and widen, for improved flow of air and blood. C02 relaxes smooth muscles of airway passages (bronchi) and blood vessels.


Hyperventilating is rapid or deep breathing or breathing more than the body needs. The result is lower levels of C02, blood supply to the brain reduces(vasoconstriction) with associated feelings of light headedness.


Breathing is largely unconscious and under the control of the autonomic nervous system within the brain stem. Nerve signals direct inspiratory muscles to breath in and out. Receptors in arteries monitor oxygen and carbon dioxide levels and instruct the body to take appropriate action.     

It’s when we step outside our normal unconscious breathing that C02 tolerance training is said to come into its own. If when we begin to breathe harder during exercise, and we don’t have sufficient control over our respiratory processes, we begin to breathe harder and lose C02.

One of the consequences is the body becomes more alkaline through respiratory alkalosis. Chemoreceptors in the carotid arteries will signal vasoconstriction and bronchoconstriction, narrowing arteries and airways. This will also tend to signal the heart rate to increase, prompting the sympathetic nervous system.     

I think we must assume most of our students won’t have sufficient control to breathe deeply and slowly enough, or to retain the breath with the control needed to maintain the balance between C02 and 02.

Co2 TOLERANCE TEST (two examples)

First example.

Take 4 full breaths

A 3-5 second inhale then a 5-10 second relaxed exhalation. Pause for a second between breaths. 

At the top of the 4th inhalation time the exhale. Make the exhale as slow as possible.

Second example.

Take four full controlled breaths

At the top of the 4th inhale, with the lungs full time the exhalation.  

Then exhale as slowly as possible (from your nose). 

RESULTS (example)

80 seconds: Elite. Excellent breath control, excellent stress control.

60-80 seconds: Advanced. Healthy pulmonary system, good breath control and relatively good stress control.

40-60 seconds: Intermediate. Generally improves quickly with focus on CO2 tolerance training.

20-40 seconds: Average. Moderate to high stress/anxiety state, breathing needs improvement.

20 seconds: Poor. Very high anxiety and stress sensitivity, mechanical restriction possible, poor pulmonary capacity.

Martin Thompson.  


“It is not lactic acid but the increased acidity in your blood that is to blame for the burning sensation you might feel during intense exercise. And the acidity is caused by the release and build-up of hydrogen ions, not lactic acid.”

CO2 tolerance