In Part 1 I briefly explained that the urge to breathe is regulated by a build up of CO2 sensed by the body, rather than an actual lack of oxygen (that the body cannot directly determine).

In this post I look at an example of the complex interactions that occur within the body during apnea and what happens if you try to disturb the balance.

One of the ways in which a freediver is able to hold his breath for an apparently impossible time (the current record is over 10 minutes) is that the air in his lungs only represents a portion of the oxygen at his disposal.  Unlocking the hidden reserves are key to a long and safe dive.

Let’s look at the sequence of events that occur in a normal breath hold :

1. The athlete prepares himself (more on this fundamental step next time)

2. A final deep breath fills the lungs to capacity and signals the start of the apnea

3. As the oxygen in the lungs is used, the resulting CO2 is dissolved into the bloodstream causing the uncomfortable sensations that we associate with a need to breathe

4. The dissolved CO2 decreases the pH of the blood and the acidity stimulates the haemoglobin in the muscles to liberate the oxygen that it has stored.

5. The athlete’s ability to remain relaxed despite the pain of the increasing CO2 as oxygen is gradually liberated determines the final length of the apnea attempt.

Hyperventilation is the act of ‘over breathing’ that many of us tried as a child in order to induce lightheadedness. For many years it was a mainstay of freedivers’ preparation due to the subsequent decrease in discomfort experienced during the breath hold.

What was not understood at the time is that hyperventilation does not increase the amount of oxygen available, but instead purges the body of CO2.  Superficially this is great as lower initial CO2 means a significant delay in the onset of the urge to breathe and an improvement in the comfort felt by the athlete.

Unfortunately, lower CO2 also means more alkaline blood, and the store of oxygen in the muscles therefore remains untapped.  Unlike a natural breath hold where the athlete trades discomfort for more oxygen, a forced breath hold using hyperventilation means a more comfortable but ultimately worse performance.

Now, here’s the important bit.  As the body cannot directly detect O2 levels in the blood, freedivers rely on the discomfort as an indicator for when to stop.  Take away the sensation by hyperventilating and you just lost your instrument panel.  Essentially you feel great right up to the point that you black out.  Do that on the sofa and you may give your wife a fright, but practicing hyperventilation underwater is a death sentence.

• Freediving
• Hyperventilation