What Happens During a Breath Hold?
A breath hold may seem simple—you stop breathing for a short period of time.
But inside your body, a lot is happening.
ParaBOLT uses breath holds after a normal exhale. Here, we'll simply call them breath holds.
Your body continues using oxygen and producing carbon dioxide even though you've stopped breathing.
As the hold continues, oxygen gradually falls, carbon dioxide rises, and your body begins responding to the challenge.
These responses are part of what makes breath-hold training so interesting.
1. You Become More Comfortable With Carbon Dioxide
Carbon dioxide is constantly produced by your body and normally removed when you breathe.
When you hold your breath, carbon dioxide begins to build up. This rise in carbon dioxide is known as hypercapnia.
As carbon dioxide rises, your urge to breathe becomes stronger.
With repeated practice, you can become more comfortable with rising carbon dioxide and the sensations that come with it.
This is what we mean by improving your CO₂ tolerance.
As your tolerance improves, you can become less likely to breathe more than your body needs, supporting breathing that is slower, lighter, and more efficient.
This is also why your BOLT score can improve as your CO₂ tolerance improves.
2. Your Spleen Releases an Oxygen Reserve
Your body has another fascinating response to breath holding.
Your spleen stores a reserve of red blood cells.
During a breath hold, the spleen can contract and release some of these cells into your bloodstream.
Red blood cells contain hemoglobin, which carries oxygen around your body. Releasing more of them temporarily increases your blood's ability to carry the oxygen that's available.
Think of it as your body calling on an extra reserve when oxygen becomes more limited.
With repeated breath-hold training, the spleen can adapt by increasing its capacity to store and release red blood cells when they're needed.
3. Your Body Starts Conserving Oxygen
Even though you've stopped breathing, your body hasn't stopped using oxygen.
As the hold continues, the amount of oxygen available to your body gradually falls. This reduced-oxygen state is known as hypoxia.
Because breath-hold practice repeatedly creates periods of lower oxygen followed by recovery, this is a form of intermittent hypoxia.
As oxygen becomes more limited, your body begins conserving what it already has.
Your circulation changes.
Blood flow is reduced to areas that can temporarily manage with less, helping preserve oxygen for vital organs such as your brain and heart.
Your heart rate can also slow, helping reduce the amount of oxygen your body uses.
In simple terms, your body becomes more economical with the oxygen that's available.
With practice, this oxygen-conserving response can become better developed.
4. You Practise Staying Calm Under Pressure
There is another part of breath-hold training that isn't just about oxygen or carbon dioxide.
It's about how you respond to the challenge.
As carbon dioxide rises and oxygen falls, your body's signals telling you to breathe become stronger.
Your natural reaction may be to tense up, become uncomfortable, or immediately want to breathe.
Breath-hold practice gives you an opportunity to experience those signals while remaining relaxed and in control.
You're not trying to fight your body or ignore its signals.
You're learning to stay calm as the challenge increases.
Over time, this can make the sensations of breath holding feel more familiar and manageable.
What Does It All Add Up To?
During a breath hold, several things are happening together.
Your tolerance to carbon dioxide is being challenged.
Your spleen can release stored red blood cells.
Your body begins conserving the oxygen it has.
And you practise staying relaxed as the urge to breathe becomes stronger.
With repeated practice, your body and mind become more familiar with this challenge.
Most importantly for everyday breathing, improving your tolerance to carbon dioxide can reduce the tendency to breathe more than necessary, supporting breathing that is slower, lighter, and more efficient.
Your BOLT score helps you track this progress.
Your Total Hold Practice shows how much breath-hold practice you've accumulated along the way.
That's why every breath hold counts.
A Practice With a Long History
Although we can now explain many of these responses using modern physiology, deliberately holding the breath is not a new practice.
Breath retention has been practised for thousands of years within yogic breathing traditions.
Holding the breath after an exhale is traditionally known as bahya kumbhaka.
Modern breath-hold training may use different methods and explanations, but the basic practice of deliberately pausing the breath has a very long history.