In the last couple of months, I noticed that even some Freediving professionals are not completely understanding how breathing control in humans happens. And be honest, while I was writing this article I found out that I was not 100 % correct as well.
Hopefully, this article helps you to understand better what exactly happens with your respiratory system when you hold your breath. And if you find any mistake here – feel free to correct me – I am still learning as well!
As any other Freediving school, we are teaching about breathing control on our Freediving courses, but here I tried to put a little bit more details.
To start with – we have two different types of chemoreceptors which are detecting chemical changes in our body and sending signals to our respiratory center within Pons and Medulla Oblongata (both located in the brainstem), from where impulses send to our external intercostal muscles and diaphragm, to change the volume and frequency of our breathing (or cause “urge to breath” if you are holding your breath).
We can divide these receptors into 2 categories
- Central chemoreceptors. Why “central”? Because these receptors are part of our central nervous system and literally part of our brain (located inside Pons and Medulla Oblongata). Since these receptors are not inside blood vessels, they are responding to high CO2/H+ not within the blood, but within cerebrospinal fluid (CSF), which separated from the blood vessels by the blood-brain barrier (BBB).
Let’s make an example. You hold your breath for a few minutes. Amount of CO2 increases in your blood, the amount of H+ also increase creating low pH (respiratory acidosis). H+ doesn’t diffuse through BBB, but CO2 does. This CO2 bonds with water inside CSF and produce H+, an increased amount of which is going to be detected by central chemoreceptors.
Recently I learned that lactate has an impact on this process as well. Lactate, which is produced during anaerobic energy production, in a form of lactic acid, can go through BBB where it brakes down to lactate and H+, which eventually lead to the activation of central chemoreceptors.
Eventually, central receptors can desensitize and this is why we have the potential to become less sensitive to high H+ over some period of training with exposure to a high CO2 (whether it is a breath hold training or some form of HIIT).
- Peripheral chemoreceptors. They are not part of the central nervous system (instead, they are an extension of the peripheral nervous system) and located inside aorta (largest artery of the human body). More specifically – inside the aortic and carotid body. Interesting fact – here we have one of the highest blood flow in a human body.
Chemoreceptors inside aortic body sensitive to the change of partial pressure of CO2 and O2. If there is a change – they send the signal to Medulla Oblangata via Vagus nerve.
Chemoreceptors inside carotid body sensitive to change of partial pressure of CO2/O2 and change of pH (metabolic change, due to high lactate production for example). And if there is a significant change – send the signal to the respiratory center via Glossopharyngeal nerve.
The main function of peripheral chemoreceptors (glomus cells) is control of pO2 (in contrast with central chemoreceptors, where the main trigger is a change of pCO2/H+). As I said early, they also sensitive to the change of pCO2/H+ but secondary. It means that the sensitivity of these receptors to the low pO2 is greater when pCO2/H+ is high.
Activation of peripheral chemoreceptors are low when the partial pressure of O2 is close to the normal (100 mmHg), but when it is going below 60 mmHg the activity increases rapidly due to a decrease of hemoglobin-oxygen saturation.
Peripheral receptors are not desensitized over time.
Two common hypoxic ventilation responses (CO2/pH can stay at the normal level) – reaction to high altitude or high concentration of carbon monoxide in breathing air.
How all of this can be useful for us Freedivers? In the middle part of the breath hold, when your contractions start, it is a reaction to a high CO2/H+ sensed by central chemoreceptors. Peripheral chemoreceptors are not playing an important role at this moment since the partial pressure of O2 is close to normal. But close to the end of your MAX attempt, when pO2is going to be close to 60 mmHg and low, a reaction from them will contribute to your urge to breathe.
For further reading
Useful videos to watch