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When training for free diving, there are several physiological and psychological changes that enable this activity, however one of the changes that I do not understand is increased resistance to blood acidification.

Part of the resistance to pain is psychological but from my understanding the body itself is also less sensitive to the carbon dioxide buildup in the blood itself, so I am wondering how this happens. Is this a change in the nervous system, blood or chemoreceptors themselves?

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According to Libretexts Medicine link There are two types of chemoreceptors that help regulate breathing. Those with the most impact, the central chemoreceptors, can be desensitized. The relevant quote is:

  1. Central chemoreceptors: These are located on the ventrolateral surface of medulla oblongata and detect changes in the pH of spinal fluid. They can be desensitized over time from chronic hypoxia (oxygen deficiency) and increased carbon dioxide.
  2. Peripheral chemoreceptors: These include the aortic body, which detects changes in blood oxygen and carbon dioxide, but not pH, and the carotid body which detects all three. They do not desensitize, and have less of an impact on the respiratory rate compared to the central chemoreceptors.

From FreediveUK website. There are two types of breath training necessary. One is psychological, to learn to control the breathing reflex due to high CO2 levels and the other is to train the body to operate with lower levels of O2. They also mention the mammalian dive reflex which produces a physiological response:
When the face is submerged and water fills the nostrils, sensory receptors sensitive to wetness within the nasal cavity and other areas of the face supplied by the fifth (V) cranial nerve (the trigeminal nerve) relay the information to the brain.1 The tenth (X) cranial nerve, (the vagus nerve) – part of the autonomic nervous system – then produces bradycardia and other neural pathways elicit peripheral vasoconstriction, restricting blood from limbs and all organs to preserve blood and oxygen for the heart and the brain (and lungs), concentrating flow in a heart–brain circuit and allowing the animal to conserve oxygen.

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  • $\begingroup$ As an aside: If you go to the freediveuk website they have an article warning about using hyperventilation. You would think this would help, and it was actually taught for many years as a technique. But it turns out that this greatly lowers the concentration of CO2 and you do need a high level of CO2 to signal that it's time to surface. What can happen is that the diver stays down too long, so uses too much O2 and as the diver gets close to the surface the O2 partial pressure decreases and the O2 level in the blood falls so low that the diver blacks out. $\endgroup$
    – Rich
    Mar 13 at 4:56

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