Updated: Nov 19, 2019
This article describes the effects of peripheral and central chemoreceptors, how they function in the human body and what pivotal role they have in freediving physiology. It’s a long read, I know, but well worth it. These topics were part of my final presentation during my AIDA Instructor course.
In this article I will discuss the following points:
What physiological triggers initiate contractions and the urge to breathe during a breath-hold?How are contractions and chemoreceptors connected and does this benefit us during a breath-hold?
What are chemoreceptors?
The peripheral and central chemoreceptors are what detect changes in blood composition such as oxygen, carbon dioxide, pH, pressure, temperature, etc. Grossly stated; the peripheral chemoreceptors (aortic and carotid bodies) monitor of the arterial blood and the central chemoreceptors monitor the cerebrospinal fluid. Breathing is influenced by both the central and peripheral chemoreceptors(Maramattom & Wijdicks, 2007). The peripheral chemoreceptors monitor both CO2 and hypoxia, whereas the central chemoreceptors monitor pH through an increase in CO2(Davies & Moores, 2010).
It has been suggested that the peripheral chemoreceptors are quicker to respond to changes in partial pressure of carbon dioxide, whereas the central chemoreceptors can take up to 5 minutes. “Although slower to respond than the peripheral receptors, central chemoreceptors are responsible for about 80% of our sensitivity to carbon dioxide” (Davies et Moores, 2010). It is interesting to note especially for freedivers that the central chemoreceptors are responsible for the majority of CO2 sensitivity, but this response is delayed by several minutes. Could this be another argument to say we were meant to freedive?
Cerebral Blood Flow and Chemoreceptors: Like Wine and Cheese
So, what happens during a breath hold? Firstly, the peripheral chemoreceptors begin to detect a weak change in blood composition (i.e. carbon dioxide), since “the carotid body provides about 20% to 30% of the total hypercapnic response, and this response is fast, with a time constant in the range of 60 to 150 seconds” (Vassilakopoulos, T. 2012). A decrease in heart rate occurs well before the onset of contractions. Thus, perhaps the initial response to hypercapnia intensifies the mammalian dive response. After all, small changes in CO2 levels normally have a much more pronounced effect on breathing than small changes in O2 levels.
If you recall your biochemistry, you’ll know that CO2 is dissolved in the blood as carbonic acid, which turns into two components that cannot diffuse through the blood brain barrier (the hydrogen and bicarbonate ions):
H2O + CO2 <—> H2CO3 <—> H+ + HCO3–
During a typical breath hold, trained freedivers get contractions starting 2 – 6 minutes after the onset of a breath hold, and that is right within the range of the central chemoreceptors’ response time. This is because the central chemoreceptors have a strong (but delayed) response to CO2. In addition to that, they monitor the cerebrospinal fluid and we know that contractions help to keep the brain oxygenated.
We seem to have evolved an incredible mechanism that allows us a comfortable breath hold. The rise in systemic CO2 appears to strengthen our dive response but the same rise in CO2 in the cerebrospinal fluid signals that it is time to breathe minutes later!
Does hyperventilating delay the dive response?
If hypercapnia indeed strengthens the dive response, hyperventilating is doubly dangerous. Not only does it delay the onset of contractions, it might also reduce the dive response. In this scenario the mechanisms that conserve O2 are not as effective, contractions that increase cerebral oxygenation start later than they should, and on top of that the lowered CO2 (i.e. blood is more alkaline) does not allow for O2 to be released from hemoglobin.
Hyperventilation…the more we know about it, the worse it seems. It truly sets us up for blackout.
The peripheral chemoreceptors might help conserve O2 and allow a comfortable breath hold by increasing the dive response.The central chemoreceptors likely induce contractions after sensing high CO2 in the cerebrospinal fluid.The central chemoreceptors sense that CO2 around the brain increases to dangerous levels and therefore help preserve life. They signal the end of our ‘comfortable’ breath hold.
Is it possible to adapt our bodies to rising CO2 levels in the cerebrospinal fluid? Can we train ourselves to understand and enjoy contractions if we know that they provide more vital blood flow to the brain? Or are we just at the mercy of the central chemoreceptors, since they are the final sensors that control an on/off switch?
Although it is going to take some time to answer these questions, we can rest assured of one thing. Our bodies have adapted to hold our breath, comfortably for minutes at a time. Let’s not waste that precious gift.
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Cross, T. J. et al 2013. Cross TJ, Kavanagh JJ, Breskovic T, Zubin Maslov P, Lojpur M, et al. (2013) The Effects of Involuntary Respiratory Contractions on Cerebral Blood Flow during Maximal Apnoea in Trained Divers. PLoS ONE 8(6): e66950.
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