pCO2 at depth?

[sai]

Hey, I just read an article about the partial pressure of O2 and CO2 on deep dives. It says that you normally start with a pO2 of 100mmHg and pCO2 of 35-40mmHg, after you took your last breathe before diving down. In a dive to -20m the pO2 rises up to 250mmHg and the pCO2 to "only" ~50mmHg.
First of, is that correct?
Second, so I do actually have a bigger urge to breath at depth due to an elevated pCO2 compared to a static dive at surface?
There was a graphic attached, which showed that the pCO2 doesn't really rise after you hit the bottom, which made me wonder even more.
I'm curious because I noticed contractions starting at depth, which sometimes faded as I surfaced.
Can I conclude that if I get contractions at depth, they're not getting any worse as I prolong my bottom time?

 

[trux]

There is some confusion and simplification in the facts. It is necessary to understand that the total amount of CO2 in the body is stored in several ways: as gas in lungs, dissoved in liquids (especially in the blood plasma), molecularly bound to the hemoglobin, but most of it (70-80%) is stocked in the form of bicarbonate ions (HCO3-). The partial pressure of CO2 is then given by two principal factors - by the level of CO2, and by the ambient pressure (changing with depth). So now it depends where you measure the partial pressure - you can measure PACO2 (alveolar partial pressure), PaCO2 (arterial PP), or PvCO2 (venous PP).

The ambient pressure will rise proportionally with the depth. The level of the gaseous CO2 in lungs will not rise much, because most of the produced CO2 will be converted to bicarbonate or bound to hemoglobin.

However, the urge to breathe is not controlled by the PaCO2, but rather triggered by the pH chemoreceptors (the PaCO2 and PaO2 chemoreceptors play a certain role too, but in different phases, and to a lesser degree). And since the pH is greatly influenced by the bicarbonate level, the urge to breathe will rise even in depth (assuming you did not hyperventilate too much).

 

[sai]

OK, thanks for the clarification here. Too bad, I actually had some hope I'm not doing as bad as I thought. :blackeye
But still if I understood you correctly, the urge to breathe still comes in a bit earlier than at the surface?
When diving deep I get contractions as early as after 20 seconds (whole dive time, not bottom time), while when doing dynamics they certainly come later. I'm trying do my breathe-up and movement as similar as possible, except that I'm diving vertically of course. :t
Does that have something to do with depth? Or is it a relaxation problem?

 

[trux]

May be because the immersion is usually the most difficult phase, where you need to fight the buoyancy, hence you produce a lot of CO2

 

[Simos]

Could be what Trux said + maybe some stress and additional effort for equalisation.

But also maybe it also has to do with short surface intervals too? What are your diving patterns like?

 

[sai]

Dive time is ~1 minute to -20m and surface interval is a minimum of 3 minutes. I already start to get contractions at the bottom and then surface.