Contractions and CO2 levels

[jmnug2]

Hi all,

Just a quick question about something I've noticed but can't explain.

When I take a full breath and hold it (dry) contractions begin after a couple of minutes. However, when if I do the same breath-up then exhale and hold, contractions begin much earlier after less than 30 seconds.

I've been lead to believe that contractions are caused by the build up of CO2 in the blood stream which should be the same in both of the above cases, right? Can anyone explain why they come so much earlier on exhale?

Cheers,
Jack

 

[jschultzdanmark]

That's because the CO2 receptors are in the lungs. The CO2 buildup is the same, full or empty lungs, only the concentration of CO2 is of course higher in the latter case.
Receptor trigger = CO2/air-in-lungs x 100

 

[Ronnie]

With a lungful of air, you can still exchange gasses to some extent and remove CO2 from your circulation. With deflated lungs this ability is much reduced.

Rrspiratory drive is controlled by both hypercarbia (high CO2) and hypoxia (low oxygen), but the effects of hypercarbia is a stronger response.

 

[MarcinB]

CO2 receptors that trigger contractions and diving reflex are not in lungs. They are in carotid arteries, aorta and in some regions of the brain. It is also likely that diaphragm can sense CO2 level too. Contractions occur much earlier on empty lungs, compared to full ones, because CO2 level in blood increases faster. The rate of CO2 production is the same in both cases. However, when lung volume is low the same amount of CO2 diffuses into small volume of air. As a result the level of CO2 in alveolar air and, therefore, also in the arterial blood increases at a much faster rate and contractions start earlier.