I am attempting to model gas exchange across the alveolar membrane. My main question is there a direct exchange of O2 molecules for CO2 molecules? If so, then my model predicts (assuming alveolar PO2 107mmHg and CO2 40mmHg; and venous PO2 40, PCO2 46) an arterial PO2 107mmHg, but PCO2 of only 37mmHg (taking into account pH change and equilibration with bicarb and haemoglobin carbamates). If I remove the plasma bicarb equilibration process (as this takes ~15 seconds, but alveolar transit time is ~0.75 seconds), then the arterial PCO2 is even lower, ~33mmHg. The expected results should be somewhere in the range of an arterial PO2 of 107 and PCO2 of 40.
The only alternative I can think of is that there isn't a molecule for molecule exchange, with the O2 absorption and CO2 removal being separate processes. If this is the case then overall there will either be a reduction in alveolar air pressure and/or volume during gas exchange. However, I can find no references to inhaled tidal volume being different to exhaled tidal volume. Nor does there seem to be any evidence for N2 or H2O molecules to participate in this process (at standard atmospheric pressure).
Ultimately I want to then extrapolate model to deal with pathology (eg high alveolar CO2, high venous CO2 etc).
EDIT to include references... Mechanistic physicochemical model of CO2 transport in the blood https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5341128/
Modeling of the oxygen transfer in the respiratory process https://hal.archives-ouvertes.fr/hal-00714239/document
Modeling pulmonary gas exchange and single-exhalation profiles of CO https://www.frontiersin.org/articles/10.3389/fphys.2018.00927/full
Respiratory Physiology https://derangedphysiology.com/main/cicm-primary-exam/required-reading/respiratory-system
Other materials I have read include:
- Pilbeams Mechanical Ventilation
- Nunn's Applied Respiratory Physiology
- Guyton and Hall Medical Physiology