Ventilation by the lung is a higly regulated process. What is often not realized is,
that this regulation is mainly based on carbon dioxide (CO2) and only
secondary on oxygen (O2). The reason is, that mostly O2 is
no problem but CO2 is:
- in blood, O2 decreases from 97% to 75% so there is
a plenty reserve;
- CO2 is linked to the blood's pH through the reaction
CO2 + H2O ⇄ H+ +
HCO3 −
and pH is a critical factor in cell functioning.
Modelling can be done through the so-called Response Curves, curves of
the regulated quantity against PO2 or PCO2 but there is
a number of factors that have to be taken into account.
- mostly, the response curve is for total ventilation
˙VT instead of tidal volume
VT times breathing frequency fR needed
for modelling.
- the response starts in the body tissues but is (mainly) signaled
in the brain, considerably later.
- there are also receptors in lung and airways involved, not signalling
O2 or CO2 but other factors like stretch.
- the effect is on the tissues, also signaled considerably later.
- there is an interaction between O2 and CO2
response, so changing response curves.
Consequently, a regulated lung ventilation model is not easy. A description is in this document.