The uptake of a soluble gas during rebreathing is simulated in a compa
rtmental model. The lung is assumed to consist of two parallel units,
each one divided into three compartments: a personal dead space, an al
veolar space, and a tissue volume. These units are connected to the re
breathing bag via a common dead space. Gas exchange is incorporated in
to the model for a given cardiac output. Inert and soluble gas concent
rations are computed as a function of time in the various compartments
by means of differential equations. Using different initial condition
s, we simulate time-dependent concentration traces ''at the mouth'' an
d estimate the errors made by traditional analysis of the end-tidal ga
s concentrations with respect to the ''time O'' correction method as w
as first proposed by Sackner et al. (Am. Rev. Respir. Dis. 111: 157-16
5, 1975). We provide a theoretical basis for this correction method an
d outline the conditions that need to be fulfilled for its application
. We show that the tissue volume and cardiac output estimates are less
affected by ventilation and perfusion inhomogeneities when the time O
correction method is used. This is particularly relevant for the expe
cted increase in tissue volume in microgravity where ventilation and p
erfusion inhomogeneities are expected to be attenuated.