Figure 1.

The simplified lung microstructure assumed by MOXE, where the total width of the septum is $d$. The structure consists of two air-blood membranes of length $\delta$, placed symmetrically on either side of the pulmonary capillary. ¹²⁹Xe diffuses in from the airspaces on either side, first entering the 198-ppm membrane compartment, which originates in the air-blood membranes and the plasma component of the blood. The atoms then reach the RBCs, which gives rise to the 218-ppm signal²⁰.