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Fig. 4. Oxygen transfer across a compartment interface with additional diffusion
barrier (cuticle). The grid point referenced by the indices i and
j is located on an infinitesimal thin diffusion barrier separating
the tissue from the medium compartment. This grid point is characterized by
two variables, 
and
, which represent
the oxygen partial pressure at the inner and the outer side of the circular
diffusion barrier, respectively. To calculate the temporal changes in
and
(see equations
4,
5), the following geometrical
parameters are required: the cylindrical wall areas
Bi-0.5, Bi and
Bi+0.5, the areas of the hollow-cylindrical bases
and
, and the hollow-cylindrical
volumes 
and
. Three example
equations for calculating these parameters are given. vM,
flow velocity in the medium compartment; Pj,i-1,
Pj,i+1,
, oxygen
partial pressures of the neighbouring grid points; 
h and
r, distance between two grid points in axial and radial
direction.
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