While not strictly an arterial vessel, it is upstream of the organ and can be described in the same way as the large vessels. The second type of arterial vessel, downstream from the input tubing, is the artery. Substances flowing out of the artery is distributed to the various flow paths where it enters the third type of arterial vessel, the arterioles, and then the capillary-tissue units. The distribution of flow among the pathways is controlled by the flow heterogeneity parameters described in Section 3.3.2 . Each exchange unit drains into a venule.
All venules drain into the common vein, then into the output tubing.
The basic tissue unit in which blood-tissue exchange takes place, shown in Fig. This is considered a five region model despite the split of the ISF into two parts. For each species in each region, there is one PDE describing convection, axial diffusion, transmenbrane transport, and chemical reactions. If the reaction involves multiple species, it is reflected in the PDE's for all the involved species. Numerically, the model uses multiple segments along the capillary length for axial resolution.