It is comprised of discrete submodels that reflect the organ structure or the parts of the experimental apparatus that affect tracer concentrations and residues in the organ. For each tracer or nontracer substance, there is one inlet tube, one artery, one vein, and one outlet tube. There may be a maximum of twenty flow pathways each consisting of an arteriole, a blood-tissue exchange unit, and a venule. Each pathway has its own flow rate and receives a specific fraction of the total mass of the tracers and nontracer substances.
Flow enters each pathway after having passed through optional large vessel components.
Each pathway includes optional small vessel components and one exchange unit. Large and small vessel components account for delay and dispersion of the input function before it reaches the exchange unit. The user must enter the value of this parameter. The names shown are those that appear in the popup window after double clicking on a parameter button or field. The XSIM convention for inputs and outputs is that input parameters are shown with a lighter field background, and the outputs are displayed on a shaded background, as shown in Fig.