Peripheral compartment options
Peripheral compartments can be connected to other PK compartments, including central and elimination compartments, by using a PK flow. If there is only one central compartment, the volumes of the peripheral compartments are determined by their surrounding constraints, or flow parameters.
Click 
in the Model toolbox.
Or
Select Insert > Compartment > Peripheral from the right-click menu.
Peripheral compartment options work the same as the central and absorption compartment options. Refer to the “Central compartment options” section for details on setting these parameters.