Absorption compartment options
This PK compartment is used as an input site for doses following a first-order input function. By default, flows connecting an absorption compartment to another compartment use a first-order function. That function can be changed by specifying PK flow options in the Structure tab. Connect an absorption compartment to another compartment via a PK flow to create a 1st-order absorption PK model.
Click 
in the Model toolbox.
Or
Select Insert > Compartment > Absorption from the right-click menu.
Absorption compartment works the same as the central compartment, (except there is no Conc option to add volume and concentration parameters). Refer to the “Central compartment options” section for details on setting these parameters.
Note: Be careful not to use the same names for the volume and concentration parameters in different compartments.
Drag the dosing variable square from the absorption compartment to the central compartment dosing variable square.