You can change the type of breathing circuit by choosing an entry in this drop-down box. For the remainder of the simulation (unless you change it again), the circuit behaves according to your choice.

Although you can change the circuit type at any time during a simulation, making changes while replaying a simulation will invalidate all subsequent recorded results and adjustments. For example, rewinding a simulation (or opening a saved simulation) and changing the circuit type would erase it from the revision point forward. (This will not affect the saved version.) Gas Man® will ask you to confirm your intent whenever you modify a simulation during replay.