Non-rebreathing circuits are not used with CO₂absorbers or standard bellows ventilators. One tube carries the gas from the machine to the patient where a connection holds a facemask, nosecone or endotrachael tube. A second, larger diameter "exhaust down tube" also acts as a reservoir of breathable gas for the patient. Larger animals, 5 lbs and up, can be safely maintained on a non-rebreathing circuit by adding a "reservoir bag" between the down tube and the evacuation system.