Tube-outside-a-tube circuit. The fresh gas tube is outside the larger diameter downtube.

Non-rebreathing circuits consist of two tubes. One delivers gas from the anesthesia machine to the patient connection, then a second, usually larger tube, channels the waste gases to evacuation. In this type of circuit any gas not utilized is immediately directed toward the evacuation circuit. The waste gas tube should be easily connectable to either a disposable charcoal canister or to an in-house evacuation system. This type of circuit is usually configured with an elbow, a tee or a straight connection to the nosecone/ET tube at the patient end. The fresh gas tube can be located inside or outside the waste gas tube. The internal diameters and overall lengths of the fresh gas and waste gas tubes can vary greatly.