The anesthesia breathing system is unique among ventilators in that it is specifically designed to allow patients to rebreathe exhaled gases. This allows for the conservation of anesthetic agent along with heat and moisture. While gases are rebreathed, they are also conditioned and mixed with the fresh gas flow, which changes their composition. Rebreathing is accomplished by a unidirectional circuit, controlled by at least two one-way valves called the circle system.