ÒSemiopenÓ anesthesia breathing circuits classified as Mapleson A-F based on position of components. Advantages are simplicity, lightness, and easy cleaning. Disadvantages are wasteful gas usage, potential CO2 rebreathing, and relatively heavy valve positioned over patientÕs face. Mapleson A circuit Ð Ònoncircle anesthesia breathing system used in pediatrics; advantages are relative inexpensiveness and valvelessness. Mapleson D circuit (Bain circuit) Ð widely used ÒnoncircleÓ anesthesia breathing system with single tube-inside-a-tube design. It is valveless with no CO2 absorber.