meter 24 as well as the vaporizer selector valves 44 the oxygen flush valve 50 is used in situations where an immediate flow of oxygen is required to the common outlet to flush any anesthesia gas that might still remain there as shown in fig2 gas from the common outlet 48 is introduced into a patient breathing system indicated generally by reference numeral 56 gas from the common outlet 48 is received over an inhalation line 58 which passes through a first check valve 60 and thence to a patient wye 62 to which is connected a breathing tube 64 or a mouthpiece not shown through which the patient breathes the patient wye 62 connects the patient breathing tube 64 to the inhalation line 58 and to an exhalation line 66 the exhalation line 66 includes a second check valve 68 and is connected to a carbon dioxide co 2 absorber 70 and thence by a line 72 back to the inhalation line 58 the check valve 60 in the inhalation line 48 operates to ensure that no exhalation gas from the patient passes along the inhalation line and the other check valve 68 performs a corresponding function in the exhalation line 66 ensuring that gas is not inhaled through the exhalation line when the patient exhales an expandable volume must be provided in the breathing system for the exhaled gas if the patient is breathing spontaneously this excess volume is usually provided in the form of an expandable breathing bag 76 alternatively if the patient is incapable of spontaneous breathing a ventilator 78 is typically employed both to receive exhaled gas.