The invention relates to an anesthesia ventilating apparatus having a breathing circuit and a plurality of control loops for the anesthesia gas, the anesthesia agent and the carbon dioxide exhaled by a patient. A disconnect switch is provided for opening and closing the control loop for adjusting the anesthesia agent. When this control loop is opened, a control unit changes the desired value of the anesthetic agent to a desired flushing value thereof. The control unit includes a processor which monitors the time-dependent course of the concentration of the anesthetic agent to determine breathing circuit system parameters and then computes the adjusting parameters of a controller in the control loop for adjusting the anesthetic agent. A method of operating the apparatus is also disclosed.