when the operator enters the data or commands for the keyboard disply then to the microcontroller (which is the mind of the device) which in turn translates the command & sends them to the parts of device,,then the solenoiid valve for the oxygen gas allows it to exit from its source with certain values..also ,the solenoid valve for the air allows the air that pulled from the outside,which is filtered by an air filter, then to compressor which is turn mixes them,then presses them to take them out with a certain pressure, then goes to the pressure sensor to measure and adjust the gas pressure.. then it passes the flow sensor to measure the gas flow mixed with the inhalation process befor it leaves the machine, then to the humidifier which moisturizes gas to prevent dryness of the patient while breathing ,then by inspiratory tube to the patient,, As for the exhalation process it passes through an expiratory tube to another flow sensor to measure the rate of exhalation flow for the patient ..and finally to air filter then to room