A patient ventilator system provides a controllable flow and volume of a mixed inhalation gas to a patient, and receives exhaled gas from the patient. The ventilator system includes gas flow controllers that mix air and oxygen, and provide the mixture to the patient with a controllable inhalation cycle. The ventilator system monitors the resulting patient pressures and volume of gas exhaled by the patient, as necessary. Compressed air is provided to the gas controller in the event of an absence of an external gas supply by a compressor. A controller permits the user of the ventilator system to control ventilation mode, gas pressure, composition, and flow rate of the mixed gas with a single control knob.