A ventilation system for intensified breathing that includes both a manual ventilation system and automatic ventilation system. The gas mixture from a gas supply system is supplied to a switching valve that is operable between a manual position and an automatic position. When the switching valve is in the manual position, the inhalation gas from the supply system is guided to a manual ventilation bag. When the ventilation bag is squeezed during manual inhalation, the gas flow from the manual bag is directed to the inhalation limb of the patient circuit. When the manual ventilation bag is released, the pressure within the ventilation bag falls below the exhalation pressure, thereby indicating the beginning of the exhalation phase and the re-inflation of the manual ventilation.