An open ventilation system for intensified breathing that includes both a manual ventilation system and an automatic ventilation system. A ventilation selection switch is positioned in the system prior to the patient circuit to select between the manual and automatic ventilation systems. The manual ventilation system includes a manual bag filling valve that controls the flow of inhalation gases from the ventilation selection switch to a manual bag. The manual bag filling valve includes a valve assembly that is movable between an open position and a closed position based on the difference in the forces created by the pressures of the manual bag and the inhalation gas.