Arrangement and method for ventilating lungs

An arrangement for ventilating lungs of a subject is disclosed herein. The arrangement includes a machine ventilator circuit (14) comprising an inspiration delivery unit (20) for delivering a gas flow for an inspiration, and an expiration circuit (22) for controlling a discharge of an expiration gas. The arrangement also includes a manual ventilation circuit (18) comprising a manual bag (40) guiding a gas from the manual bag to assist the inspiration and receiving a gas flow for filling the manual bag during the expiration at least partly by the gas used to assist the inspiration and a control unit (21) for controlling an operation of the arrangement. The manual ventilation circuit also comprising a sensor (44) to detect a flow direction inside the manual ventilation circuit and to produce for the control unit a signal determining the flow direction to guide the expiration circuit. A corresponding method is also provided.