An arrangement and a method for cardiopulmonary bypass is provided. The arrangement comprises a reservoir (20) configured to receive blood from the cardiotomy of a patient (P). An oxygenator (11) receives the blood from the reservoir. An arterial connection (2) is connected to the patient for supplying the blood from the oxygenator. A pump (15) is provided for pumping the blood through the oxygenator to the arterial connection and the patient. The arrangement comprises a supply device (30) including a supply conduit (31) attached to the reservoir. The supply device supplies a displacing gas to the reservoir via the supply conduit.
