A control system controlling blood gas values in blood processed by an oxygenator, wherein the oxygenator generates arterial blood by exposing venous blood to oxygen from an oxygenation gas supply, comprises a monitoring arrangement to determine a level of the blood gas values in the arterial blood and a controller that is responsive to the monitoring arrangement and configured to control parameters of the oxygenation gas supply to the oxygenator. This allows the blood gas values to be adjusted toward a pre-determined level.