A gas exchanger with a restriction element or elements to reduce gas exchange as desired to avoid hypo-capnia and hyper-oxygenation in small patients. The gas exchanger includes a gas exchanger housing with an outer wall and a core which defines an inner wall and having a blood inlet for receiving a blood supply and a blood outlet. The gas exchanger also includes a hollow fiber bundle disposed within the housing between the core and the outer wall, and a gas inlet compartment for receiving an oxygen supply and directing the oxygen supply to the first ends of the hollow fiber bundle, wherein the gas inlet compartment includes at least one restriction element configured to allow the oxygen supply to reach only a portion of the hollow fiber bundle.