Abstract: An artificial lung includes a housing having gas inlet and outlet ports, a blood inlet chamber mounted on the upper ends of the housing, a blood outlet chamber mounted on the lower end of the housing, and a plurality of hollow fiber membranes for exchanging a gas, disposed within the housing and having ends opening into the blood inlet chamber and opposite ends opening into the blood outlet chamber. A blood inlet port communicates with the blood inlet chamber and is positioned such that the ratio of the distance from the central axis of the blood inlet chamber to the central axis of the blood inlet port to the inside radius of the blood inlet chamber ranges from 0.25 to 0.95, so that blood introduced from the blood inlet port into the blood inlet chamber will be swirled in one direction.