Abstract: An artificial lung has an axially extending housing which accommodates a bundle of hollow fibers retained within the housing by a pair of walls provided at the ends of the housing. The housing has blood inlet and outlet ports defining a blood chamber within the housing. Oxygen gas is passed through the hollow fibers and blood is passed through the blood chamber so that a gas exchange may take place through the hollow fiber walls. The artificial lung is provided with a blood reservoir chamber integral with the blood chamber and capable of being communicated with the blood chamber, so that blood which has undergone a gas exchange inside the chamber may be stored within the reservoir. A heat exchanger is disposed at a point preceding, following or intermediate the blood chamber and blood reservoir chamber, which construct a blood circuit.