A blood oxygenating device comprising an oxygenating chamber and a heat exchange chamber, the oxygenating chamber comprising a bubbler chamber and a mixing chamber. Oxygen is introduced into the bubbler chamber through a porous diffusion means situated near the inlet of the bubbler chamber. Bubbles pass from the bubbler chamber into the mixing chamber which is provided with a plurality of secondary flow-producing deflectors to promote secondary flow and rotation of blood bubbles passing therethrough. The secondary flow results in a highly efficient oxygen-carbon dioxide transfer. The blood bubbles are then passed through a heat exchange chamber and a defoaming means prior to exiting the oxygenating device.In a preferred version, blood and oxygen-rich gas are admitted at the top of the device and caused to flow substantially downwardly through a tortuous path which causes at least a portion of the blood bubbles to rotate. During the course of downward flow, the blood-gas-bubble mixture passes around a heat exchange device, which preferably is convoluted, helically wound tubing so as to form the tortuous path, to bring the blood to a desired temperature. The oxygenator is equipped with a defoaming means such that the blood bubbles are converted back into liquid blood, which is then returned to the patient. Vent means are provided for removal of carbon dioxide and other vent gases. Means for promoting secondary flow in the blood are preferably provided in the upward region of the oxygenator.