A blood oxygenating device having an oxygenating chamber and a heat exchange chamber. The oxygenating chamber consists of 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 a mixing chamber which is provided with a plurality of secondary flow-producing deflectors to promote secondary flow 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.