A blood oxygenator is disclosed comprising a housing, a blood inlet, a blood outlet, a spiral volute, a gas inlet, an oxygenator fiber bundle, and a gas outlet. The housing encloses the fiber bundle and provides the structure for the blood flow path and connectors. The fiber bundle comprises gas-exchange membranes which transfer oxygen to the blood and remove carbon dioxide when the blood flows across the membranes. The spiral volute guides the blood to flow through the fiber bundle. A gas flow chamber receives sweep gas containing oxygen and distributes the sweep gas into the fiber membranes, which gas is then exchanged with the blood being oxygenated.