Abstract: An oxygen supply unit for use with a blood oxygenator comprises an oxygen concentrator and a carbon dioxide scrubber. In an on-line operational mode, oxygen-rich gas from the oxygen concentrator is predominantly supplied to the blood oxygenator with a reduced flow of recycled gas from the concentrator. In an off-line operational mode where the oxygen supply unit is being powered by battery only, a larger flow of recycled gas from the blood oxygenator is passed through the carbon dioxide scrubber and combined with a lesser amount of oxygen-rich gas from the oxygen concentrator. The oxygen supply unit may be used in combination with a blood pump and oxygenator to provide ambulatory blood oxygenation to patients with compromised lung function.