Abstract: A method for improving oxygenation in subjects having essentially healthy lungs, as evidenced by the absence of a diagnosis of lung disease or injury, but having reduced alveolar gas exchange area. This reduction may be caused by such acute circumstances as an unnatural body position, or may be, for example, chronic as caused by obesity. The method employs the administration of nitric oxide (NO) into the breathing gases of such subjects. NO provided to alveoli collapsing during expiration is small compared to those remaining open, thereby to provide net reduction in the shunt and thus an oxygenation improvement. This result may be gained either by precise control of the inspired NO concentration or by pulsed administration of the NO.