The measurement of co uptake (VCO and DLCO) from alveolar gas is a unique way to noninvasively assess pulmonary vascular function, specifically the functional volume of the pulmonary capillary bed. Proper interpretation of results, however, needs to account for inherent assumptions regarding co distribution and timing procedures. Moreover, reasonable airway mechanics, lung volumes, and patient cooperation are required for accurate measurements. Potential clinical utility may be increased if measurements are made in different positions or under exercise conditions.