The estimated diffusion coefficients for nitric oxide and carbon monoxide in human plasma and red cells can be inserted into the equations for Fick's 1st and 2nd laws. The surface area of the alveolar membrane and exposed red cells can then be calculated from DLNO and DLCO (in hyperoxia). This yields a value of 18.7 m2 for alveolar capillary membrane via DLNO, 19.6 m2 for red cell surface area via DLCO in a seated human at rest and = 2.03m2 for a membrane oxygenator. The values for a seated human are substantially less than morphometric values. This is likely due to a considerable functional reserve in the lung.