An iterative computerized classical physiological model of respiration and O2 transport (MACPUF) has been written for simultaneous variation of fractional inhaled oxygen (F1O2) and fractional inhaled carbon dioxide (F1CO2). MACPUF has been updated and released in a microprocessor version. The formation of carboxyhemoglobin (COHb) as a consequence of carbon monoxide (CO) inhalation has been described by a physiological model called the Coburn-Forster-Kane equation (CFKE). The CFKE predicts COHb for a wide variety of CO exposure conditions. Incorporation of the CFKE into MACPUF and the modification of MACPUF to include changes in oxyhemoglobin (O2Hb) dissociation curve was done to make available an iterative algorithm capable of estimating the blood gases, as a function of time, for a wide variety of exposure scenarios involving F1O2, F1CO2 and fractional inhaled CO (F1CO). The value of the lung diffusivity for CO (DLCO) was also recomputed to include the effects of exercise.