A method for non-invasively determining functional cardiac output (FCO) and/or venous blood CO2 partial pressure (PvCO2). The amount of CO2 (VCO2N) released from the blood and end capillary blood CO2 content (CcCO2N) are determined from measurements from exhaled breathing gases. The CO2 content of the breathing gases inhaled by the subject is increased and values for VCO2R and CcCO2R are obtained. A regression analysis is performed using the obtained VCO2N, VO2R, CcCO2N, and CcCO2R values. The regression line is extrapolated to obtain a value for CcCO2 when (VCO2) is zero so that CvCO2 becomes known. The CvCO2 thus determined can be inserted in a non-differential form in the Fick equation, along with VCO2 and CcCO2 values from normal breathing, to determine FCO. To determine PvCO2, CvCO2 is altered in accordance with the amount of oxygen in the venous blood, to correctly indicate PvCO2. The continuing validity of the FCO measurement can be examined on a breath-by-breath basis by noting changes in an indicator variable, such as VCO2 or end tidal CO2 amounts.