The quantitative evaluation of pathological CO2 elimination and O2 uptake are discussed. CO2 elimination may be described by the concept of "functional dead space ventilation" (VD/VT) in a manner sufficient for clinical use. For this purpose it is necessary to measure PaCO2 and the concentration of CO2 in the mixed expired gas. For a quantitative definition of oxygen uptake it is necessary to relate PaO2 to inspiratory O2 concentration (F1O2), and this is best achieved using the "intrapulmonary right-to-left shunt". To calculate this ratio, mixed venous oxygen saturation as well as F1O2 and PaO2 have to be measured, which means that a pulmonary artery catheter has to be in place. A new interpretation of the N2 wash-out curve demonstrates that pathological changes of intrapulmonary gas diffusion may be responsible for difficulties in the interpretation of O2 uptake. Monitoring of pulmonary hemodynamics calls not only for a pulmonary artery thermodilution catheter but also monitoring of the course of body weight. However, recent work shows that the measurement of ventricular filling pressures must be related to the individual ventricular volumes in systole and diastole which are measurable today.