Relative hysteresis of airspaces and bronchi in excised lungs.

In a steady state the rate of removal of COz from the blood into the alveoli equals its rate of removal from alveoli to atmosphere. There is a gradient of COz concentration from mixed venous blood to alveolar gas or arterial blood and then to mixed expired gas. The mixed venous to alveolar (or arterial) difference depends upon cardiac output (Fick) and the alveolar (or arterial) to mixed expired difference depends upon physiological dead space (Bohr). Mixed venous COz tension (rebreathing), mixed expired COz tension and COz production can easily be measured. If an arbitrary value for arterial COz tension is selected the simultaneous equations for cardiac output and dead space may be solved, and if this process is repeated coordinates may be obtained relating cardiac output, dead space and arterial C02 tension. A digital computer can be programmed to construct a graph relating cardiac output, dead space and arterial Pcoz for any given C02 production, mixed venous and mixed expired C02 tension. It is able not only to convert COz tensions into the required concentrations but also to make allowances at all stages for alterations in the COz dissociation curve due to such factors as saturation, base excess and haemoglobin.