Thermodynamic considerations of arteriovenous gradients of hydrogen ion concentration and carbon dioxide tension

Abstract

It is shown that, in a multicompartmental homeostatic system, the extent of interaction between any two compartments can be assessed by determination of the difference in free energy change of one particular reaction, or a series of coupled reactions, operative in both of the compartments under consideration. Hydrogen ion concentration and carbon dioxide tension have been used to determine free energy change difference relationships between the venous and arterial compartments (- G(a-v)) of the circulatory system. Data from the literature (from two studies of congestive heart failure and one study of experimentally induced cardiac arrest) are used to calculate - G(a-v). It was found that in control subjects - G(a-v) is close to zero, whereas in congestive heart failure or cardiac arrest, the value rises to 150 calmol-1 or more, whereas in blood, the approach towards equilibrium between hydrogen and bicarbonate ions and dissolved carbon dioxide (aqueous CO2) is known to be only moderately rapid. It is concluded that, in the system under study, and with respect to the reaction H+ + HCO3- = CO2 + H2O, a high value for the free energy change difference between the two compartments (high - G(a-v)) must be due to an insufficient blood circulation rate. Accordingly, - G(a-v) is probably a quantitative measure of cardiac insufficiency.