Figure 1 illustrates the most important equilibrium reactions of carbon dioxide in blood relating to acid-base physiology:
Because many different non-bicarbonate buffers are present in human blood, the final equilibrium state reached at any given pCO2 is highly complex and cannot be readily predicted using theory alone.
By depicting experimental results, the Davenport diagram provides a simple approach to describing the behavior of this complex system.
A small sample of blood is taken from a healthy patient and placed in a chamber in which the partial pressure of carbon dioxide (PCO2) is held at 40 mmHg.
The fall in pH is also not surprising, since the formation of a bicarbonate molecule is concomitant with the release of a proton (see Fig.
The buffer line can be used to predict the result of varying the PCO2 within a range close to the experimentally determined points.
In the Davenport diagram, these titration curves are called isopleths, because they are generated at a fixed partial pressure of carbon dioxide.
It is instructive to note that the slope of the bicarbonate line will never actually reach zero (i.e. will never be horizontal) under equilibrium conditions, even in the complete absence of non-bicarbonate buffers.
Likewise, an increase in pH for similar reasons must occur with some minimal decrease in bicarbonate concentration.
7 can be thought of as a topographical map—that is, a two-dimensional representation of a three-dimensional surface—where each isopleth indicates a different partial pressure or “altitude.” A more accurate depiction would involve three axes.
As with the isopleths, buffer lines in their actual three-dimensional orientation are confined to the surface representing the values of PCO2, [HCO3−] and pH that satisfy equilibrium for the system.
One of the most important features of the Davenport diagram is its usefulness in depicting movement from one point on the equilibrium surface to another following changes in respiration and/or metabolism.
On the other hand, if one were to hyperventilate, then fresh air would be drawn into the lungs and carbon dioxide would rapidly be blown out.
Note that these two situations, those of respiratory depression and hyperventilation, produce effects that are immediately analogous to the experiment described previously, in which the partial pressures of carbon dioxide were varied and the resulting changes in pH observed.
As indicated by the Davenport diagram, respiratory depression, which results in a high PCO2, will lower blood pH.
Additionally, hydroxide ions will abstract protons from carbonic acid in solution, causing the bicarbonate concentration to increase.
Alternatively, if protons are added to the bloodstream in the form of acidic metabolites, as occurs during diabetic ketoacidosis, then pH will fall, along with bicarbonate concentration.