The Davenport Diagram Is A Two-Dimensional Representation of A Three-Dimensional Surface
Recall that the relationship represented in a Davenport Diagram is a relationship between three variables: PCO2, bicarbonate concentration and pH. Thus, Fig. 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. Figure 9 shows a Davenport Diagram in three dimensions. The light blue lines indicate the isopleths as we would normally encounter them, confined to a two-dimensional plane. The dark blue curves show the actual locations of the isopleths in three dimensions. Thus, the light blue lines are simply the projections of the isopleths in three-dimensional space onto a two-dimensional plane. Again, recall that an isopleth is just a titration curve—that is, the path that is traced if pH is varied while PCO2 is held constant. The green surface describes all combinations of PCO2, and pH that satisfy equilibrium for the system. All isopleths in their actual three-dimensional orientations must be confined to this surface.
A second key concept is that movement along a buffer line must correspond to a change in PCO2. Thus, like the isopleths, the buffer line as drawn on a typical Davenport Diagram (e.g., Fig 6) is actually the projection of a line existing in three-dimensional space onto a two-dimensional plane. As with the isopleths, buffer lines in their actual three-dimensional orientation are confined to the surface representing the values of PCO2, and pH that satisfy equilibrium for the system. In Fig. 10, the dark red lines are the actual buffer lines in three-dimensional space, while the light red lines are the projections of the buffer lines onto a two-dimensional plane. (We will see later how multiple, parallel buffer lines can be determined for a given system).
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