Isohydric shift

The transformations described, so far (Figure 46-9, reactions 1 through 5) are referred to as the isohydric shift (i.e., a shift in which the hydrogen ion concentration remains unchanged). 

This anomalous pH behavior results from the presence of polyborates, which dissociate into B(OH)2 and B(OH) as the solutions are diluted. Below pH of about 9 the solution pH increases on dilution the inverse is tme above pH 9. This is probably because of the combined effects of a shift in the equihbrium concentration of polymeric and monomeric species and their relative acidities. At a Na20 B202 mol ratio equal to 0.41 at pH 8.91, or K20 B202 mol ratio equal to 0.405 at pH 9 the pH is independent of concentration. This ratio and the pH associated with it have been termed the isohydric point of borate solutions (62). 

Figure 46-9 Scheme demonstrating the isohydric and chloride shift.The encircled numbers refer to the reactions described in the text. For details, see text. 

In blood, the main buffer system is bicarbonate at a concentration of [HCOj"] = 0.02-0.03 M (20-30 mEq/ I). Hemoglobin provides a further 10 mEq/l buffer capacity, and phosphate makes a small contribution of 1.5 mEq/l. The 5 liters of blood in an average adult human are thus able to absorb about 0.15 mole before the pH becomes dangerously low. The major buffers of the body are, however, present in other tissues. The total musculature of the body, for example, can neutralize about 5 times as much acid as the blood, and the blood HCO37CO2 system represents only about a tenth of the total buffer capacity of the body. Since all the buffer systems of the body are able to interact and buffer each other, all changes in the acid/ base balance of the body are reflected in the blood. This mutual buffering by the shift of H from one body system to another is known as the isohydric principle. 

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