Carbamino compounds

Total carbon dioxide (CO2) content of plasma consists of carbon dioxide dissolved in an aqueous solution (dCOa), CO2 loosely bound to amine groups in proteins (carbamino compounds), HCO3 and vanishingly small amounts of CO ions, and carbonic acid (H2CO3). Bicarbonate ions make up ail but 2 mmol/L of the total carbon dioxide of plasma (22 to 31 mmol/L). Measurement of the tota CO2 as part of an electrolyte profile is useful chiefly to evaluate HCO3 concentration in assessment of acid-base disorders. 

Bicarbonate is the second largest fraction (behind Ci ) of plasma anions ( 25 mmol/L). Conventionally, it is defined to include (1) plasma bicarbonate ion, (2) carbonate, and (3) CO2 bound in plasma carbamino compounds (Figure 46-7). At the pH of blood, the plasma carbonate concentration is 25 pmol/L, which is -1/700 to 1/1000 of the total bicarbonate concentration. C02-bound carbamino compounds (RCNHCOOH) are 0.2 mmol/L in plasma and 1.5 mmol/L in erythrocytes. Actual bicarbonate ion concentration is not measured, but rather calculated from the Henderson-Hasselbalch equation as described below (and discussed in detail in Chapter 27). Also, as described in Chapter 27, the analyte usually measured in plasma is total COa, which includes bicarbonate and dissolved CO (dC02). The dC02 fraction is defined to include both the undissociated carbonic acid and physically dissolved, free CO2. At the pH of the blood, the amount of dissolved CO2 is 700 to 1000 times greater than the amount of carbonic acid and therefore... 

The occurrence of CO2 chemically combined with hemoglobin, as carbamino compounds. 

About 20 percent of the CO2 in blood is carried as carbamino compounds formed by the reaction with amino groups of hemoglobin. The reaction may be represented as... 

In this way, the loading of CO2 as a carbamino compound results in the additional unloading of Oj at the tissue level. 

The carbamino compound is then combined with ammonia, and dehydrated to form the uramido acid, citrulline, HjN.CO.NH.R. 

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