X

HCO,

(blood)

A loss of H+ in the urine is equivalent to adding new HCO3 ~ to the blood. The same is true if H+ is lost from the body via another route, such as by vomiting of acidic gastric juice. This process leads to a rise in plasma [HCO3~]. Conversely, a loss of HCO3~ from the body is equivalent to adding H+ to the blood.

Cell Model For Hco3 Reabsorption
kA cell model for HCO32 reabsorption. Filtered HCO3~ combines with secreted H+ and is reabsorbed indirectly. Carbonic anhydrase (CA) is present in the cells and in the proximal tubule on the brush border.

Titratable Acid Excretion. Figure 25.9 shows a cell model for the formation of titratable acid. In this figure, H2PO4~ is the titratable acid formed. H+ and HCO3~ are produced in the cell from H2CO3. The secreted H+ combines with the basic form of the phosphate (HPO42-) to form the acid phosphate (H2PO4~). The secreted H+ replaces one of the Na+ ions accompanying the basic phosphate. The new HCO3~ generated in the cell moves into the blood, together with Na+. For each mEq of H + excreted in the urine as titratable acid, a mEq of new HCO3~ is added to the blood. This process eliminates H+ in the urine, replaces ECF HCO3~, and restores a normal blood pH.

The amount of titratable acid excreted depends on two factors: the pH of the urine and the availability of buffer. If

Peritubular blood

Tubular epithelium

Tubular urine

HCO3"

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