Bicarbonate Ion Is Secreted by Several Digestive Glands and Must Be Recovered from the Gut if Body Acid-Base Balance Is to Be Maintained
Much bicarbonate is in essence “absorbed” by the neutralization of HCl from the stomach. Sodium bicarbonate entering the intestine reacts with HCl to form water, carbon dioxide, and sodium chloride, effectively resulting in the absorption Ofbicarbonate (HCO3 ) and hydrogen (H+) ions.
(See Chapter 28 for an explanation of the counterbalancing effects of gastric acid secretion and pancreatic bicarbonate secretion.) However, considerable bicarbonate remains in the intestine after the neutralization of stomach acid. This remaining bicarbonate is reabsorbed, primarily in the ileum and colon via an ion-exchange mechanism.Bicarbonate anions in the gut are electrically balanced, primarily with sodium cations, and reabsorbed essentially as sodium bicarbonate. In the absorptive process, Hf and HCO3" ions are first generated within the enterocytes from water and carbon dioxide. H* is then exchanged for Na+ across the apical membrane. Within the cell, Na' is electrically balanced by the remaining HCO3", whereas the HCO3 remaining in the gut lumen is neutralized by the secreted Hf (Figure 30-19). The result is that sodium is transferred through the membrane. However, luminal bicarbonate is converted to water and carbon dioxide in the gut lumen, whereas bicarbonate anion
FIGURE 30-19 ■ Absorption of bicarbonate (HCO3 ) is facilitated by sodium-hydrogen (Na*∕H*) exchange at the apical membrane. The bicarbonate ion is regenerated by the action of carbonic anhydrase.
is regenerated intracellularly. The net effect is the absorption of sodium bicarbonate. This mechanism is essentially one half of chloride-coupled sodium absorption» except that the bicarbonate ion remains within the enterocyte rather than being exchanged for chloride.