The epithelial cells lining the small intestine and colon absorb most of the delivered electrolytes and water in isoosmolar concentrations. Thus, the fluid that is absorbed is always isosmotic. The absorption of Na⁺ and Cl⁻ in the small intestine is similar to that of the proximal tubule, and involves the following transport mechanisms:

Same transport mechanisms as those in the jejunum and additionally a Cl/HCO₃ exchanger, which facilitates absorption of Cl⁻ and secretion of HCO₃ ⁻ into the lumen are used

Thus, HCO₃ ⁻ absorption occurs in the jejunum, whereas its secretion occurs in the ileum

Like principal cells, the epithelial cells of the colon contain Na⁺ and K⁺ channels separately

Absorption of Na⁺ and secretion of K⁺ occurs via their respective channels

Aldosterone regulates both Na⁺ and K⁺ transport

The epithelial cells lining the intestinal crypts secrete both electrolytes and water

The apical membrane of crypt cells contains Cl⁻ channels, and the basolateral membrane contains Na/K-ATPase, Na/K/2Cl cotransporter, and a K⁺ channel. Na⁺, K⁺, and Cl⁻ enter the cells from blood via these transporters. Cl⁻ is secreted into the lumen via Cl⁻ channel, whereas Na⁺ enters the lumen passively via the paracellular pathway. Subsequently, water moves into the lumen following NaCl secretion

Usually, Cl⁻ channels are closed, but remain open following activating substances. These substances bind to their receptor at the basolateral membrane, leading to the stimulation of adenylate cyclase and production of cAMP in crypt cells. cAMP then keeps Cl⁻ channel open, facilitating its secretion into the lumen

Although HCO₃ ⁻ is secreted in the colon, all of it is not excreted in the stool. Most of this HCO₃ ⁻ is used up by the production of organic acids such as propionic acid, butyric acid, acetic acid, and lactic acid. These acids are the products of unabsorbed carbohydrates that are fermented by bacteria

Titration of these acids by NaHCO₃ generates sodium propionate, sodium butyrate, and other organic acids, which enter the liver for regeneration of HCO₃ ⁻. Therefore, the stool contains low HCO₃ ⁻

Table 30.2 shows the normal values of electrolytes in various fluids of the GI tract. The information is useful in assessing the acid–base disturbances due to GI disorders

It is evident from Table 30.2 that the GI tract as a whole absorbs all the secreted Na⁺ and Cl⁻, leaving very few milliequivalents in the stool

More specifically, the jejunum absorbs about 100 mEq of Na⁺ and 3 L of water, whereas the ileum absorbs 400 mEq each of Na⁺ and Cl⁻ as well as 3.5 L of water

Finally, the colon is the most efficient segment of the intestine, absorbing > 90 % of 200 mEq of Na⁺, 100 mEq of Cl⁻, and 1.4 L of water delivered to it. Because of this tremendous absorptive capacity of the colon, the stool contains < 100–200 mL of water, low quantities of Na⁺, Cl⁻, and HCO₃ ⁻; however, K⁺ concentration in stool is more than the other electrolytes because of its secretion in the colon

Diarrhea is defined when stool weight exceeds > 200 g/day or > 200 mL/day, when secretions of fluids exceed their absorption

Diarrhea is the most common nonrenal cause of hyperchloremic metabolic acidosis

Unlike renal acidoses where hyperchloremic metabolic acidosis is due to defects in transport mechanisms, diarrhea or GI disorders-induced hyperchloremic metabolic acidosis is due to loss of HCO₃ ⁻ and other electrolytes in the stool

The composition of the diarrheal fluid varies depending on the etiology of diarrhea (Table 30.3)

Diarrhea is usually classified into the following types: