Fluid and Electrolyte Absorption

Bidirectional Fluid Flux

During a 24-hour period, 7 to 10 L of water enters the small intestine ( Fig. 12.1 ). Fluid derived from food and drink accounts for approximately 2 L. The other 7 L is derived from the secretions of the gastrointestinal (GI) tract: saliva, 1 L; gastric juice, 2 L; pancreatic juice, 2 L; bile, 1 L; and small intestinal secretions, 1 L. Of the amount entering, only approximately 600 mL per 24-hour period reaches the colon, a finding indicating that most water is absorbed in the small intestine. Because the average daily fecal weight is approximately 150 g, of which 100 g is water, 500 mL of fluid is absorbed daily from the colon. This volume represents 10% to 25% of the absorptive capacity of the colon, which can absorb approximately 4 to 6 L of fluid per day. Malabsorption of solutes and water in the small intestine may permit enough fluid to enter the colon to overwhelm its absorptive capacity and cause diarrhea. This, in turn, can precipitate severe electrolyte deficiencies. Although it is possible that ions and water may be added to the feces from colonic mucosa, the major source of water and electrolytes in a diarrheic stool is the small intestine (see Fig. 12.1 ).

It is evident from the foregoing account that several liters of fluid are secreted into the GI tract daily, and several liters are absorbed. The volume of fluid moving from blood to lumen (secretion) is less than that moving from the lumen to the blood (absorption), thus resulting in net absorption. Absorption generally results from the passive movement of water across the epithelial membrane in response to osmotic and hydrostatic pressures. Because of these so-called Starling forces, the consequent bulk flow of fluid is analogous to the flow of fluid across capillary walls. In the absence of food, ions are the most important contributors to osmotic pressure in the intestinal lumen. The ionic composition of the luminal contents may vary along the length of the intestine and is different from that in feces. Luminal fluid generally remains isotonic with plasma, however, because of the relative permeability of the mucosal membrane. The continued production of solutes by colonic bacteria, together with the relative impermeability of the colonic membrane to water, usually causes stool water to be hypertonic, 350 to 400 milliosmoles (mOsm)/L, to plasma.

Ionic Content of Luminal Fluid

Osmotic equilibration with plasma of material entering the small bowel occurs rapidly in the duodenum. Water is absorbed from hypotonic solutions and enters hypertonic solutions. Na ⁺ and Cl ⁻ also leave hypertonic solutions. Most of this movement occurs through the relatively permeable junctions between the epithelial cells of the proximal small intestine.

Proceeding from the duodenum to the colon, the Na ⁺ and Cl ⁻ concentrations in the lumen progressively become lower than the plasma concentrations. In the duodenum, the concentration of Na ⁺ is approximately 140 mEq/L, equal to the serum concentration. The major anion is Cl ⁻ . Na ⁺ concentrations decrease in the jejunum and reach approximately 125 mEq/L in the ileum. The major anions in the ileum are Cl ⁻ and ${\text{HCO}}_3^{-}$ . Na ⁺ decreases to 35 to 40 mEq/L in the colon, whereas the K ⁺ concentration increases to 90 mEq/L, up from 9 mEq/L in the ileum. The major anions in the colon are Cl ⁻ and ${\text{HCO}}_3^{-}$ .

These values indicate an effective absorption process for Na ⁺ that becomes increasingly efficient toward the distal portions of the gut. This is caused in part by a decrease in the permeability of the epithelium that prevents the back-diffusion of ions absorbed in the distal portions. Whereas the absorption of Na ⁺ in the distal gut is effective, the conservation of Cl ⁻ is even more so. Cl ⁻ is exchanged for metabolically derived ${\text{HCO}}_3^{-}$ .

K ⁺ is absorbed passively by the small intestine as the volume of intestinal contents decreases. The concentration of K ⁺ remains roughly equal to that in the serum (4 or 5 mEq/L). In the colon, net K ⁺ secretion occurs. Because of K ⁺ secretion and the exchange of Cl ⁻ for ${\text{HCO}}_3^{-}$ in the colon, prolonged diarrhea results in hypokalemic metabolic acidosis.

Transport Routes and Processes

Ions move between the gut lumen and the blood through transcellular and paracellular pathways . The passive movement of Na ⁺ , both into and out of the lumen, is largely through the lateral spaces. This movement is determined by the tight junctions or zonulae occludens . The rate of this passive component is affected by electrochemical gradients and Starling forces. Normally these forces are small and account for only a small fraction of the net transport. They can, however, be altered under certain conditions, with marked effects.

The tight junction is about twice as permeable to Na ⁺ and K ⁺ as it is to Cl ⁻ . Thus electrical potentials can arise across this structure. If, for example, NaCl is moving across the tight junction, the Cl ⁻ will be retarded relative to the Na ⁺ , and the surface toward which the movement is occurring will become positive relative to the other surface. Ions slightly larger than Na ⁺ and K ⁺ are much more restricted in their movement.

The pores through which transcellular diffusion takes place are probably larger (7 to 8 angstroms [Å]) in the proximal bowel than in the ileum (3 to 4 Å). This restricts the passive transport of solutes in the distal gut and allows these solutes to exert a more effective osmotic pressure. In turn, the reduced permeability makes carrier-mediated transport a more important contributor to net transport out of the lumen.