• About 85% of the 9 L of water and 30 g of NaCl presented to the gastrointestinal (GI) tract daily is absorbed by the small intestine. All but 100-200 mL of the remaining fluid is reabsorbed by the colon.
• Solutes and water may cross the epithelial cell lining of the GI tract through transcellular and paracellular routes. The transcellular route involves movements across the apical and basolateral membranes arranged in series. The paracellular route involves movements through the zonula occludens, or tight junctions, and the underlying intracellular space.
• The permeability of the paracellular pathway is highest in the duodenum and decreases progressively in the aboral direction.
• The Na+-K+ pump, which extrudes three Na+ ions in exchange for two K+ ions per adenosine triphosphate (ATP) hydrolyzed, is the principal mechanism responsible for Na+ movement across the basolateral membranes of the epithelial cells. This mechanism is responsible for the low intracellular Na+ and high intracellular K+ activities of these cells.
• Some chloride is absorbed in the intestinal tract by diffusion through paracellular pathways driven in part by the transepithelial electrical potential difference established by Na+ absorption.
• Some cells possess a neutral chloride entry mechanism that mediates the movement of 1 Na+, 1 K+, and 2 Cl~ ions across the apical membrane, energized by the Na+ gradient across that barrier.
• Potassium absorption by the small intestines is by means of diffusion through paracellular pathways. K+ absorption by the large intestine is an active process mediated by a K+,H+-ATPase
Essential Medical Physiology, Third Edition
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