GI Water and Electrolyte Absorption

Overview

The GI system secretes nearly 8L of fluid into the GI tract which is combined with the nearly 2L of ingested fluid; however, only a 0.1-0.2L of fluid is excreted in the feces. Therefore, the GI system must absorb nearly 9L of fluid, composed of water and electrolytes, each day. Water and electrolyte absorption primarily occurs in the small and large intestines. Dysregulation of this absorption can either lead to diarrhea or constipation. We discuss the absorption of water and different electrolytes separately below.

Sodium Resorption

Luminal membrane Na⁺ resorption occurs via a number of symporters and antiporters throughout the small and large intestines. In all cases, the luminal membrane resorption is powered by a NaK ATPase on the basolateral enterocyte membrane. This NaK ATPase actively transports Na⁺ past the basolateral membrane, thus reducing intracellular Na⁺ concentration, which subsequently creates an electrochemical gradient for inward Na⁺ transport on the luminal membrane.

Luminal resorption occurs on a variety of Na-Nutrient symporters, which include monosaccharides, as described in Carbohydrate Digestion and Absorption, as well as amino acids, dipeptides, and tripeptides, as described in Protein Digestion and Absorption. These mechanisms of sodium resorption are constitutively active and are not physiologically regulated.

Na⁺ absorption in the large intestine is very similar to that occurring in the Principal Cells during late distal tubule and collecting duct transport. Briefly, diffusion of Na⁺ through luminal membrane ion channelss is powered by a basolateral NaK ATPase. As in the late distal tubule and collecting duct aldosterone significantly enhances sodium resorption in the large intestine by increasing expression of the basolateral NaK ATPase and luminal Na⁺ ion channels.

Water Absorption

Water is always absorbed in the alimentary tract through passive osmosis via a mostly paracellular route between enterocyte tight junctions. Consequently, water absorption is primarily actuated by active absorption of osmotic electrolytes, especially sodium. In cases where a high concentration of unabsorbable solutes remain in the GI lumen, water cannot be resorbed and thus causes an osmotic diarrhea.

Chloride Resorption

Absorption of Cl^- largely occurs through passive diffusion via a paracellualr route. Substantial resorption of Na⁺ may create a lumen negative charge, thus creating a strong electrochemical gradient for passive resorption of Cl^-. The majority of chloride is resorbed in the small intestine, especially the duodenum and jejunum.

Bicarbonate Resorption

A large amount of bicarbonate is secreted during pancreatic secretion and maintenance of proper acid-base balance requires that some must be resorbed. A CO₂ molecule is converted to H⁺ and HCO₃^- by carbonic anhydrase in the enterocyts. The HCO₃^- is transported past the basolateral membrane while the H⁺ is transported into the intestinal lumen on an Na⁺-H⁺ Antiporter. The H⁺ probably then combines with luminal bicarbonate, and is then converted to CO₂ which then diffuses into the blood and is breathed off by the respiratory system. The net effect is resorption of a bicarbonate ion.

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