Passive Increases in Luminal Osmotic Pressure Occur During Hydrolytic Digestion and Result in Water Secretion

Food entering the intestine may be hyperosmotic because of its composition, such as salty foods and foods with high sugar content. Alternatively, food may become hyperosmotic after digestion.

Digestion of foods creates many osmotically active molecules from one giant precursor molecule; thus the osmotic activity of ingesta is increased initially by digestion. When starchy meals, for example, first enter the duodenum, intraluminal digestion creates thousands of osmotically active disaccharide and trisaccharide molecules from single starch molecules. These osmotically active saccharide molecules draw water from the lateral spaces into the intestinal lumen. Water in the lateral spaces is quickly replaced by water from the intestinal capillaries, so water is essentially drawn into the intestine from the vascular system. As digestion proceeds, the saccharide molecules are absorbed, thus reducing the number of particles and lowering the osmotic pressure of the intestinal lumen. As solute molecules are absorbed, water follows them osmotically back through the epithelium and into the blood vascular system. The cardinal rule of water movement in the intestine is that water moves in whatever direction necessary to keep ingesta iso-osmotic, entering the gut when ingesta is hyperosmotic and leaving the gut when ingesta is hypo- osmotic. This fact has important clinical implications in the pathophysiology of diarrhea, as discussed later.

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