Digestive System

The Small Intestine: Introduction and Index

The small intestine is the portal for absorption of virtually all nutrients into blood. Accomplishing this transport entails breaking down large supramolecular aggregates into small molecules that can be transported across the epithelium. An exception to this statement is seen in herbivores, where large amounts of short chain fatty acids are absorbed at other sites.

By the time ingesta reaches the small intestine, foodstuffs have been mechanically broken down and reduced to a liquid by mastication and grinding in the stomach. Once within the small intestine, these macromolecular aggregates are exposed to pancreatic enzymes and bile, which enables digestion to molecules capable or almost capable of being absorbed. The final stages of digestion occur on the surface of the small intestinal epithelium.

The net effect of passage through the small intestine is absorption of most of the water and electrolytes (sodium, chloride, potassium) and essentially all dietary organic molecules (including glucose, amino acids and fatty acids). Through these activities, the small intestine not only provides nutrients to the body, but plays a critical role in water and acid-base balance.

Core concepts in small intestinal physiology are presented as the following topics:

Advanced and supplemental topics related to physiology of the small intestine:

Pathophysiology and diseases affecting the small intestine:

Send comments to Richard.Bowen@colostate.edu