The Digestive System




The small intstine extends from the stomach to the large intestine. The pyloric sphincter between the stomach and duodenum regulates the emptying of stomach contents into the intestine. At the other end of the small intestine, the ileocolic (ileocecal) valve allows intestinal contents into the cecum of the large intestine.



The length of the small intestine is divided into the three segments, duodenum, jejunum, and ileum. Superficially, the three segments look similar, but there are differences at the tissue level. The duodenum is unique in that it has special mucous glands in the submucosa called brunner's glands. The ileum wall has special lymphatic nodules along one side of the organ called peyer's patches. The jejunum has neither of these features.



Otherwise, all of the small intestinal regions have a similar structure. The outer serosa has a covering of simple squamous epithelium (mesothelium) with connective tissue beneath. The muscularis externa has an outer longitudinal layer and an inner circular layer. The submucosa supports the folds of the mucosal lining.



On the folds of the mucosal lining are numerous villi. Villi are finger-like extensions of the mucosal surface. These villi are covered with a layer of simple columnar cells which are mostly absorptive cells, or enterocytes. Each of these cells will have many microcilli on the exposed surface of the cell to enhance absorption of nutrients from the digested food in the lumen. Mucous cells are also found in the epithelial lining. Mucous cells increase in number along the length of the intestine and form the dominat cell type of the large intestine.

Intestinal glands are located between the bases of the villi, and within each villus is a simple sraight lymphatic vessel (lacteal) and a capillary network.

The length of the small intestine, the circular folds, the villi and the microvilli all serve to increase the absorptive surface area.



Introduction to Human Digestive System

Human Oral Cavity

Structure of a Tooth

Tooth Development

Human Salivary Glands

Divisions of the Pharynx

Human Esophagus

Human Stomach

Human Small Intestine

Human Large Intestine

Human Biliary System and Pancreas

Mouth Development

Formation of Pharyngeal Derivatives

Development of Liver, Gall Bladder, and Pancreas

Development of Intestines

Urorectal Fold and Separation of Cloaca

A Comparative Look at Vertebrate Teeth

A Comparative Look at the Vertebrate Tongue

A Comparative Look at Digestive Tubes