Anatomy of the Small Intestine
The small intestine is divided into three regions, the duodenum, the jejunum, and the ileum. The duodenum is the shortest region, is retroperitoneal. It starts at the pyloric sphincter of the stomach and extends about ten inches until it merges with the jejunum. The meaning of Duodenum is ‘12’. That part of intestine is is named since it is about the width of twelve fingers. The jejunum is about one inch long and extends to the ileum. Jejunum means ‘empty’, which is how it is found at death. The final and longest region of the small intestine, the ileum, measures about six feet and joins the large intestine at a smooth muscle sphincter known as the ileocecal sphincter.
Histology of the Small Intestine
Small intestine wall is composed of the same four layers that make up most of the gastrointestinal tract: serosa, muscularis, submucosa, and mucosa. The mucosa is composed of a layer of epithelium, lamina propria, and muscularis mucosae. The epithelial layer of the small intestinal musoca consists of simple columnar epithelium that contains many types of cells. Absorptive cells of the epithelium absorb and digest nutrients in small intestinal chime. Also present in the epithelium are goblet cells, which secrete mucus. The small intestinal mucosa contains many deep crevices lined with glandular epithelium. Cells lining the crevices form the intestinal glands and secrete intestinal juice. Besides absorptive cells and goblet cells, the intestinal glands also contain paneth cells and enteroendocrine cells. Paneth cells secrete bactericidal enzyme, lysozyme and are capable of phagocytosis. Paneth cells may have a role in regulating the microbial population in the small intestine. Three types of enteroendocrine cells are found in the intestinal glands of the small intestine: K cells, CCK cells, and S cells, which secrete the hormones secretin, cholecystokinin, and glucose dependent insulinotropic peptide respectively.
The submucosa of the duodenum contains duodenal glands, which secrete alkaline mucus that helps neutralize gastric acid in the chime. Sometimes the lymphatic tissue of the lamina propria extends through the muscularis mucosae into the submucosa. The muscularis of the small intestine consists of two layers of smooth muscle. The outer, thinner layer contains longitudinal fibers; the inner, thicker layer contains circular fibers. Except of a major portion of the duodenum, the serosa completely surrounds the small intestine.
The wall of the small intestine is composed of the same four basic layers as the rest of the gastrointestinal tract; special structural features of the small intestine facilitate the process of absorption and digestion. These structural features include villi, circular folds, and microvilli. Villi are also present in the small intestine, which are fingerlike projections of the mucosa that are one millimeter long. They vastly increase the surface area of the epithelium available for digestion and absorption and give the intestinal mucosa a velvety appearance. Circular folds are folds of the mucosa and submucosa. These permanent ridges, which are about ten millimeters long, begin near the proximal portion of the duodenum and end at about the midportion of the ileum. Circular folds enhances absorption by increasing surface area and causing the chime to spiral, rather than move in a straight line, as it passes through the small intestine.
Besides villi and circular folds, the small intestine also has microvilli, which are projections of the free membrane of the absorptive cells. When viewed through the light microscope, the microvilli are too small to be seen individually; instead they form a fuzzy line, called the brush border, extending into the lumen of the small intestine. The brush border also contains various brush border enzymes that have digestive functions.