The small intestine is a tubular structure within the abdominal cavity that carries the food in continuation with the stomach up to the colon from where the large intestine carries it to the rectum and out of the body via the anus. The main function of this organ is to aid in digestion.
How big is the small intestine?
As a person grows the small intestine increases 20 times in length from about 200 cm in a newborn to almost 6 m in an adult. The length of the small intestine is approximated by three times the length of the infant, or height of the child or adult.
The duodenum is about 25 cm (10 inches) long; the jejunum is about 2.5 m (8 feet) long and the ileum is about 3.6 m (12 feet) long.
Anatomy of the small intestine
The small intestine begins with the duodenum. The duodenum begins at the duodenal bulb and goes around the head of the pancreas and ends as it returns to the peritoneal cavity at the ligament of Treitz. The peritoneal cavity is a thin membrane cavity that covers the organs within the abdomen with some exceptions.
The remainder of the small intestine is suspended within the peritoneal cavity by a thin, broad-based mesentery that is attached to the posterior abdominal wall. This allows free movement of the small intestine within the abdominal cavity.
After the duodenum comes the next 40% of the mobile small intestine called the jejunum. The remaining 60% is the ileum.
The jejunum occupies the left upper portion of the abdomen while the ileum is positioned in the right side and upper part of the pelvis.
The inner walls of the small intestine show mucosal folds. These are called the plicae circulares. The plicae are more numerous in the early jejunum and reduce in numbers in the later part and are completely absent in the ileum.
Absorption takes place via primary cell type of the epithelial layer. Goblet cells, located throughout the epithelial layer, secrete mucus that helps protect the epithelial layer from digestion.
Enteroendocrine cells secrete hormones into blood vessels that penetrate each villus. Paneth cells, located in the epithelial layer facing the intestinal crypts, secrete lysozyme, an enzyme that destroys bacteria. An inner core of lamina propria (connective tissues) contains blood capillaries and small lymphatic capillaries called lacteals.
The submucosa under the mucosa contains Brunner's (duodenal) glands, found only in the submucosa of the duodenum. It secretes an alkaline mucus that neutralizes the gastric acid in the incoming chyme.
Aggregates of lymphoid follicles are scattered throughout the small intestine but are found in highest concentration within the ileum, where they are designated Peyer's patches. These are more prominent among children and infants. The small intestine ends at the ileocecal valve that leads it to the colon. The ileocecal valve provides a barrier to the back flow of the colonic contents into the small intestine.