Structure and Function of the Bowel

Structure and Function of the Bowel

The bowel is divided into the small bowel, the large bowel and the rectum. The small bowel (small intestine), a coiled tube of about 16 feet in length, begins right after the stomach and is subdivided into the duodenum, the jejunum and the ileum. At the end of the ileum, in the right lower abdomen, the small bowel leads into the large bowel (colon) which is about 5 feet long and encloses the small bowel like an inverted “U”. Above this opening, at the cecum and the vermicular appendix, the large bowel begins to ascend (ascending colon) to the right upper abdomen just below the liver, transverses the upper abdomen until just below the spleen (transverse colon) and descends (descending colon) towards the left lower abdomen. The large bowel eventually terminates in an S-shaped curve (colon sigmoideum) to the rectum which is closed by the anal sphincter.

The wall of the colon is made up of four layers of tissue. The sleek outermost layer (serosa) is followed by two muscle layers with an adjacent inner layer of connective tissue (submucosa). In this inner tissue layer there is a complex meshwork of blood vessels, nerves and lymph vessels. The innermost layer (mucosa) forms mucus-secreting villi. Next to other factors, it is the degree of tumor invasion through the wall of the colon which determines the stage of the colorectal cancer. The location of the colon tumor and its blood supply determine the extent of the surgery and the technique used for removal.

Function of the Bowel
After the initial digestion in mouth and stomach, the food gets digested further in the small bowel. The small bowel breaks down the food through enzymes and gives its nutrients to the blood circulation. The colon thickens the waste matter from the food by absorbing water and salt. Strong intestinal movements (peristalsis) form and transport the waste matter, assisted by a lubricant made by the intestinal mucosa. The S-shaped colon (sigmoideum) and the rectum serve as a reservoir for the waste matter until it passes out of the body through the anus.