Overview of Intestinal Anatomy
The absorption of ingested food primarily takes place in the small intestine. Other materials that cannot be digested are eliminated as feces through the large intestine after as much water as possible has been recovered. The structures of these two continuous organs make them well-suited for their purposes. Both the small and large intestines have folds that are connected to the abdominal wall through the mesentery. The mesentery is a layer of connective tissue that also holds the blood vessels and nerves that supply the intestine.
The Small Intestine
The small intestine is made up of the duodenum, the jejunum, and the ileum, respectively. Its length varies but is generally between 12 feet and 21 feet. The length often depends on the degree to which it is stretched out. The duodenum receives the acidic contents of the stomach and the digestive enzymes from the pancreas. If problems arise, duodenal ulcers may occur. Continuous with the duodenum is the jejunum and the ileum. There is no landmark to separate the latter two. In general, the first 40% is considered the jejunum and the last 60% the ileum. Unlike the duodenum, which is fixed, the jejunum and ileum are very mobile.
The small intestine is very well adapted for absorption. It is long and intricately folded, and to greatly increase its surface area, it is lined with finger-like projections called villi and microvilli. The folds, villi, and microvilli create an enormous surface area for absorbing materials.
The Large Intestine
The large intestine has four parts: the cecum, the colon, the rectum, and the anal canal. It is further divided into the ascending, transverse, descending, and sigmoid colon. The large intestine is more suited for eliminating undigested material than it is for absorption. The villi seen in the small intestine and specialized for absorption are not seen in the large intestine.
After food and nutrients reach the terminal ileum, there is passage into the large intestine. The point of entry is at the cecum (part of the ascending colon). It continues through the transverse colon to the descending colon, the sigmoid colon, through the rectum and finally the anus, to out of the body as feces.