The Intestines

The Intestines.

    The absorption of nutrients occurs primarily in the intestine. It's a long transit (8-10 meters) where nutrients pass from the gut lumen into the blood stream. The function of the digestive tract is to stir and push the chyme from the duodenum to the rectum. Throughout, the various segments will participate in absorption of various nutrients.

    The first and main segment of the digestive tract is the small intestine (6-7 meters) which comprise the duodenum, the jejunum and the ileum. We already saw that the duodenum is relatively short and receive the chyme from the stomach. The jejunum has a horseshoe shape around the pancreas and gradually become the ileum which is the major segment of the intestine. Then, the ileum pours into the colon which is the large intestine.

    The colon has four segments: 1) the ascending colon ascending from the lower right of our abdominal cavity, where the appendix is, toward the tip of the liver, 2) the transverse colon which crosses horizontally toward the left side of the body, 3) the descending colon that descends along the left side, and 4 ) the sigmoid colon (S-shaped) ends up at the anus.

    The intestines contract and relax continuously. These movements serve to mix and propel the food. Firstly, small concentric waves of contractions propagate along a network of nerve fibers located between muscle layers (myenteric plexus) and help for mixing food. Secondly, other waves of longitudinal contraction, the peristalsis, propel the chyme toward the anus. The speed of those peristalsis waves can reach 5cm/sec at the beginning of the small intestine down to 0.5cm/sec in the terminal portion. In the colon, these contractions are weak and travel only short distances so that the chyme does not travel more than 1cm/min. All this activity is partly controlled by the autonomic nervous system, the sympathetic or the parasympathetic, whose respective activation slows down or activate digestion.

    The villi extending inside the intestine serve to increase the contact surface between the chyme and the intestinal mucosa. It is at this epithelial surface that the absorption of nutrients occurs.

Cross section of the intestinal wall.