The small intestine, consisting of the duodenum, jejunum and ileum, is the site where digestion is completed and nutrients are absorbed. The small intestine is the t portion of the GI tract between the pyloric sphincter of the stomach and the iliocecal valve that opens into the large intestine. It is positioned in the central and lower part of the abdominal cavity and is supported except for the first portion by the mesentary. The fan shaped mesentary permits movement of the small intestine but prevents it from becoming kinked or twisted.
Enclosed within the mesentary are blood vessels, nerves and lymphatic vessels that supply the intestinal walls. The small intestine is about 12 feet long (3 m) in a living person but will measure twice that length in a cadaver due to relaxation of the muscular wall. It is called “small” due to its small diameter relative to the large intestine. The small intestine is the body’s major digestive organ and the main site of nutrient absorption. It contains digestive enzymes which aid in the final breakdown and absorption of food.
1. Duodenum is a fixed C shaped tube measuring 10 inches from the pyloric sphincter of the stomach to the duodenojejunal flexure. It receives bile secretions from the liver and gall bladder and pancreatic secretions from the pancreatic duct.
2. Jejunum extends from the duodenum to the ileum, is approximately 3 feet long. It has a larger lumen and more internal folds than the ileum.
3. Ileum (not to be confused with the ilium of the os coxae) makes up the remaining 67 feet of the small intestine. It empties into the cecum of the large intestine through the ileocecal valve. Lymph nodes called mesentary patches are abundant in the walls of the ileum.
Structural modifications of the Small Intestine Digested food are absorbed across the lining of the intestinal mucosa. Absorption occurs mainly in the jejunum, although some occurs in the duodenum and ileum also. Absorption is aided by structures that increase the surface area of the intestine.
1. Plicae circulares are large macroscopic folds of mucosa 2. Villi are finger-like macroscopic folds of the mucosa that project into the lumen 3. Microvilli are microscopic projections formed by the folding of epithelial cell membranes.
The villi have specialized goblet cells which secrete mucous. Additionally, the center of the villi contain capillaries and lymphatic vessels called lacteals. Proteins and carbohydrates are enter the capillaries and fatty acids enter the lacteals.
Contractions of the longitudinal and circular muscles of the small intestine produce 3 distinct types of movement: rhythmic segementation, pendular movement and peristalsis.
1. Rhythmic segmentations are local contractions of the circular muscle layer. They occur at the rate of 12-16 per minute in regions containing chyme. Rhythmic segmentations churn the chyme with digestive juices and bring it into contact with the mucosa. During these contractions, the vigorous motion of the villi stirs the chyme and facilitates absorption.
2. Pendular movements primarily occur in the longitudinal muscle layer. In this motion, a constrictive wave moves along a segment of the intestine and then reverses and moves in the opposite direction, moving the chyme back and forth. Pendular movements also mix the chyme but do not seen to have a particular frequency.
3. Peristalsis is responsible for the propulsion of chyme through the small intestine. These wave-like contractions are weak and relatively short, occuring at a frequency of about 15-18 per minute. Chyme requires about 3-10 hours to travel the length of the small intestine. Both muscle layers are involved in peristalsis.
The large intestine receives food that is undigested or undigestible from the small intestine
The large intestine begins at the end of the ileum in the lower right hand quadrant of the abdomen. From there it leads superiorly on the right side to a point just below the liver; it then crosses to the left, descends into the pelvis and terminates at the anus. A specialized portion of the mesentary, the mesocolon supports the transverse portion of the large intestine along the posterior abdominal wall.
The large intestine has little or no digestive function. It absorbs water and electrolytes from the remaining chyme. It also functions to form, store and expel feces from the body.The large intestine is divided into the cecum, colon rectum and anal canal. The cecum is a dilated pouch positioned slightly below the ileocecal valve. The ileocecal valve is a fold of mucous membrane at the junction of the small and large intestine that prevents back flow of chyme.
A finger like projection of the cecum called the appendix is attached to the inferior margin of the cecum. It contains an abundance of lymphatic tissue but it serves no discernible function. It is thought to be a vestigial remnant of an organ that was functional in our ancestors. Because it is a blind pouch and waste material can accumulate within, inflammation and infection can occur. If not treated, rupture will lead to further infection of the peritoneal cavity, resulting in peritonitis.
The superior portion of the cecum is continuous with the colon, which consists of the ascending, transverse, descending and sigmoid portions. The ascending portion extends superiorly from the cecum along the right abdominal wall to the inferior surface of the liver. The point where the colon bends here is called the hepatic flexure. From this bend it becomes the transverse colon until it reaches another right angle bend on the left side called the splenic flexure.
From this point it becomes the descending colon as it tranverses inferiorly on the left. At the bottom of the descending colon it angles again in an S shaped bend known as the sigmoid colon. The end of the line, the last 7.5 inches of the tract is the rectum. The final inch (2-3 cm) is the anal canal. The anus is the external opening of the anal canal. Two sphincter muscles are found in this opening: the internal anal sphincter which is smooth muscle and the external anal sphincter which is skeletal muscle.
Three types of movements occur throughout the large intestine: peristalsis, haustral churning and mass movement. In Haustral churning, the relaxed haustrum fills with food residue until a point of distension is reached that stimulates contraction of the muscle.
This movement churns the food residue and exposes it to the mucosa where the water and electrolytes are absorbed. As this happens food residue becomes solid or semisolid and becomes feces.
Mass movement is a strong peristaltic wave which moves the feces towards the rectum. Mass movement occurs only 2-3 times a day, generally after a meal. In infants this response to eating is called the gastrocolic reflex and results in a bowel movement during or shortly after eating.
The defecation reflex normally occurs when rectal pressure rises to a particular level that is determined by individual habit. At this point the internal anal sphincter relaxes to admit feces into the anal canal. During defecation , the longitudinal rectal muscles contract to increase rectal pressure and the internal and external anal sphincters relax. This process is aided by contraction of the abdominal muscles which raise intra-abdominal pressure and help push the feces through the anal canal and out the anus.