The large Intestine and its mechanical action

The large intestine has little or no digestive function. It absorbs water and electrolytes from the remaining chyme

The large intestine receives food that is undigested or undigestible from  the small intestine, absorbs the water and electrolytes from the chyme and passes it as feces out of the GI tract. The large intestine measures about 5 feet in length and 2.5 inches in diameter.

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.

Mechanical Action of the Large Intestine

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.

The large Intestine and its mechanical action The large Intestine and its mechanical action Reviewed by gafacom on June 03, 2019 Rating: 5

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