Large intestine


The large intestine has three main responsibilities. It accepts the food remains that were not digested in the small intestine. It receives excess water and electrolytes from the chime. It also passes solid waste material out of the gastrointestinal tract. The large intestine is more than twice the diameter of the small intestine, measuring about 2 ½ inches across, while measuring about 5 feet in length. It is considered the large intestine based on its diameter, not its length. In the lower right quarter of the abdominal cavity, where the ileum of the small intestine ends, the large intestine begins. It can then be traced along the right side until it reaches just below the liver. At this point, it crosses the abdomen and descends to its terminating point at the anus. The transverse segment of the large intestine is structurally supported by the mesocolon. The mesocolon is actually a specialized segment of the mesentery structure.

The large intestine might add just a fraction of a percentage of digestive function, simply absorbing the excess fluids and electrolytes from the left over chyme. Its basic function revolves around fecal matter, its storage, and its expulsion from the body.


The large intestine can be structurally and functionally segregated into various sections, mainly the cecum, colon, the rectum, and the anal canal. Just under the ileocecal valve there is a noticeably dilated pouch associated with the large intestine. This pouch is known as the cecum. The ileocecal valve is the definite border between the small intestine and the large intestine, and is the primary functional structure which prevents backwash from the large intestine to enter the small intestine. The appendix, if still in the body, can be found along the inferior medial margin of the cecum as a finger like projection. While there has been much medical debate about the importance of the 3 inch appendix, many physicians and medical science professionals are now positive that the appendix is there to help ward off serious infection. This notion is based on the ample amounts of lymphatic tissue that is found within the structure. Others believe, while this may be true, that it is more likely a marker of ancestral history. The appendix may be little more than vestigial remnant of earlier humans.


The colon and the superior section of the cecum are continuous with each other. The colon is subjected to portions that ascend, transverse, ascend, and sigmoid sections. Along the right abdominal wall, running against the inferior surface of the liver, the ascending colon can be traced dropping superiorly from the cecum. When the colon reaches the liver, it takes a quick turn to the left along the hepatic flexure and then becomes the transverse colon as it heads across the upper region of the abdominal cavity. The splenic flexure follows when the colon reaches the right abdominal wall. This is a right angle turn that creates the descending colon, which traces the right abdominal wall down into the pelvic area. When the colon reaches the pelvic area, it once again traverses across the abdominal cavity, this time in an S pattern just along the border to the pelvic region, where it is now markedly known as the sigmoid colon. The remaining 7 ½ inches of the colon is known as the rectum. Here, the anal canal takes up the very last 3 centimeters of the colon.


Large intestine
Image: Large Intestine


The rectum can be located anterior to the sacrum, where the peritoneum keeps it firmly secured in its place. The opening of the anal canal which leads to the exterior of the body is the anus. Two strong sphincter muscles maintain wastes in the anal canal until the human is ready to deposit it appropriately, as these sphincter muscles are both voluntary and involuntary. The internal sphincter muscle is created by smooth muscle tissue. The external sphincter muscle is derived from skeletal muscle.


Anal columns refer to the long folds of significantly vascular mucous membranes that line the anal canal. The large intestine and the small intestine share the same basic tunic structure, with a few different adaptations. The large intestine is devoid of intestinal villi. It additionally contains ample goblet cells which continuously secrete protective mucous in the mucosal layer. Running the length of the large intestine the muscularis layer creates three highly distinctive bands known as taeniae coli. Along the length, as well, are numerous bulges known as haustra or sacculations along the walls. Superficially attached to the teaniae coli are epiploic appendages that are basically pouches that are filled with fat.

The superior and inferior mesenteric plexus sympathetically innervates the large intestine. The celiac plexus also lends a certain amount of innervation. The pelvic splanchic nerves and the vagus nerves parasympethetically innervate the large intestines.

Bowel pressure initiates the urge to defecate and triggers sensory fibers that signal a full colon. The superior and inferior mesenteric branches provide for the blood supply of the large intestine. The superior and inferior mesenteric veins are responsible for the venous drainage. These veins channel into the hepatic portal vein and therefore the venous drainage ultimately enters the liver.


With every rhythmical opening and closing of the ileocecal valve, the large intestine receives about 15 milliliters of the pasty chyme that has escaped digestion. When food enters the body initially, peristaltic action is increased and the rhythmical opening and closing of the ileocecal valve increases. This phenomenon is known as gastroileal reflex. The large intestine allows the material to enter and buildup in the large intestine and the ascending colon.

The large intestine experiences three basic forms of functional motion. These include peristalsis, haustral churning, and mass movement. Peristalsis is nearly identical to the peristalsis that occurs within the small intestine, the only major variation is the speed of which this occurs. Everything tends to occur at a slower rate in the large intestine.


Potential waste material enters the large intestine and ascending colon with a relaxed haustrum. As the build up of material increases, distention begins to occur and this signals contraction of the muscularis. This functional action is known as haustrum churning. This accomplished two agendas. First, the material is moved downward in the system, to the next haustrum. Secondly, excess amounts of water and electrolytes are then absorbed as the material is exposed to increasing encounters with the mucosa. As the excess water and electrolytes are absorbed during this process, feces are formed as a result of the material becoming devoid of the watery substances.

This fecal matter is now formed into a solid waste and needs to be excreted from the body. Large waves of peristaltic action encourage the fecal matter to move closer to the rectum. This is known as mass movement, and it is known to usually occur about two or three times per day after a meal or large snack is introduced to the body. This estimate is based on a healthy adult. Mass movement can often happen even while the meal is still being ingested. This action is referred to as the gastrocolic reflex. Infants have easily observed gastrocolic reflexes, as they are known to defecate either during or just after a feeding.

Na+, K+, and water are continuously absorbed as waste material and fecal matter move through the colon and closer to the rectum. The mucosa of the colon is known to absorb as much as 850 milliliters of water in any given 24 hour period.


As fecal matter enters the rectum, the urge to defecate becomes obvious, and the body prepares for the release of the waste material. However, if the fecal matter id voluntarily denied exit through the anus, the internal anal sphincter maintains its closed status and allows the material to consistently back up into the colon. When necessary, the waste fecal matter will remain in the sigmoid colon. As rectal pressure rises, the urge to defecate becomes stronger. Habitual response to rectal pressure can cause the sphincter to prepare release on a timely scale. When the internal sphincter releases and allows the feces into anal canal, most humans have difficulty denying the urge to defecate.

Defecation occurs when the long muscles of the of the rectum contract and increase rectal pressure. Both sphincter muscles relax and the abdominal and pelvic skeletal muscles contract to assist in the excretion of waste. This intra-abdominal pressure encourages the waste fecal matter to leave the body. The fecal matter traverses the rectum through the anal canal and then leaves the body through the anus.
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