Epiploic appendages, pieces of fat-filled connective tissue , are attached to the outer surface of the colon. Unlike the small intestine, the large intestine produces no digestive enzymes.
Chemical digestion is completed in the small intestine before the chyme reaches the large intestine. Functions of the large intestine include the absorption of water and electrolytes and the elimination of feces. The rectum continues from the sigmoid colon to the anal canal and has a thick muscular layer. It follows the curvature of the sacrum and is firmly attached to it by connective tissue. The rectum ends about 5 cm below the tip of the coccyx , at the beginning of the anal canal.
The last 2 to 3 cm of the digestive tract is the anal canal, which continues from the rectum and opens to the outside at the anus. The mucosa of the rectum is folded to form longitudinal anal columns. The smooth muscle layer is thick and forms the internal anal sphincter at the superior end of the anal canal.
This article, the fifth in a six-part series exploring the gastrointestinal tract, describes the anatomy and functions of the large intestine. Citation: Nigam Y et al Gastrointestinal tract 5: the anatomy and functions of the large intestine. Nursing Times [online]; 10, Authors: Yamni Nigam is professor in biomedical science; John Knight is associate professor in biomedical science; Nikki Williams is associate professor in respiratory physiology; all at the College of Human Health and Sciences, Swansea University.
With the exception of ingestion, the small and large intestines carry out all the major functions of the digestive system. The intestines take up most of the space in the abdominal cavity and constitute the greatest portion of the gastrointestinal GI tract in terms of mass and length.
Part 4 in this six-part series on the GI tract described the anatomy and function of the small intestine. Part 5 describes the anatomy and functions of the large intestine, as well as common pathologies that affect both the small and large intestine. The large intestine is approximately 1.
The structure of the large intestine is very similar to that of the small intestine see part 4 , except that its mucosa is completely devoid of villi. Chyme that has not been absorbed by the time it leaves the small intestine passes through the ileocaecal valve and enters the large intestine at the caecum. On receipt of the contents of the ileum, the caecum continues the absorption of water and salts. The caecum is about 6cm long and extends downwards into the appendix, a winding tubular sac containing lymphoid tissue.
The appendix is thought to be the vestige of a redundant organ; its narrow and twisted shape makes it an attractive site for the accumulation and multiplication of intestinal bacteria. At its other end, the caecum seamlessly joins up with the colon, this is the longest portion of the large intestine Fig 1.
Food residue starts by travelling upwards through the ascending colon, located on the right side of the abdomen. The ascending colon bends near the liver at the right colic flexure or hepatic flexure and becomes the transverse colon, passing across to the left side of the abdomen.
Just above the spleen at the left colic flexure or splenic flexure , the transverse colon becomes the descending colon, which runs down the left side of the abdomen. Before the next bend, the descending colon transforms into the sigmoid colon. The colon has a segmented appearance; its segments, which are caused by sacculation, are called haustra.
The ascending colon, descending colon and rectum are located in the retroperitoneum outside the peritoneal cavity. The transverse and sigmoid colon are attached to the posterior abdominal wall by the mesocolon.
Distally, the large intestine opens into the rectum, which is continued by the anal canal. The rectum forms the final 20cm of the GI tract. It is continuous with the sigmoid colon and connects with the anal canal and anus Fig 2, page The rectum ends in an expanded section called the rectal ampulla, where faeces are stored before being released; the rectum is usually empty since faeces are not normally stored there for long.
The anal canal located in the perineum outside the abdominopelvic cavity , is 3. It has two sphincters:. Meals pass from the small to the large intestine within hours of ingestion. The large intestine absorbs most of the remaining water, a process that converts liquid chyme residue into semi-solid stools or faeces. The large intestine has three major functions:. The presence of food residues in the colon stimulates haustral contractions, which occur approximately every 30 minutes and last about one minute each.
With each contraction, each haustrum distends and contracts, pushing the food residues into the next haustrum. The contractions also mix the food residues, thereby facilitating the absorption of water. The large intestine also absorbs electrolytes. Antiperistaltic contractions move food residues back towards the ileocaecal valve, slowing transit down and giving more time to the large intestine to absorb water and electrolytes. Of every ml of food residue that enters the caecum each day, about ml become faeces.
These contain mostly bacteria, old epithelial cells from the intestinal mucosa, inorganic waste, undigested food matter and fibre, as well as water to help it pass smoothly through the GI tract. They also contain small quantities of fats and proteins. Their characteristic brown colour is due to the presence of stercobilin and urobilin, breakdown products of haemoglobin from old red blood cells.
Since chyme residue lingers in the large intestine for hours, most of the 1. This small quantity of fluid gives faeces their semi-solid consistency. Faeces are also softened by dietary fibre. Mucus, secreted by goblet cells lining the entire colon, helps to bind dehydrated chyme and also lubricates the passage of faeces.
Normal bowel emptying patterns vary greatly between individuals, from three times per day to three times per week Walter et al, Peristalsis in the ileum forces chyme into the caecum. Distention of the caecum triggers the gastric colic reflex and colonic mass movements begin.
Stimulated by stomach distension and colonic irritation, mass movements usually occur three or four times a day, often during or immediately after meals. These strong waves, which can last up to 30 minutes, start midway through the transverse colon. Helped by haustral contractions, they push the now largely dehydrated contents along the colon toward the rectum. These clefts can be seen from the inside of the colon as the semilunar folds: folds that encircle only part of the colon's circumference.
Taeniae of the colon. The mesocolic taenia runs near the attachment of the mesentery. The free tenia L: taenia libera runs at the opposite side of the attachment of the mesentery, the 'anti-mesenteric' side. The omental taenia runs at the posterolateral side of the colon. The taeniae start at the base of the appendix. Description: The characteristics of the large intestine are discussed and shown.
Anatomical structures in item: Colon. Requirements for usage You are free to use this item if you follow the requirements of the license: View license.
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