Clinical Practice Keywords Colon/Fluid/Absorption/ Electrolytes/Defecation Systems of life This article has been GI tract double-blind peer reviewed In this article... ● Anatomy and functions of the large intestine ● Processes of fluid absorption and faeces formation ● Transport of faeces in the large intestine Gastrointestinal tract 5: the anatomy and functions of the large intestine Key points Authors Yamni Nigam is professor in biomedical science; John Knight is associate The large intestine professor in biomedical science; Nikki Williams is associate professor in respiratory comprises the physiology; all at the College of Human Health and Sciences, Swansea University. caecum, colon, rectum, anal canal Abstract In the large intestine – the final section of the gastrointestinal tract – and anus absorption of water and electrolytes takes place and colonic bacteria complete the process of chemical digestion. The large intestine is also where faeces are formed Haustral and from the remains of food and fluid combined with by-products of the body. Intestinal antiperistaltic content is pushed back and forth by haustral contractions and antiperistaltic contractions help it contractions, until faeces are finally pushed towards the anal canal by mass absorb water and movements. This article, the fifth in a six-part series exploring the gastrointestinal electrolytes tract, describes the anatomy and functions of the large intestine. The large intestine is Citation Nigam Y et al (2019) Gastrointestinal tract 5: the anatomy and functions of where faeces form the large intestine. Nursing Times; 115: 10, 50-53. from food residues, water and bodily by-products ith the exception of inges- Caecum and appendix tion, the small and large Chyme that has not been absorbed by the Mass movements intestines carry out all the time it leaves the small intestine passes push faeces towards Wmajor functions of the through the ileocaecal valve and enters the the anus and create digestive system. This is where the ‘real large intestine at the caecum. On receipt of an urge to defecate business’ of digestion takes place. The the contents of the ileum, the caecum con- intestines take up most of the space in the tinues the absorption of water and salts. Constipation can abdominal cavity and constitute the The caecum is about 6cm long and indicate serious greatest portion of the gastrointestinal extends downwards into the appendix, a physiological (GI) tract in terms of mass and length. Part winding tubular sac containing lymphoid disturbance or 4 in this six-part series on the GI tract tissue. The appendix is thought to be the disease described the anatomy and function of the vestige of a redundant organ; its narrow small intestine (Bit.ly/NTGITract4). Part 5 and twisted shape makes it an attractive describes the anatomy and functions of site for the accumulation and multiplica- the large intestine, as well as common tion of intestinal bacteria. pathologies that affect both the small and large intestine. Colon At its other end, the caecum seamlessly Anatomy of the large intestine joins up with the colon, this is the longest The large intestine is approximately 1.5m portion of the large intestine (Fig 1). Food long and comprises the caecum, colon, residue starts by travelling upwards rectum, anal canal and anus (Fig 1). The through the ascending colon, located on structure of the large intestine is very sim- the right side of the abdomen. The ilar to that of the small intestine (see ascending colon bends near the liver at the part 4), except that its mucosa is com- right colic flexure (or hepatic flexure) and pletely devoid of villi. becomes the transverse colon, passing Nursing Times October 2019 / Vol 115 Issue 10 50 www.nursingtimes.net across to the left side of the abdomen. Just Fig 1. Anatomy of the large intestine above the spleen at the left colic flexure (or splenic flexure), the transverse colon Transverse colon becomes the descending colon, which Right colic Left colic runs down the left side of the abdomen. flexure flexure Before the next bend, the descending colon transforms into the sigmoid colon. The colon has a segmented appearance; its segments, which are caused by saccula- tion, are called haustra. The ascending Ascending colon, descending colon and rectum are colon located in the retroperitoneum (outside Segment or haustrum Descending the peritoneal cavity). The transverse and colon sigmoid colon are attached to the posterior abdominal wall by the mesocolon. Ileocaecal valve Rectum, anal canal and anus Distally, the large intestine opens into the rectum, which is continued by the anal canal. The rectum forms the final 20cm of the GI Caecum tract. It is continuous with the sigmoid colon Appendix and connects with the anal canal and anus Sigmoid colon (Fig 2, page 52). The rectum ends in an Rectum expanded section called the rectal ampulla, where faeces are stored before being released; the rectum is usually empty since faeces are Anus not normally stored there for long. The anal canal located in the perineum (outside the abdominopelvic cavity), is 3.8- The large intestine also absorbs electro- entire colon, helps to bind dehydrated chyme 5cm long and opens to the exterior of the lytes. Sodium ions are actively absorbed by and also lubricates the passage of faeces. body at the anus (Fig 2). It has two sphincters: the action of the sodium/potassium pump; Transit in the colon is slow: it takes three ● Internal anal sphincter, which is this moves sodium and potassium ions in days to clear 70% of a meal and complete controlled by involuntary muscles; opposite directions across cell membranes, expulsion of all remnants can take up to a ● External anal sphincter, which is made fostering sodium absorption and potassium week; transit is faster in men than women of skeletal muscle and is under loss by releasing the hormone aldosterone. (Degen and Phillips, 1996). Normal bowel voluntary control. Antiperistaltic contractions move food emptying patterns vary greatly between Except during defecation, both anal residues back towards the ileocaecal valve, individuals, from three times per day to sphincters normally remain closed. slowing transit down and giving more three times per week (Walter et al, 2010). time to the large intestine to absorb water Functions of the large intestine and electrolytes. Colonic mass movements Meals pass from the small to the large Peristalsis in the ileum forces chyme into intestine within 8-9 hours of ingestion. The Formation and transport of faeces the caecum. Distention of the caecum trig- small intestine will have absorbed about Of every 500ml of food residue that enters gers the gastric colic reflex and colonic 90% of the ingested water. The large intes- the caecum each day, about 150ml become mass movements begin. Stimulated by tine absorbs most of the remaining water, a faeces. These contain mostly bacteria, old stomach distension and colonic irritation, process that converts liquid chyme residue epithelial cells from the intestinal mass movements usually occur three or into semi-solid stools or faeces. The large mucosa, inorganic waste, undigested food four times a day, often during or immedi- intestine has three major functions: matter and fibre, as well as water to help it ately after meals. These strong waves, ● Absorption of water and electrolytes; pass smoothly through the GI tract. They which can last up to 30 minutes, start ● Formation and transport of faeces; also contain small quantities of fats and midway through the transverse colon. ● Chemical digestion by gut microbes. proteins. Their characteristic brown Helped by haustral contractions, they push colour is due to the presence of stercobilin the now largely dehydrated contents along Absorption of water and electrolytes and urobilin, breakdown products of hae- the colon toward the rectum. Fibre in the The presence of food residues in the colon moglobin from old red blood cells. diet increases the power of colonic contrac- stimulates haustral contractions, which Since chyme residue lingers in the large tions propelling faeces towards the anus. occur approximately every 30 minutes and intestine for 12-24 hours, most of the 1.5L of Colonic mass movements fill the last about one minute each. With each fluid entering the large intestine every day is rectum, creating an urge to defecate. It is contraction, each haustrum distends and absorbed, leaving less than 100ml to pass out important to act on this urge, as once the contracts, pushing the food residues into in the faeces. This small quantity of fluid movements have passed, the urge also the next haustrum. The contractions also gives faeces their semi-solid consistency. ceases. If the urge to defecate is ignored for mix the food residues, thereby facilitating Faeces are also softened by dietary fibre. an extended period of time, the rectum the absorption of water. Mucus, secreted by goblet cells lining the overfills, the large intestine absorbs more LAMB PETER Nursing Times October 2019 / Vol 115 Issue 10 51 www.nursingtimes.net Clinical Practice Systems of life water and faeces become harder and drier. Fig 2. Anatomy of the anal canal This can cause constipation. Physiology of defecation As faeces begin to fill the rectum, the rectal Rectal ampulla wall stretches, which sends an impulse to nervous centres in the spinal cord to ini- tiate the spinal defecation reflex. This results in the relaxation of the internal anal sphincter, which allows a small quantity of faeces to pass into the anus. The anus detects whether the material is gaseous or solid and acts accordingly. If the material is solid, the external anal sphincter opens Anal canal up and defecation takes place. However, the external anal sphincter is controlled by Internal anal sphincter voluntary muscles, so it can be consciously restrained to delay defecation until a more External anal sphincter convenient time.