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Home , Anal canal, Anus, Descending colon, External anal sphincter, Gastrointestinal tract, Haustrum (anatomy)

Copyright EMAP Publishing 2019 This article is not for distribution except for journal club use

Clinical Practice Keywords Colon/Fluid/Absorption/ / Systems of life This article has been GI tract double-blind peer reviewed In this article... ● and functions of the ● 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 Health and Sciences, Swansea University. caecum, colon, , Abstract In the large intestine – the final section of the – and absorption of and electrolytes takes place and colonic complete the process of chemical . 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 [online]; 115: 10, 50-53. from food residues, water and bodily by-products ith the exception of inges- Caecum and tion, the small and large that has not been absorbed by the Mass movements intestines carry out all the time it leaves the 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 , the caecum con- intestines take up most of the space in the tinues the absorption of water and . can 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 . The appendix is thought to be the disease described the anatomy and function of the vestige of a redundant ; 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. 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 , located on structure of the large intestine is very sim- the right side of the . The ilar to that of the small intestine (see ascending colon bends near the at the part 4), except that its mucosa is com- right colic flexure (or hepatic flexure) and pletely devoid of villi. becomes the , passing

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across to the left side of the abdomen. Just Fig 1. Anatomy of the large intestine above the at the left colic flexure (or splenic flexure), the transverse colon Transverse colon becomes the , 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 . 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 Descending the peritoneal cavity). The transverse and colon sigmoid colon are attached to the posterior 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 (outside the ), 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. ions are actively absorbed by and also lubricates the passage of faeces. body at the anus (Fig 2). It has two : the action of the sodium/potassium pump; Transit in the colon is slow: it takes three l Internal anal , 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 l External anal sphincter, which is made fostering sodium absorption and potassium week; transit is faster in men than women of and is under loss by releasing the 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 in the ileum forces chyme into intestine within 8-9 hours of . 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 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, l Absorption of water and electrolytes; pass smoothly through the GI tract. They which can last up to 30 minutes, start l Formation and transport of faeces; also contain small quantities of fats and midway through the transverse colon. l Chemical digestion by 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

PETER LAMB PETER the absorption of water. , secreted by goblet cells lining the overfills, the large intestine absorbs more

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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 to ini- tiate the spinal defecation reflex. This results in the relaxation of the , 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 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. Children have usually learned this behaviour by the age of two or Anus three years. People with severe dementia may no longer know how to do this. Faeces are normally passed by con- abdominal cramps, and diarrhoea. increase absorption of essential sodium tracting the rectal muscles, helped by a Symptoms range from mild discomfort to and water in the presence of glucose. voluntary procedure called Valsalva’s severe pain. One of the gases produced by the If a person has diarrhoea, it is essential manoeuvre. This involves contracting the bacterial of in the colon to quickly replenish fluids and electrolytes diaphragm and abdominal wall muscles, is , so people who have lactose intol- by administrating a containing which increases intra-abdominal pressure erance exhale hydrogen. The hydrogen the correct balance of glucose and electro- and pushes faeces out of the rectum. breath test can be used to help diagnose the lytes (for example, Dioralyte). Drinks such If the between the external anal condition (Argnani et al, 2008). as lemonade or squash may not contain the sphincter and the defecation centre in the correct balance. medulla are damaged – as may be the case If diarrhoea leads to acute after a stroke, in multiple sclerosis, or after Coeliac disease is an intolerance to gluten, hyponatraemia (serum sodium concentra- spinal injury – the ability to suppress defe- a found in wheat, barley and rye. If tion <135mmol/L), this must be corrected cation may be lost, resulting in faecal people with coeliac disease eat gluten, promptly. Treatment may include the incontinence. Also, with ageing, the intestinal immune cells (T cells) release administration of hypertonic saline, but ability of the anus to detect whether it con- inflammatory mediators that cause a flat- care must be taken to ensure that blood tains gas or faeces may become impaired tening of the intestinal mucosal lining, sodium levels are not allowed to increase and faecal matter may be treated as gas, impairing the ability to digest and absorb too quickly, as this can cause a sudden shift causing faecal incontinence. foods. Symptoms range from mild to of water in brain cells that may lead to the severe and include diarrhoea, abdominal fatal complication central pontine mye- Chemical digestion by gut microbes pain, bloating and , linolysis (Rusoke-Dierich, 2018). The large intestine does not secrete its own and constipation; in severe cases the con- digestive : in this part of the GI dition can lead to malnutrition. Constipation tract, chemical digestion occurs exclu- Constipation is the infrequent and diffi- sively through the action of millions of Diarrhoea cult or painful evacuation of faeces due to colonic bacteria. Through fermentation, Diarrhoea (loose and watery stools) is most slow movement of hard, dry faeces. It may these bacteria break down some of the commonly caused by , noro- lead to and pain remaining , which releases virus or food poisoning but can also be due and, if left untreated, faecal impaction and the hydrogen, and to food intolerances or , irritable GI obstruction. The condition can be due that create flatus (gas). Colonic bacteria bowel syndrome, inflammatory bowel dis- to irregular bowel habits, a diet low in also protect the intestine from potentially ease, coeliac disease and diverticular disease. fibre, and immobility. Certain medica- harmful bacteria coming from the external If the intestines do not absorb fluids, the tions, disorders and the overuse of environment and can synthesise certain body can lose several litres of fluid per day, may also cause – or compound – . Their role will be more fully with consequences such as , constipation. Including 20-60g of fibre/ explored in part 6 of this series. loss of electrolytes (potassium and sodium day in the diet and drinking one or two ions) and increased risk of blood clotting. glasses of fluid with each meal may help to Disorders of the intestines Large losses of potassium ions, for example, prevent constipation. Nurses need to bear can cause cardiac arrest. The only absorp- in mind that constipation can indicate People with lactose intolerance cannot digest tion mechanism that is not disturbed by serious physiological disturbance or dis- dietary lactose. The undigested lactose fer- diarrhoea is glucose/sodium co-transport, ease such as , obstruction due

PETER LAMB PETER ments in the large intestine, producing gas, which means people with diarrhoea can a tumour or paralytic .

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Bowel obstruction Crohn’s disease Colorectal A tumour, adhesions in the intestinal Until recently, Crohn’s disease was thought is the second most walls, foreign bodies or impacted faeces to be an autoimmune disorder in which common cause of cancer death in the UK may cause the intestines to become par- the attacked the body’s (Bit.ly/CRUKMortality) and may be signalled tially or completely blocked and intestinal own gut lining. Today, there is evidence to by constipation or diarrhoea, cramping, contents to back up. This may result in suggest that the immune system overzeal- and rectal bleeding – which abdominal swelling, pain, cramps, vom- ously attacks a microbial on the may be either visible or hidden in the faeces iting and severe constipation or diarrhoea. gut lining (Torres et al, 2017). Most people (occult). , excessive con- Another cause of is par- with Crohn’s disease are diagnosed before sumption and a diet high in fat and alytic ileus, a dramatic slowing of the the age of 30. Intestinal obstruction, a proteins have been linked to an increased normal peristaltic movement of the intes- common complication, arises from risk of colorectal cancer. Some studies sug- tines. Paralytic ileus can be caused by bac- swelling and formation of scar tissue, a gest that increasing dietary fibre intake terial or fungal , mesenteric thickening of the bowel wall and a nar- could reduce the risk (Yang and Yu, 2018). ischaemia, , abdominal sur- rowed intestinal passage (strictures). Per- Since most colon arise from gery and certain . foration of the bowel may occur as a result benign mucosal growths called polyps, pre- of an abscess or . vention focuses on identifying these polyps. Diverticulitis Screening for occult faecal blood is cur- Pea-size pouches called diverticula some- Ulcerative rently offered every two years to people aged times form on weakened spots of the intes- is characterised by 60-74 in most of the UK, although in Scot- tinal walls as a result of increased pressure; and ulceration in the lining land it is offered from age 50 and Public for example, while straining during defe- of colon and rectum, and rectal urgency England last year announced plans to extend cation. They are most common in the sig- that can result in painful, bloody diarrhoea its screening programme in stages to cover moid colon, but both the location and the up to 20 times a day. Symptoms may come 50-74 years of age (PHE, 2018). NT prevalence varies with age, for example in and go but 5-10% patients have constant the US it is reported that the prevalence of symptoms. Perforation is a potential com- ● Part 6 of our six-part series on the was 35% in those younger plication, since chronic inflammation and anatomy and physiology of the GI tract than 50 years, 40% in individuals 50–59 ulceration may weaken the intestine wall will describe the role of gut microbes in years old, and 58% in those aged over 60 to such an extent that a hole may form. digestion, protection against invading years (Peery et al, 2016). People who have This is generally linked with toxic mega- , and synthesis. diverticula but mild or no symptoms are colon, an emergency condition where the said to have the benign condition divertic- colon loses all contractile function and gas References Argnani F et al (2008) Hydrogen breath test for ulosis. Complications can occur in about builds up. Perforation can result in life- the diagnosis of lactose intolerance, is the routine 20% of people with diverticulosis, who will threatening . sugar load the best one? World Journal of develop diverticulitis – an inflammation Gastroenterology; 14: 40, 6204-6207. Degen LP, Phillips SF (1996) Variability of and of the diverticula. This tends syndrome gastrointestinal transit in healthy women and men. to occur when bacteria have built up in Malabsorption syndrome covers a number Gut; 39: 2, 299-305. Peery AF et al (2016) Distribution and diverticula blocked by waste. Diverticular of disorders in which the small intestine is characteristics of colonic diverticula in a United bleeding may occur, as well as chronic unable to absorb enough of certain nutri- States screening population. Clinical injury to the small blood vessels next to the ents (proteins, fats, minerals, vitamins Gastroenterology and ; 14: 980–985. Public Health England (2018) Bowel screening to diverticula and colonic obstruction. and/or carbohydrates) and fluids, resulting start at 50. Bit.ly/PHEBowelScreen in deficiencies, malnutrition and wasting. Rusoke-Dierich O (2018) Damage caused by Inflammatory bowel disease In patients who have had more than 50% of external factors. In: Rusoke-Dierich O. Diving . Cham: Springer. Inflammatory bowel disease (IBD) is the small intestine removed, Torres J et al (2017) Crohn’s disease. Lancet; 389 uncontrolled inflammation and bowel absorption will be severely compromised. (10080): 1741-1755. injury in the large intestine resulting in Walter SA et al (2010) Assessment of normal bowel habits in the general adult population: the severe discomfort, with symptoms such as Appendicitis Popcol study. Scandinavian Journal of abdominal cramps, bloating, gas, liquid If the appendix becomes blocked it Gastroenterology; 45: 5, 556-566. and diarrhoea. Often there is a becomes inflamed, causing appendicitis. Yang J, Yu J (2018) The association of diet, and colorectal cancer: what we eat severe urgency to defecate and there may Obstruction causes a pressure build-up, may imply what we get. Protein Cell; 9: 5, 474-487. be anal/ or bleeding. Severe which may compress the blood supply to

IBD may result in loss of appetite, loss of the gut wall, resulting in ischaemic injury CLINICAL weight and deficiency anaemia. and bacterial infection. The classical SERIES Gastrointestinal tract series The two main types of IBD are Crohn’s dis- symptom is acute pain beginning at the Part 1: The and oesophagus Jun ease and ulcerative colitis. While ulcerative umbilicus and spreading to the right iliac Bit.ly/NTGITract1 colitis often manifests as continuous areas of . , and possibly fever Part 2: The stomach Jul inflammation and can usually be cured by may ensue. If this is not treated, the Bit.ly/NTGITract2 removing the affected areas, Crohn’s disease appendix may rupture, causing dangerous Part 3: The , liver and Aug tends to cause a patchy distribution of peritonitis and allowing bacterial infection Bit.ly/NTGITract3 inflamed ulceration that can affect any part to rapidly spread through the peritoneal Part 4: The and ileum Sep of the GI tract, but most commonly the ter- cavity, potentially leading to death within Bit.ly/NTGITract4 Part 5: The large intestine Oct minal ileum, or the colon, making treatment hours. Appendicitis is one of the com- Part 6: Gut microbes Nov and more difficult. monest causes of acute abdominal pain.

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