The Anatomy, Histology and Function of the Normal Adult Gastrointestinal Tract and the Effects of Coeliac Disease
I. Introduction
This report summarises the anatomy, basic histology, and normal function of the adult gastrointestinal (GI) tract. The report then briefly describes the impact of gluten-sensitive enteropathy or as it is more commonly known, coeliac disease, on the GI tract, looking at the effect it has on the histology and function of the small intestine in particular.
Coeliac disease, an autoimmune disease in genetically susceptible individuals, where the ingestion of gluten (the primary protein in wheat, barley, rye and related grains ) provokes an immune response, has an incidence rate of about 1% in most populations. The avoidance of gluten-containing foods has become a worldwide phenomenon today, with a range of associated gluten disorders including coeliac disease, wheat allergy, irritable bowel syndrome, and non-coeliac gluten sensitivity. The dividing line between these disorders is not always clear, which can make diagnosis and treatment challenging. However, of these conditions, coeliac disease is medically well-established, and diagnosis rates have been increasing. This seems to be due not to increased awareness and detection, but rather to a true rise in incidence itself. Gluten-sensitive enteropathy is therefore a topical research area of increasingly widespread relevance.
The report shall take a brief look at the effect of gluten-sensitive enteropathy on the histology and function of the small intestine, after presenting the functional anatomy and histology of the normal GI tract.
II. Functional Anatomical and Histological Overview of the Gastrointestinal Tract
The function of the GI tract is to break down and absorb food in order to provide the body with the necessary energy required for its metabolism and survival.
The GI tract, also referred to as the digestive tract, is primarily composed of a tube – the alimentary canal – where food digestion and nutrient absorption occur. The alimentary canal begins with the oral cavity, where food enters the body, and terminates at the anus, where waste material is expelled. From proximal to distal, the primary parts of the alimentary canal are: the oral cavity,
pharynx, oesophagus, stomach, small intestine, and the large intestine, which terminates with the anus.
The basic histological structure of the majority of the GI tract (from the oesophagus to the anal canal) exhibits four main layers, listed from deep (adjacent to the lumen) to superficial:
i. Mucosa or mucous membrane – consisting of epithelium, connective tissue, and the muscularis mucosae (a thin layer of smooth muscle)
ii. Submucosa – connective tissue
iii. Muscularis externa – consisting of two thick layers of smooth muscle, the outer longitudinal layer and the inner circular layer
iv. Serosa or adventitia – consisting of two thin outer layers, one of connective tissue and the outermost layer of epithelial tissue.
Superimposed on to the basic structure, each region of the alimentary canal has distinctive histological characteristics that change along its length, reflecting the changes in function across its different parts. This report shall primarily concern itself with the anatomy, however salient superficial histological characteristics will be highlighted, most notably for the small intestine, which sees histological changes associated with coeliac disease.
Accompanying the alimentary canal are its associated glands – the salivary glands, liver, and pancreas. These glands play crucial roles in aiding digestion and absorption of nutrients by secreting enzymes, which break down large, complex food molecules into smaller ones that the body can absorb.
The report now turns to a closer anatomical and superficial histological look at each of the individual parts of the alimentary canal and their functions.
The Oral Cavity
The oral cavity is the beginning of the alimentary canal and is where the tongue and the teeth manipulate food before the act of swallowing, deglutition.
The oral cavity begins anteriorly with the lips, is contained laterally by the cheeks, and ends at the isthmus of the fauces, opening up to the oral part of the pharynx posteriorly. The alveolar arches, gums, and teeth divide the oral cavity into an outer vestibule and an inner mouth cavity, or the mouth cavity proper . The maxilla, mandible, and teeth limit the inner mouth cavity anteriorly and laterally, containing within them the tongue as the most obvious feature of the inner mouth cavity.
The roof of the oral cavity, consisting of hard palate anteriorly and the soft palate posteriorly, separates the oral cavity from the nasal cavity superiorly. The floor of the oral cavity consists primarily of the two mylohyoid muscles spanning the area between the mandible and the hyoid bone, and the geniohyoid and digastric muscles.
The oral cavity is lined by a mucous membrane, stratified squamous non-keratinizing epithelium, which is keratinized in certain parts for added protection. In contrast to the simple columnar epithelium with its regional specializations that characterizes the majority of the length of the GI tract (including the stomach, small, and large intestines, excepting the anal canal), stratified squamous epithelium offers a form of barrier structure for protection. Membrane cells are replaced continuously to counter the abrasion experienced from chewing food. The flattened surface cells overlying the multiple layers of more cuboidal cells underneath are too thick to play an efficient role in transport – absorption or secretion – and is thus the choice of epithelium structure in those areas of the GI tract where protection against abrasion is functionally paramount: the oral cavity, pharynx, oesophagus, and anal canal.
Function
After manipulation of food by the tongue, cheeks, and teeth, a conscious decision is made to swallow, during which the tongue squeezes the chewed bolus of food superiorly and posteriorly against the hard palate into the oropharynx. From then on the process of swallowing becomes involuntary.
The soft palate in the roof of the oral cavity rises during the act of swallowing to obstruct the nasopharynx from the oral part of the pharynx, thereby directing food down the right way. All of the floor muscles – the two mylohyoid, the geniohyoid, and the digastric muscle – act in conjunction to raise the hyoid bone during swallowing, or to open the mouth by depressing the mandible.
Three main pairs of salivary glands – the paratoid, submandibular, and sublingual glands – as well as numerous smaller glands around the inner mouth cavity secrete saliva and assist the act of chewing and deglutition by moistening the food, which then passes into the pharynx. It can be noted here that saliva does contain an enzyme, ptyalin, which begins the breakdown of starch – however its overall contribution to digestion is minimal.
The Pharynx
The pharynx is a muscular tube around 14cm long, extending inferiorly from the base of the skull to the level of the C6 vertebra, where it becomes the oesophagus. The anterior wall of the pharynx has large openings into the nose, mouth, and larynx, dividing the pharynx into naso-, oro-, and laryngo-pharynges. Of these, the oropharynx and laryngopharynx are part of the alimentary canal, and are lined by mucous membrane – stratified squamous epithelium – similar to that in the mouth.
The oropharynx extends inferiorly from the terminal end of the soft palate of the oral cavity to the upper border of the epiglottis, where it becomes continuous with the laryngopharynx . It communicates with the oral cavity anteriorly through the faucial isthmus, the lateral walls of which have two vertical folds of mucous membrane – the palatpharyngeal arch and the palatoglossal arch – with the palatine tonsil lying between them. The laryngopharynx extends inferiorly from the upper border of the epiglottis to the oesophagus. Anteriorly, the larynx presses backwards into the laryngopharynx, causing deep grooves on either side called the piriform fossae.
Apart from the mucous membrane, the walls of the pharynx also have a submucous coat and muscular coats – namely the superior, middle, and inferior constrictor muscles from the top of the pharynx to the bottom. Descending down the pharynx, the constrictor muscles are stacked on top of each other, with the middle constrictor being stacked on the superior, and the inferior constrictor in turn being stacked on the middle constrictor. Each constrictor muscle spreads out from its anterior attachment and passes around the pharynx to attach posteriorly in a midline pharyngeal raphe extending from the occipital bone to the oesophagus. The lowest parts of the inferior constrictor muscle form the circular cricopharyngeal sphincter – the narrowest part of the entire alimentary canal, apart from the appendix – at the junction of the pharynx with the oesophagus.
Function
The pharyngeal muscles effect the act of swallowing, with each muscle contracting in sequence in a descending order to push ingested food and fluids from the oropharynx, down the piriform fossae of the laryngopharynx, and past the cricopharyngeal sphincter, which relaxes in its turn, so the food can enter the oesophagus. As briefly alluded to previously, the contractions of the muscle, peristalses, that effect this descent of food down the alimentary canal during swallowing, are involuntary.
The Oesophagus
The oesophagus is a tubular channel around 25 cm long, extending from the pharynx in the neck at the level of the C6 vertebra, inferiorly through the thorax, and conveying food to the stomach. The narrowest part of the oesophagus is normally the superior-most part at the junction with the pharynx.
As it descends its lower part curves anteriorly, away from the vertebral column, to pierce the diaphragm at the level of the T10 vertebra. The final 3cm, the abdominal oesophagus which lies inferior to the diaphragm, enters the abdomen to the left of the midline and joins the stomach at the cardio-oesophagal junction, the cardiac orifice.
The mucous membrane of the oesophagus is easily distinguishable by stratified squamous epithelium characterized by abundant folds, and a prominent muscularis mucosae which follows the shape of the mucosal folds. The outer muscular wall is composed of skeletal muscle for the first third, and smooth muscle for the lower two-thirds.
Function
Peristalsis continues to drive the food through the oesophagus into the stomach and during swallowing the lower oesophageal sphincter muscle relaxes to allow the passage of food. Infrequent groups of mucous glands in the submucosa secrete mucus in order to lubricate the passage. Reflux of gastric contents from the stomach is prevented by a lower oesophageal sphincter (a circular ring of muscle at the junction with the stomach) as well as by the oesophagus’ oblique angle of entry into the stomach.
The Stomach
The widest part of the alimentary canal, the stomach is where food and fluid are received, mixed, and the process of digestion of protein commenced. It is normally J-shaped, lies underneath the left dome of the diaphragm in the left hypochondrial and epigastric regions of the abdomen, and can easily accommodate 1- 1.5 litres of content in adults. The terminal end of the stomach lies slightly to the right of the midline at the level of the inferior part of the L1 vertebra.
The stomach is divided into different parts: the fundus is the part inferior to the greater curvature of the stomach that lies superior to the level of the cardiac orifice; the body of the stomach, or corpus, is the main part extending from the fundus to the angular notch of the lesser curvature of the stomach; and the pyloric part, or pylorus, extends from the notch to the pyloric sphincter, which separates the stomach from the duodenum of the small intestine. The proximal pyloric part is a dilated pyloric atrium while the distal pyloric part is tubular pyloric canal, ending in a muscular thickening at the gastroduodenal junction called the pyloric sphincter or pylorus, which regulates the passage of contents from the stomach to the duodenum.
Macroscopically, gastric mucosa form rugae, irregular deformable folds, which become flattened by gastric content. In contrast to the more proximal parts of the alimentary canal, gastric mucosa is characterized by columnar surface epithelium of mucus-secreting cells, which invaginate inwards into gastric pits. The pits in turn provide access to gastric glands, both pits and glands varying in their characteristics across different parts of the stomach.
Function
The stomach functions through contractions and relaxations of its muscular walls, peristaltic waves passing down from the cardiac orifice to the pylorus at the frequency of about three a minute. This assists in breaking down and churning food, with particular force in the pylorus, with gastric secretions that start the process of protein digestion. Gastric secretions originate from the epithelial cells of the stomach’s mucous membrane – the primary secretions being hydrochloric acid and pepsinogen, which react to form pepsin. Pepsin initiates the breakdown of protein polymers into smaller molecules, and is further assisted by the acidic environment of the gastric lumen, which helps dissolve food particles and release their polysaccharides and proteins.
The resulting mixture of digestive secretions and food is termed chyme. Food entering the stomach causes the muscles to distend to accommodate the content without increasing muscle tension, a process called receptive relaxation. This stimulates larger peristaltic waves, which not only churns the chyme, but also serves to squeeze 2-3ml of gastric content through the pyloric sphincter into the duodenum. The force of the contractions and the rate of emptying of the stomach are proportional to the volume of the gastric content, both decreasing as the contents decrease. Falling gastric content and the ensuing presence of chyme in the small intestine causes a cessation in gastric secretion . The stomach takes around 3 hours to empty after the average meal.
The Small Intestine
The small intestine consists of three parts, the duodenum, jejenum, and ileum, extending from the pyloric sphincter to the ileocaecal junction, where the large intestine begins. The small intestine furthers the digestion of the acid chyme that the duodenum receives from the pyloric sphincter, and is the primary site of nutrient absorption in the alimentary canal.
In contrast to the pits and glands of the stomach, the mucous membrane in the small intestine is characterized by permanent transverse folds, called plicae circulares, on which numerous finger-like projections called villi extend from the mucosa and submucosa into the lumen. The villi in turn have thousands of microscopic microvilli on their surface. Between the villi, tubular glands called crypts descend to the muscularis mucosae. These characteristics all serve to enlarge the surface area of the small intestine, greatly facilitating its absorptive and secretory capacities required to fulfill its digestive role.
The mucosal epithelium is simple columnar consisting, most abundantly, of absorptive cells called enterocytes, mucus-secreting goblet cells, as well as less frequent enteroendocrine cells, which play local stimulatory and inhibitory functions. All this is shared across most of the length of the small intestine, however the duodenum, jejenum, and ileum additionally also exhibit distinctive histological features specific to their function.
Duodenum
The duodenum, about 25 cm long, is the shortest but widest part of the small intestine, and the most fixed – being retroperitoneal and having no mesentery for the most part. It runs from the pyloric sphincter on the right side to the duodenojejunal juncture on the left side, lying on the posterior abdominal wall in a C-shaped curve (open to the left) around the head of the pancreas.
The duodenum is divided into four parts for descriptive purposes:
i. The first or superior part is 5 cm long and ascends to the right from the pyloric sphincter, being overlapped by the liver and gallbladder. The first 2 cm of the superior part is called the ampulla, and has mesentery and is mobile. The distal 3 cm of the first part, as well as the remaining three parts of the duodenum, have no mesentery and are retroperitoneal, and therefore immobile.
ii. The second or descending part is 8cm long and turns down, curving inferiorly around the head of the pancreas, to the right of the 3rd lumbar vertebra. Its posteromedial wall has an opening shared by the main pancreatic duct and the bile duct, the major duodenal papilla, located about 10 cm from the pyloric sphincter.
iii. The third part, also known as the inferior or horizontal part, is 10 cm long and extends left across the posterior abdominal wall, passing over the inferior vena cava, aorta, and 3rd lumbar vertebra.
iv. The fourth or ascending part turns upwards, running superiorly to the inferior border of the pancreas, where it curves anteriorly and joins the jejenum at the duodenojejunal juncture on the left of the 2nd lumbar vertebra. The duodenojejunal juncture is supported by a suspensory muscle of the duodenum, the ligament of Treitz, which contracts to widen the angle of the juncture in order to assist movement of intestinal contents.
The histologically distinctive characteristic of the duodenum is aggregations of submucosal Brunner’s glands, diminishing distally along its length. These glands secrete approximately 200mL/day of an alkaline mucus-like fluid that neutralizes the acidity of the chyme received from the stomach.
Jejunum and Ileum: Jejuno-ileum
The remaining two parts of the small intestine, the jejunum and the ileum, do not have a clear external demarcation between them – as captured in the combined term jejuno-ileum. The jejunum begins at the duodenojejunal juncture, at which point the intestinal tract becomes intraperitoneal, with most of it occupying the left upper quadrant of the infracolic compartment. The ileum ends at the ileocaecal junction (where the caecum, the first part of the large intestine, begins), with most of it occupying the right lower quadrant of the infracolic compartment. The terminal ileum and the caecum are not direct continuations of each other in the manner the stomach continues into the small intestine via the pyloric sphincter. Rather, the ileocaecal junction joins the large intestine at its side.
Being intraperitoneal, the jejuno-ileum is not pinned against the posterior abdominal wall like the duodenum, but rather lies suspended in coils in the mesentery (a fold of visceral peritoneum) below the stomach and liver, occupying most of the abdominal and pelvic cavities. Together, they are between 6-7m long, with the jejunum measuring two-fifths of that length, and the ileum the remaining three-fifths.
Despite the lack of clear external demarcation, some distinct features listed in Table 1, including colour, pattern of vascular arcades, and pattern of mesenteric fat, help distinguish a given segment of the intestine from the more proximal jejunum and the more distal ileum.
Histologically, the jejenum is distinguished by having the tallest villi projecting from the plicae circulares of the mucosa and submucosa, and correspondingly deeper folds of the epithelium that form crypts. The core of the villus, or lamina propra, is an extension of the mucosal connective tissue above the muscularis mucosa and, with its component cells, plays vascular, neural, and immunological roles. The crypts are the source of the epithelial cells that line the mucosal surface – cells which only survive a week and must therefore be constantly replaced. Brunner’s glands are not present in the jejenum.
The presence of goblet cells, on the other hand, increases distally from the duodenum to the ileum, lubricating the epithelial surface by secreting a mucous gel, which protects against abrasion as well as acts as an antibody binding site for trapped microorganisms. The ileum is additionally characterized by fewer and shorter villi, and its distal end exhibits aggregated lymphoid follicles called Peyer’s patches, which play immune defense functions.
Function: The Small Intestine
The mechanical function of the small intestine involves two different types of movement – segmentation movements, an alternate squeezing and relaxing which mixes intestinal contents, and peristalsis, which advances the contents onwards towards the large intestine. Segmentation movements predominate after the intake of food, to mix the luminal contents and bring them into contact with the mucosa for maximal absorption. Peristalsis predominates after absorption, originating from a peristaltic center in the first part of the duodenum.
Digestion further proceeds in the small intestine, furthering the stomach’s peptic breakdown of protein and initiating carbohydrate and fat digestion, through secretions triggered by the presence of contents in the intestinal lumen:
• The intestinal breakdown of protein is effected by the enzyme trypsin, derived from the trypsinogen of pancreatic juice (secreted into the secondary duodenum via the major duodenal papilla) and enterokinase from the intestinal epithelial cells. Peptide breakdown into amino acids are effected by peptidases, both in the small intestinal lumen before absorption, as well as in the epithelial cell membranes and cell lumen after absorption.
• Amylase from pancreatic juice, as well as numerous disaccharides from the intestinal epithelium, contributes to the digestion of carbohydrates. As noted earlier, this process had already been initiated by amylase in the saliva, but minimally. This reduces starch and other carbohydrates to absorbable sugars – primarily glucose, fructose, and galactose.
• Lipases from the pancreas and the intestinal epithelium, and bile from the liver (also secreted into the secondary duodenum via the major duodenal papilla) serve to digest fats, producing absorbable fatty acids and monoglycerides.
Absorption of the products of digestion occurs across the small intestine, however most notably in the jejunum, with a few exceptions such as bile salts and vitamin B12, which are absorbed exclusively in the terminal ileum. A large amount of water, approximately 9.5 liters, moves through the entire GI tract on a daily basis – including ingested fluids, salivary secretions, bile, and gastric, intestinal, and pancreatic secretions. The majority of this is absorbed by the small intestine, which allows only about 1.5 liters of water to pass on to the large intestine.
The Large Intestine
The large intestine extends from the ileocaecal junction to the anus, measuring about 1.5 m long. From proximal to distal it is divided into the caecum, appendix, colon (further divided into the ascending, transverse, descending and sigmoid colon), rectum, and the anal canal. It receives and accumulates the remaining indigestible liquid chyme from the ileum, from which it absorbs more water, in the process turning it into a semi-solid mass, faeces, which it then excretes from the body.
Caecum
The caecum is the first part of the large intestine inferior to the ileocaecal junction, which, as mentioned previously, joins the large intestine at its side. The caecum is a blind intestinal sac, roughly 7.5 cm in diameter and length, which lies in the right iliac fossa in the right lower abdominal quadrant, and is continuous with the ascending colon. It is almost entirely intraperitoneal, however has no mesentery. The terminal ileum, which joins the caecum obliquely, invaginates into it with the ileal orifice being placed between superior and inferior ileocolic lips. The lips are part of folds that meet to form lateral ridges known as the frenula of the ileal orifice.
The histology of the caecum and appendix are briefly described below with that of the colon.
Appendix
The appendix is another blind diverticulum, 6-10cm long, formed from the posteriomedial wall of the caecum that lies inferior to the ileocaecal orifice. It is the narrowest part of the alimentary canal, normally measuring less then 0.5cm wide. Its base, where it opens up into the caecum, is fixed in position; however its tip is highly mobile and can be found in a variety of positions, most commonly in a retrocaecal position. It is intraperitoneal and connected to the terminal ileum by mesentery, the mesoappendix. It appears not to play any role in the digestive system, however its walls possess much lymphoid tissue, which suggests that it plays some role in the immune system.
Colon
The colon is superior to, and continuous with, the caecum and encircles the small intestine via four parts that together form an arch – the ascending colon (positioned to the right of the small intestine), transverse colon (superior and/or anterior to the small intestine), descending colon (to the left of the small intestine), and sigmoid colon (inferior to the small intestine).
The ascending colon, which passes superiorly on the right side of the abdomen, is about 15 cm long. It is narrower than the caecum and is retroperitoneal, pushed against the posterior abdominal wall, like the duodenum. At the right lobe of the liver, deep to the 9th and 10th ribs, it turns left at the right colic flexure and becomes the transverse colon.
The transverse colon, the longest part of the large intestine, is about 50 cm long and runs across the abdomen from the right colic flexure to the left colic flexure, being suspended by a fold of peritoneum called the transverse mesocolon. Being intraperitoneal, it is also the most mobile part of the large intestine, and its position is variable across body types. At the left colic flexure, which lies anterior to the inferior part of the left kidney, the transverse colon turns inferiorly and becomes the descending colon.
The descending colon, which is the narrowest part of the colon, is about 30 cm long. It lies in the left lateral region , extending from the left colic flexure, which lies more superior than the right colic flexure, to the left iliac fossa, where it becomes the sigmoid colon. Like the ascending colon it is retroperitoneal, in contact with the posterior abdominal wall.
The sigmoid colon, the final part of the colon, is characterized by an S-shaped loop about 40 cm long. It runs from the left iliac fossa to anterior to the 3rd sacral vertrebra, where it joins the rectum at the rectosigmoid junction about 15 cm from the anus, marked by the termination of the taeniae coli. Being intraperitoneal, the sigmoid colon has a mesentery, the sigmoid mesocolon, and possesses considerable freedom of movement.
Histologically, the mucosa in the colon (along with the caecum and appendix) is similar to that of the small intestine, except that it has no villi. Its distinguishing feature is numerous crypts, or colonic glands – straight tubular glands that descend to the muscularis mucosae. There are plenty of absorptive enterocytes and goblet cells. The appendix has two distinguishing features: the crypts are far less abundant, and it has a circular arrangement of lymphoid follicles.
The walls of the colon, including the caecum, are distinguishable from the walls of the small intestine and rectum by having three longitudinal bands of smooth muscle on its outer muscularis externa – the taeniae coli. The taeniae coli, which begin at the base of the appendix, are shorter than the remainder of the colonic wall, causing the wall to be sacculated into haustra in between the taeniae. They run the entire length of the colon, and at the rectosigmoid junction, where the colon ends, the taeniae broaden and merge into a continuous longitudinal layer around the rectum, thereby terminating the alternating contraction-sacculation which characterizes the colonic wall. Further to this, the colonic wall also has little tags of fat, appendices epiploicae, that distinguish it from other parts of the large intestine.
Rectum
The rectum, the pelvic part of the digestive tract, lies in the posterior pelvis and extends from the sigmoid colon proximally to the anal canal distally. It is about 12 cm long, beginning anterior to the 3rd sacral vertebra with the sacral flexure of the rectum curving antero-inferiorly, following the curve of the sacrum and the coccyx, and ending antero-inferior to the tip of the coccyx where the anal canal begins at the anorectal junction. The anorectal junction is characterized by a postero-inferior curve of about 80° called the anorectal flexure, which pierces the pelvic diaphragm.
The rectum begins intraperitoneally, becoming retroperitoneal and then subperitoneal as it descends, with its lower third lying extraperitonially in pelvic fascia. Together with the sacral flexure superiorly and the anorectal flexure inferiorly, the rectum is characterized by an S-shape when viewed laterally. When viewed anteriorly, the rectum is characterized by three lateral flexures of the rectum called transverse rectal folds, two on the left side and one on the right side, which overlie thickened parts of circular muscle in the rectal wall. The terminal part of the rectum, the ampulla, is dilated and lies directly superior to the pelvic diaphragm, which supports it.
The histology of the rectal mucosa is similar to that of the colon with simple columnar epithelium, however there are more goblet cells and the crypts are longer than in the colon.
Anal Canal
The anal canal is the terminal 3-4 cm of the alimentary canal, descending from the anorectal junction and turning posteriorly through the pelvic floor to open at the anus, through which defaecation occurs.
Histologically, at the anorectal junction the epithelium reverts to stratified squamous surface epithelium (last seen in the oesophagus) in which the distinctive foregoing characteristics of intestinal histology – villi, crypts, Brunner’s glands, and Peyer’s patches – are all absent.
A prominent internal feature of the upper part of the anal canal is plexuses of veins causing three cushion-like swellings of the mucous membrane, which remain in contact with one another to keep the canal closed. The upper two-thirds of the wall of the canal are characterized by the involuntary internal anal sphincter, a thickening of the inner circular muscle, encircled by the voluntary external anal sphincter in the lower two-thirds of the canal. The external sphincter has deep, superficial, and subcutaneous parts from proximal to distal, with the deep part surrounding the middle anal canal, the superficial part surrounding the lower anal canal, and the subcutaneous part being a ring of muscle surrounding the terminal end, the anal orifice.
Function: The Large Intestine
The large intestine is the site for further fluid absorption from the liquid chyme received from the small intestine. Of the approximately 1.5 liters of water that pass into it from the small intestine, the colon absorbs all but 100 milliliters, which is passed on with faeces. In order to compensate for the loss of fluid, goblet cells secrete mucus, which lubricates the passage of faecal matter. Additionally, to supplement the bulk of the digestive process which takes places in the small intestine, the large intestine hosts resident bacteria which break down cellulose undigested by intestinal enzymes.
Mechanically, the large intestine operates through segmentation movements and mass contractions, particularly in the descending and sigmoid colons and the rectum, to propel faecal content onwards. These are all involuntary, intrinsic nervous and muscular activity, with the exception of the process of defaecation, which is consciously initiated.
The ampulla in the rectum receives and accumulates faeces, being able to accommodate a certain amount of contents without any increase in pressure. The 80° angle of the anorectal flexure, the ampulla’s ability to relax and accommodate additional arrival of faecal material, as well as the contraction of the external anal sphincter muscles all serve to maintain faecal continence. When defecation is allowed to occur, a combination of processes, including an increase in abdominal pressure via the diaphragm and anterior abdominal muscles, the straightening of the anorectal flexure via relaxation of the puborectalis muscle, relaxation of the anal sphincters, and a mass contraction in the terminal part of the colon and the rectum, together function to expel faecal contents from the body through the anus.
III. Gluten-Sensitive Enteropathy (Coeliac Disease) and its Effects on the GI Tract
The report now turns to a brief look at the effect of coeliac disease on the histology and function of the GI tract.
Although coeliac disease has been reported to affect mucosal sites in the oesophagus, stomach, and large intestine, its diagnosis has long been based on histological findings in biopsies of the proximal small intestine – the duodenum or jejunum. There are a number of histological abnormalities associated with coeliac disease, with the indicators decreasing distally, and in some cases having a patchy distribution –
Mucosal Architectural changes:
i. Villous atrophy
ii. Crypt hyperplasia
iii. Thickening of basement membrane under surface epithelium
iv. Fewer goblet cells
Mucosal Inflammation:
v. Increase in intraepithelial lymphocyte count
vi. Increase in lamina propria cell density
Enterocyte changes
vii. Cuboidal
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