Large Intestine

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The large intestine is the final part of the digestive system. It measures approximately 1.5 metres in length and 6.5 cm in diameter. It is comprised of the Caecum, Appendix, Ascending Colon, Transverse Colon, Descending Colon, Sigmoid Colon, Rectum and Anal Canal.  The overall functions of the large intestine are to complete absorption, maintain fluid levels along with electrolyte balance, allow bacterial degradation of complex carbohydrates, formation of faeces and facilitation of its expulsion.


The gut develops from a primitive endodermal tube and comprises of the fore, mid and hind-gut areas. The caecum, ascending and right 2/3 of the transverse colon are part of the mid-gut whereas the distal 1/3, descending, sigmoid, rectum and anal canal are hind-gut structures.

Gross Anatomy




The developing mid-gut elongates to form a loop that projects into the proximal umbilical cord due to lack of space. The loop has a cranial and caudal end, with both meeting at an apex attaching to the vitelline duct. The caudal limb develops an outpouching that becomes the caecum.


Surface Anatomy


The mid-point of a line drawn from the right anterior superior iliac spine to the superior point of the symphysis pubis approximately marks the middle of the lower border of the caecum.


Macro Anatomy


The caecum is a blind-ending pouch situated in the right iliac fossa. Converging at the base of the caecum on the appendix are Taenia Coli. These are flattened bands of muscle which pass from the caecum to the rectosigmoid region. These taenia are shorter than the rest of the bowel, hence giving the large bowel its characteristic sacculated appearance. The bulk of the caecum lies just inferior to the ileocaecal valve. Anterior to the caecum is the abdominal wall, greater omentum and ileal coils. The iliacus and psoas muscles lie posteriorly with the caecum usually completely covered by peritoneum. The appendix usually arises approximately 2.5 cm below the ileocaecal valve on the posteromedial aspect of the caecum.

Micro Anatomy


The colonic wall is made up of (from the lumen outwards) the mucosa, submucosa, muscular layer and serosa. The muscular layer is made up of the circular muscle and 3 longitudinal muscle bands known as the Taenia coli. Note that the taenia coli are absent in the rectum.


The muscular wall is thick, ideal for powerful peristaltic movements. In contrast to the small intestine there are no villi or permanent circular folds in the mucosal layer. The mucosal layer consists of simple columnar epithelium, lamina propria and muscularis mucosae. The cell types are mostly absorptive and goblet cells. These are consistent with the primary functions of the bowel which are the absorption of water from the faeces, and the lubrication of the faeces before expulsion.

Blood Supply


The ileocolic artery is a branch of the superior mesenteric artery and supplies the caecum and initial ascending colon.







As discussed previously, the caudal limb of the mid-gut loop elongates to form the caecum and the appendix.


Surface anatomy


The appendix usually lies one-third along a line from the anterior superior iliac spine to the umbilicus. This is known as McBurney’s point.




Also known as the Vermiform process (worm-like) the appendix is attached to the posteromedial surface of the caecum. It is thick walled and varied in length from 5-10cm. The appendix has a lumen which narrows with increasing age. The position of the appendix can be variable but it usually lies retrocaecally but can be pelvic or in rare instances anterior or posterior to the terminal ileum. The taenia coli describe earlier converge at the root of the appendix. The appendix has a mesentery extending from the ileal mesentary with the attachment in continuity with the appendiceal serosal layer. This enables a degree of mobility. The mesoappendix contains within it  the appendicular artery.




The histology generally conforms to that of the rest of the large intestine but one characteristic feature is the presence of prominent lymphoid tissue in the mucosal and submucosal layers.



Blood supply


The appendicular artery, a branch of the ileocolic artery, is an end artery, and descends behind the terminal ileum and runs within the mesoappendix to supply the appendix.

Ascending colon



The ascending colon develops from the caudal limb of the mid-gut loop.


Surface anatomy


From the caecum, the ascending colon passes upwards through the right lumbar region. It is lateral to the right lateral line. Between the intersection of the subcostal and right lateral line lies the right colic (hepatic) flexure.




Beginning at the level of the iliocaecal valve the ascending colon runs vertically upwards and takes a left turn under the liver to become the transverse colon. As stated earlier the ascending colon is usually a retroperitoneal structure. The ileal loops, greater omentum and abdominal wall lie anteriorly to it. Posteriorly its relations include the iliacus, transversus abdominis and quadratus lumborum muscles, as well as the perirenal fascia over the lateral aspect of the kidney.




The micro anatomy of the large bowel has been described earlier.  The puckered appearance of the large colon comes from contraction of the longitudinal bands (Taeniae coli) and the resultant formation of a series of pouches is termed "haustra".


Blood supply


The ascending colon is supplied by the ascending and descending branches of the right colic artery (a branch of the superior mesenteric artery).

Transverse Colon



The caudal limb of the mid-gut loop forms the proximal two-thirds of the transverse colon.

The distal third of the transverse colon is formed from the hind-gut.

Surface anatomy


The transverse colon runs within the region of the umbilical and epigastric regions. Its lower border is at a level just above the umbilicus and its upper border just below the greater curvature of the stomach. It is however important to remember that the large intestine is a mobile structure.




The transverse colon extends from the right (hepatic flexure) to the left colic (splenic flexure) and is suspended by the transverse mesocolon. Its middle portion hangs down and across the umbilical region. At the splenic flexure the transverse colon takes an inferior turn just below the splenic hilum to become the descending colon. The superior relations of the transverse colon are the liver, gallbladder, greater curvature of stomach and spleen. Inferiorly lie coils of the small intestine. Anteriorly lies the greater omentum and posteriorly the kidneys, pancreas, second part of the duodenum and small intestine.




As described for the large intestine.


Blood supply


The middle colic artery (a branch of the superior mesenteric artery) divides into right and left branches in the mesentery and supplies the proximal two-thirds of the transverse colon.


The left colic artery (a branch of the inferior mesenteric artery) divides into ascending and descending branches. The ascending branch supplies the distal one-third of the transverse colon including the left colic flexure.

Descending Colon



The hind-gut gives rise to the descending colon.


Surface anatomy


The left colic flexure is situated in the upper left angle of where the left lateral and transpyloric lines intersect. The descending colon runs down the left lumbar region lateral to the left lateral line and around the iliac crest.




As discussed above, the descending colon runs from the left splenic flexure to the iliac crest then turns medially to continue as the sigmoid colon. It is covered anterolaterally by peritoneum.

Posterior to the descending colon lies the left kidney, quadratus lumborum and iliacus. Anteriorly the upper portion lies covered by coils of jejunum whereas the lower part lies adjacent to the abdominal wall where it can sometimes be palpable.




This is as for the rest of the large bowel.


Blood supply


The first branch of the inferior mesenteric artery, the left colic artery runs to the left giving ascending and descending branches. The descending branch supplies the descending colon and the initial segment of the sigmoid colon.

Sigmoid Colon



The developing hindgut gives rise to the sigmoid colon.


Surface anatomy


Slightly lateral to the left lateral line the descending colon turns medially to at a point just beyond the left lateral line as the sigmoid colon.




The sigmoid colon ends just anterior to the third sacral vertebra at the rectosigmoid junction. It has a variable length and it is mobile on its mesentery. During its initial course it crosses the external iliac vessels and left ureter. In males the sigmoid loop rests on the bladder and in females it is related to the uterus and posterior fornix of the vagina.




This is as for the rest of the large intestine.


Blood supply


The sigmoid branches run directly from the inferior mesenteric artery and travel along sigmoid mesentery to supply the sigmoid colon.




The rectum is formed from the hindgut.

Surface anatomy


The commencement of the rectum lies anterior to the third sacral vertebra. It ends 2.5 cm anterior to the coccyx. It is a midline structure.




Positioned in the posterior part of the pelvic cavity, the rectum is approximately 12 cm long. After its course anterior to the coccyx, the rectum turns sharply backwards and downwards piercing the pelvic floor and terminating at the anorectal junction. The upper two-thirds has some peritoneal covering however the lower third is extraperitoneal.


Anteriorly in the male is the rectovesical pouch, base of the bladder and prostate. In the female this is named the rectouterine pouch and posterior wall of the vagina. Posterior to the rectum is the sacrum, coccyx, sacral nerves and middle sacral artery. Laterally below the peritoneal reflections lie the levator ani and coccygeus muscles.




The histology and cell types are as that for the rest of the large intestine. An interesting point is that the rectum is devoid of taeniae coli and appendices epiploicae.


Blood supply


The superior rectal artery, a continuation of the inferior mesenteric artery, supplies the rectum. The internal iliac arteries may also give rise to the middle rectal branches which can supplement the blood supply to the rectum.

Anal Canal and Sphincters


The cloaca is the terminal part of the gut tube. It is related to a cloacal membrane, which is made up of the endoderm of the cloaca and the ectoderm of the proctodeum - the anal pit. Mesenchyme develops into the cloaca establishing itself as the urorectal septum. This creates a division of the cloaca into the bladder anteriorly and rectum posteriorly. The urorectal septum reaches the cloacal membrane and divides it into anal and urogenital membranes.   Therefore the superior two-thirds of the anal canal are formed from the hindgut whereas the inferior third from the surface ectoderm (proctodeum).

Surface anatomy


The anal canal is a midline structure.




The anal canal is the terminal end of the large intestine and is approximately 4cm long. From the anorectal junction it courses postero-inferiorly as far as the anus. The superior part bears several longitudinal ridges known as anal columns. The lower part is lined by skin. The smooth mucle of the anal canal thickens to form the internal anal sphincter. This is met on its outer wall by striated fibres of the levator ani muscle. The lower two-thirds of the anal canal has within it the external anal sphincter made up of 3 parts. The upper part blends with the levator ani, the superficial part is attached to the coccyx with an anterior attachment to the perineal body and lastly a subcutaneous part. The deep part of the external sphincter with its attachment to from levator ani is known as the anorectal ring, which is palpable during digital rectal examination.





A distinctive feature of the anal canal are the longitudinal anal columns or Columns of Morgagni that line its midpoint. Propulsion here aided by circumanal mucus-producing glands prevents clogging. The upper half of the canal is lined by columnar epithelium whereas the lower half is lined by stratified squamous epithelium. At the lower end of the external sphincter the mucosa makes a gradual transition from stratified epithelium to skin consisting of a large number of sebaceous and apocrine sweat glands.


Blood supply

The blood supply to the superior part of the anal canal comes from the superior rectal artery, a branch of the inferior mesenteric artery. This may be supplemented in some cases by the middle rectal artery, a branch off the internal iliac artery.  The majority of the canal is supplied by the inferior rectal artery, a branch of the internal pudendal artery. Branches of the superior rectal artery anastomose with those of the inferior rectal artery. The position of these explains the occurrence of haemorrhoids at the 3, 7 and 11 o clock areas when the patient is in the lithotomy position.


The upper half of the blood supply drains back into the portal venous system whereas the lower half drains into the systemic venous system. This is an important site of portocaval anastomosis.

Nerve Supply


Sympathetic and parasympathetic innervation to the caecum, ascending colon and most of the transverse colon comes from the Superior Mesenteric plexus and in the latter areas also the inferior Mesenteric Plexus.


Parasympathetic innervation in the latter part of the transverse colon comes from the Vagus nerve and Pelvic Splanchnic Nerves.


In the descending colon, sympathetic innervation is from the lumbar splanchnic nerves and inferior mesenteric plexus. Parasympathetic nerve supply arises from the pelvic splanchnic nerves.


The lower part of the anal canal also has somatic innervation from the inferior rectal nerve. The external sphincter is innervated by the inferior rectal branch, pudendal nerve (S2,S3) and perineal branch of S4.




As discussed previously the functions of the large intestine are mass peristalsis, the action of colonic flora, absorption (water, ion, vitamins) and defecation.


Chyme (partially digested liquid stool) passes into the caecum following a gastroileal reflex after a meal. This process initiates colonic movement. A characteristic feature of the large intestine is haustral churning. When haustra distend from accumulation of chyme they are in a relaxed state. At a certain level of distension, the wall contracts and contents are squeezed into the next haustrum. Peristalsis also occurs but a much slower rate than in the small intestine. Food in the stomach initiates the gastrocolic reflex which causes another type of movement - mass peristalsis. This occurs from the midpoint of the transverse colon and forces the contents quickly into the rectum.


Chemical digestion is aided by luminal bacteria and mucus. This produces gases leading to flatus. Proteins are converted to amino acids by the bacteria and decomposes bilirubin to give faeces its characteristic brown colour. B vitamins and vitamin K are absorbed in the large bowel.


The colon is the final place for water reabsorption. Sodium and water and reabsorbed by osmosis.


Defecation occurs when the rectum fills to a certain level where the urge is initiated. Sympathetic, parasympathetic and somatic nervous input controls the process. The reflex arc consists of:

rectal distension - Stretch receptors in the rectum send signals via parasympathetic fibres

Concious awareness - Ascending sensory pathways allow the distinction between solid faecal matter and flatus

Parasympathetic impulse: Increase in colonic tone and relaxation of the internal sphincter

Inconvenient to defecate: The external sphincter contracts via descending somatic impulses.

Convenient to defecate: External sphincter relaxes and allows faeces through the anus.

Clinical Anatomy

Appendicitis is inflammation of the appendix. It is caused by obstruction of the appendicular lumen by chyme, a foreign body or caecal tumours. Typically the pain is central and thereafter localises to the right lower quadrant of the abdomen. There is usually a loss of appetite accompanied by nausea and vomiting. The patient may have a fever and raised white cell count on serology. Progression of the pathology can lead to gangrene and perforation and so early appendicectomy is recommended.


Polyps are growths that can be sessile or pedunculated and benign or malignant. Sessile polyps are more likely to become malignant.


Colorectal cancer has one of the highest incidence rates and mortality after lung cancer. Most cancers develop from adenomas. Its aetiology can be genetic (familial adenomatous polyposis), environmental (low fibre/high fat diet), due to inflammatory bowel disease, polyps and exposure to radiation. Staging systems used are Dukes and TNM 9tumour, noes and metastases). Complications of colorectal cancers include obstruction, perforation, fistula formation and compressive symptoms.


Inflammatory bowel conditions that can affect the large bowel include Crohn’s and Ulcerative colitis. The aetiology of Crohns is unclear. Characteristically it can form skip lesions and a firming of the bowel wall. Ulcerative colitis involves the rectum as well as other parts of the colon. The speculated aetiologies include an abnormal immune response and genetic factors. Varying lengths of colitis may include proctitis and terminal ileitis. There is mucosal reddening and contact bleeding. Both inflammatory conditions can cause bleeding, obstruction, perforation but only Crohn’s causes fistulation.


Diverticular disease is an outpouching of the colonic mucosa through a muscular defect. It is commonly attributed to a low-fibre diet. The pathophysiology is said to be increased luminal pressure and 90% of cases involve the sigmoid colon. It can lead to inflammation, perforation, bleeding and obstruction.


Diarrhoea - This is frequent passage of loose/liquid stool. The pathophysiology can be classified as osmotic, secretory, inflammatory or abnormal motility.


Constipation - This is infrequent of difficult passage of stool which can be slightly harder/firm. The causes are:





Anal pathology - fissures, painful haemorrhoids





Drugs such as opiates

Foreign body

Quick Anatomy

Key Facts

Develops from mid and hindgut structures

Consists of the caecum/appendix, ascending, transverse and descending colon, rectum and anus.

Longitudinal muscle bands line the colon - Taeniae coli. They are absent in the rectum.

Blood supply arises from the superior and inferior mesenteric arteries with some distal supply from branches of the internal iliac artery.


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The main physiological functions are: Haustral churning, peristalsis, bacterial degradation and synthesis of vitamins, absorption of water and ions, and defecation.


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