Large intestine, rectum and anal canal

What is the large bowel?

The large bowel is the terminal part of the gastrointestinal tract which deals with any residual undigested food, absorbs water and electrolytes and sustains a variety of bowel flora.

What is the function of bowel flora?

Bowel flora forms over 90% of faeces.

What are the important clinical points of the large bowel?

The large bowel is susceptible to a variety of clinical conditions such as inflammatory bowel disease, diverticular disease, and colorectal cancer.

What is the structure of the large bowel?

The large bowel is divided into the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum.

What is the blood supply, lymphatic drainage, and nerve supply of the large bowel?

The large bowel is supplied by the superior and inferior mesenteric arteries, drained by the superior and inferior mesenteric veins, and innervated by the sympathetic and parasympathetic nervous systems.

What is the microbiome?

The genetic pool of bacteria in the bowel.

What is the microbiome-gut-brain axis?

An area of exciting research.

Is the microbiome unique to each person?

Yes, it seems to be.

What can happen if the bowel flora is upset?

It can be fatal.

What is the structure of the large bowel?

It is situated at the periphery of the abdominal cavity like an inverted “U”.

What is the length of the large bowel compared to the small bowel?

It is shorter, about 1.5 meters in adults compared to 5 meters in the small bowel.

How does the caliber of the large bowel compare to the small bowel?

It is bigger.

Is the large bowel more or less fixed in position compared to the small bowel?

It is more fixed in position.

What is the function of the sphincter in the anal canal?

It controls the expulsion of faeces and maintains continence.

Where does the large bowel end?

It ends in the perineum as the anal canal and the anal opening.

What are the three longitudinal bands of the colon called?

Taeniae coli.

What are appendices epiploicae?

Small fatty projections scattered over the colon's free surface.

What are haustrations?

Sacculations or incomplete septations visible on plain radiographs arising from the colon's wall.

What embryonic structures form the colon?

The distal midgut, all of the hindgut, and the proctodeum.

Which parts of the colon have a mesentery?

Some parts have a mesentery.

Which parts of the colon do not have a mesentery, and how is the peritoneum related to them?

Some parts do not have a mesentery, and the peritoneum covers them.

What is unique about the caecum compared to the rest of the large bowel?

The caecum has the largest diameter and thinnest wall and can rupture in large bowel obstruction.

What allows trained endoscopists to identify which part of the large bowel they are in?

There are features in the lumen that allow trained endoscopists to identify which part of the large bowel they are in with some degree of accuracy.

What is Laplace's law?

According to Laplace's law, the wall tension will be twice as large for a balloon of twice the radius.

What are the primary functions of the large intestine?

The large intestine has 4 primary functions: 1. Reabsorption of water and mineral ions such as sodium and chloride. 2. Formation and temporary storage of faeces. 3. Maintaining a resident population of over 500 species of bacteria. 4. Bacterial fermentation of indigestible materials.

What is the blood supply in the mesentery?

Lifting up a loop of bowel - see the blood supply in the mesentery.

What are the structural adaptations for function in the large bowel?

The large intestine has structural adaptations for function, including a larger diameter, more haustra, and taenia coli.

What can the bowel and particularly the rectum absorb?

The bowel and particularly the rectum can absorb certain medications as well, allowing us the delivery of substances via the rectum.

What are the layers of the wall of the large bowel?

The layers of the wall of the large bowel are the mucosa, submucosa, muscularis propria, and serosa.

What are the features in the lumen that allow trained endoscopists to identify which part of the large bowel they are in?

There are some features in the lumen that allow trained endoscopists to identify which part of the large bowel they are in with some degree of accuracy.

What are the external differences between the small and large intestine?

There are external differences between the small and large intestine.

What are the primary functions of the large intestine?

The large intestine has 4 primary functions: 1. Reabsorption of water and mineral ions such as sodium and chloride. 2. Formation and temporary storage of faeces. 3. Maintaining a resident population of over 500 species of bacteria. 4. Bacterial fermentation of indigestible materials.

What are the structural adaptations for function in the large bowel?

The large intestine has structural adaptations for function, including a larger diameter, more haustra, and taenia coli.

What is the appendix cross-section?

Appendix cross-section.

What are the endoscopic views of the bowel?

Endoscopic views of bowel.

What is the difference between the large and small intestine internally?

Externally, there are differences between the small and large intestine, but internally, there are no significant differences.

What happens to most nutrients and water by the time indigestible materials reach the colon?

Most nutrients and up to 90% of the water have been absorbed by the small intestine.

What are the features of the rectum?

The rectum holds the feces awaiting elimination by defecation. It has a larger size, an alkaline environment, haustrations that allow areas of "stasis" for bacteria to live and grow, mucous glands to keep it moist and lubricate to allow faeces to move down, mucosa that has the ability to absorb water and electrolytes actively, and muscle layers that can move the bowel contents in one direction.

What are the supportive structures of the bowel?

Arterial blood supply and venous drainage, caecum and appendix, and portal system.

What structures will be anastomosed after bowel resection?

The sides of each part of the bowel will be sutured or stapled together.

Is there any obvious problem with anastomosing at the area of the splenic flexure?

Yes, the area of the splenic flexure is the least well supplied by blood, so it should be avoided.

What does resection mean?

Resection means the removal of a part of the bowel.

What are the three types of anastomosis?

Side-to-side anastomosis, end-to-end anastomosis, and end-to-side anastomosis.

What are the three types of anastomosis?

Side-to-side anastomosis, end-to-end anastomosis, end-to-side anastomosis.

What is the lymphatic drainage of the large bowel?

Not provided in the given text.

What is the nerve supply of the large bowel?

The large bowel receives an autonomic nerve supply from the sympathetic nerves and parasympathetic nerves. The parasympathetic innervation is from the Vagus until the distal third of the transverse colon and the rest is supplied by the sacral parasympathetic outflow from the sacral nerves (S2, S3, S4).

What are some clinically important points related to the intrinsic enteric nervous system?

Acute appendicitis, caecal perforation, bowel obstruction.

What are the positions of the appendix?

Not provided in the given text.

What is an open appendicectomy?

Not provided in the given text.

How does the rectum differ from the large bowel?

It is straighter and has no taenia coli.

What are the differences between the rectum and the large bowel?

The rectum is straighter, has no taenia coli, has no haustrations, has no appendices epiploicae, has three half folds of mucosa, and has three curves.

Where does the rectum begin and where does the anal canal begin?

The rectum begins at the level of the 3rd sacral vertebra, and the anal canal begins at the pelvic floor.

What is the length of the normal rectum?

The length of the normal rectum is about 15 cm.

What is the length of the normal anal canal in females and males?

The length of the normal anal canal is about 4 cm in females and about 3 cm in males.

What is the arterial supply to the rectum?

The arterial supply to the rectum is the superior rectal artery.

What is the rectal venous plexus, and what is its significance?

The rectal venous plexus is a site of communication between the portal and systemic venous systems.

What is the drainage pathway for the upper two-thirds of the rectum and the internal part of the rectal venous plexus?

They drain mainly to the superior rectal vein and, from there, to the inferior mesenteric vein.

What is the drainage pathway for the middle third of the rectum?

It drains by one or more middle rectal veins into the internal iliac vein.

What is the drainage pathway for the lower third of the rectum and anal canal?

It drains via inferior rectal veins into the internal pudendal veins.

Where does the anal canal begin and end?

It begins at the end of the rectum at S3 vertebral level and ends at the anal verge.

What is the angle formed between the rectum and anal canal?

The rectum comes forwards and anteriorly, but the anal canal travels backwards and inferiorly, partly due to the puborectalis muscle acting like a sling.

What is the importance of the angle formed between the rectum and anal canal?

It is important for faecal continence.

What separates the middle third of the anal canal from the membranous urethra in males and from the lower vagina in females?

The perineal body.

What surrounds the anal canal laterally and posteriorly?

The ischio-anal fossae.

What is the function of the ischio-anal fossae?

It allows expansion of the anal canal but can also cause infection and abscess formation.

roscopic structure?

The anal canal consists of an inner epithelial lining, a vascular subepithelium, the internal and external anal sphincters, and fibromuscular supporting tissue, as well as dense neuronal networks of autonomic and somatic origin.

What is the function of the anal canal?

Functionally, it represents a zone of high pressure.

How long is the anal canal in adults?

It is between 2 and 5 cm long in adults

What is the shape of the anal canal at rest?

At rest, it forms an oval or tri-radiate slit in the anteroposterior plane rather than a truly circular canal.

What sort of skeletal muscle is the anal sphincter made of?

The sphincter muscles are made up of striated skeletal muscle.

What is the dentate line?

The dentate line is a line which divides the upper two thirds and lower third of the anal canal.

What is the developmental significance of the dentate line?

Developmentally, the dentate line represents the hindgut-proctodeum junction.

What are the anal columns?

In the mid-anal canal, there are 6-10 vertical mucosal folds called the anal columns.

What is the internal hemorrhoidal venous plexus?

Dilated submucosal veins in the upper anal canal form an internal hemorrhoidal venous plexus.

What are the anal cushions?

The submucosal vessels at the 3, 7, and 11 o'clock positions form three "anal cushions" that help to seal the anal canal and contribute to continence.

What are anal cushions?

Anal cushions are three structures that help to seal the anal canal and contribute to the maintenance of continence to flatus and fluid.

What is the role of anal cushions in the pathogenesis of haemorrhoids?

Anal cushions are important in the pathogenesis of haemorrhoids.

What are anal valves?

Anal valves are small crescentic folds formed by the lower ends of the anal columns.

What are anal sinuses?

Anal sinuses are small recesses located between the anal columns and above the anal valves.

What are anal crypts?

Anal crypts are small depressions in the anal valves where anal glands open.

What is the lining of the upper part of the anal canal?

The upper part of the anal canal is lined by reddish columnar epithelium similar to that of the rectum.

What is the epithelium like distally in the anal canal?

Distally, the epithelium becomes cuboidal and darker just above the level of the anal valves.

What is the composition of the subepithelial tissues in the anal canal?

The subepithelial tissues in the anal canal are mobile and relatively distensible, and contain submucosal tissue.

What is the microscopic structure of anal glands?

Anal glands are branched and lined by stratified columnar epithelium.

What is the possible extension of cystic dilations in anal glands?

Cystic dilations in the glands may extend through the internal anal sphincter and even into the external sphincter.

What are internal and external haemorrhoids?

Internal haemorrhoids are located above the dentate line and are covered by mucosa, while external haemorrhoids are located below the dentate line and are covered by skin.

Can you understand the diagram below?

This is not a question, but a prompt for the reader to refer to a diagram.

What is the dentate line?

It is the line that separates the upper part of the anal canal from the lower part.

What is the difference between the epithelium above and below the dentate line?

Above the dentate line, the epithelium is darker and contains somatic sensory nerve endings, while below the dentate line, the epithelium is smooth and parchment-colored.

What is the anal transition zone?

It is the junction between the columnar epithelium above and the squamous epithelium below the dentate line.

What is the subepithelial tissue below the dentate line?

It contains dilated veins that contribute to an external hemorrhoidal venous plexus.

What is the intersphincteric groove?

It is a palpable depression at the lower border of the internal sphincter.

What is the internal anal sphincter?

What is the muscle fiber arrangement of the internal anal sphincter?

What is the external anal sphincter?

How does the innervation of the internal anal sphincter differ from the external anal sphincter?

What is the external anal sphincter?

It is an oval tube of striated muscle composed mostly of type I slow twitch muscle fibres adapted for prolonged contraction.

What is the innervation of the external anal sphincter?

It is innervated bilaterally by the inferior rectal branch of the pudendal nerve (S234).

What is the arterial blood supply of the anal canal?

The anal canal is supplied by terminal branches of the superior rectal artery and the inferior rectal branch of the internal pudendal artery, together with a small contribution from the median sacral artery.

What is the venous drainage of the anal canal?

The venous drainage of the anal canal parallels the arterial supply. The upper canal is drained predominantly by the superior rectal veins, tributaries of the inferior mesenteric vein and the portal venous system.