Large intestine Flashcards
What are the 3 parts of the large intestine?
- Caecum (to which the appendix is attached)
- Colon (ascending, transverse, descending, sigmoid)
- Rectum
Which feature prevents the reflux of chyme from the large intestine into the small intestine during peristalsis?
ileocecal valve
Where is the ileocecal valve located?
In the ileocecal junction in the right iliac fossa. Located at the junction between the caecum and ascending colon.
Where is the caecum located?
In the right iliac fossa between the ileum (distal small intestine) and the ascending colon. It is inferior to the ileocecal junction.
When can the caecum be palpated?
When it is enlarged enough from faeces, inflammation or a malignancy
Which parts of the large intestine are retroperitoneal (behind peritoneum)?
Ascending and descending colon
Which parts of the large intestine are intraperitoneal (completely wrapped in visceral peritoneum)?
Caecum, transverse colon and sigmoid colon
Which part of the large intestine is the least fixed and can therefore dip down into the pelvis in tall, thin individuals?
Transverse colon
Between which flexures does the transverse colon lie?
The right colic / hepatic flexure and the left colic / splenic flexure
How long is the colon?
About 1.5m
Which structures meet at the hepatic / right colic flexure?
Ascending colon turns 90 degrees when it meets the right lobe of the liver to move horizontally as the transverse colon.
Which structures meet at the splenic / left colic flexure?
Transverse colon turns 90 degrees to point inferiorly which becomes the descending colon.
Which section of the abdomen is the spleen located in?
Left hypochondrium
Which 3 structures are used to differentiate between the small and large intestine?
The large intestine contains taenia coli, haustra / haustrations and appendices epiploicae / omental appendices
What is the taenia coli?
3 longitudinal bands of smooth muscle on the outside of the ascending, transverse, descending and sigmoid colon. Taenia coli contract to shorten the large intestine walls, propelling the faecal matter within by peristalsis.
What are haustra / haustrations?
Pouches / sacculations created by semilunar folds on the internal surface of the large intestine. They aid the circumferential contraction of the muscular layer of colon.
What are appendices epiploicae / omental appendices?
Fat filled pouches of the peritoneum attached externally to the walls of the colon (not the rectum). Function is unclear.
Which is the most proximal part of the large intestine?
Caecum
Which is the most distal part of the large intestine?
Rectum
How is the rectum macroscopically distinct from the large intestine?
Absence of taenia coli, haustra and appendices epiploicae / omental appendices
Structure of the rectum and anus
The rectum is continuous with the sigmoid colon and contains rectal valves. The rectum terminates into the anal canal which contains the internal hemorrhoidal plexus, rectal sinus and sphincters. Terminates at the anus.
Function of the rectum
Temporary store of faeces
Function of rectal valves
Support the weight of the faeces and slows the movement into the anal canal
What are rectal valves made of?
Inner circular muscle
Features of the anal canal
Internal hemorrhoidal plexus, rectal sinus, internal and external sphincters
What is the internal hemorrhoidal plexus?
A venus plexus (collection of veins) that surrounds the rectum.
Why are drugs given rectally faster acting than orally?
The drug enters the internal hemorrhoidal plexus which drains into the portal and systemic venus system (towards liver and heart) bypassing the the gut (oesophagus, stomach, small intestine).
Causes of haemorrhoids
Dilation of internal hemorrhoidal plexus
Which sphincter in the anal canal is smooth muscle?
Internal sphincters - involuntary control by ANS
Which sphincter in the anal canal is striated muscle?
External sphincters - voluntary control gained about age 2.
How would you diagnose disorders of the large intestine, rectum and anal canal?
Barium enema - liquid containing barium is injected into the rectum and coats the colon lining. X-rays are taken.
Examples of disorders of the large intestine, rectum and anal canal that can be investigated by barium enema
Colonic tumours, polyps, diverticula (out-pouches), anatomical abnormalities
What forms the ileocaecal valve?
A fold of mucosa
Where is the vermiform (worm-like) appendix found?
Attached to the caecum
Similarities in the basic structure of the small and large intestine
Both have simple columnar epithelium with goblet cells, intestinal crypts of Leiberkuhn, and mucosal glands (mucous secretions)
Differences in the structure of the small and large intestine
The large intestine doesn’t contain villi, has an incomplete layer of longitudinal muscle (taenia coli), has a ring-like arrangement of circular muscle (haustra) and has appendices epiploicae.
Which 2 types of epithelial cells line crypts and intestinal villi?
Goblet cells (secrete mucus) and enterocytes (secrete water and electrolytes - doesn’t stain)
What are enterocytes?
Simple columnar epithelial cells lining the small and large intestine. Secrete water and electrolytes and absorb nutrients.
Where is the anal canal located?
Extraperitoneal between the recto-anal junction and anus
What is the change in epithelium at the recto-anal junction?
In the rectum / large intestine the epithelium is simple columnar which changes to stratified squamous epithelium in the external anal canal.
What are the 3 zones of the anal canal?
Colorectal (columnar) zone - anal columns, anal valves, sinuses, crypts of Morgagni, pectinate line
Intermediate zone - anoderm
Squamous / cutaneous zone - perianal skin
How is the anal column divided?
The superior 2/3 is divided from the inferior 1/3 by the pectinate / dentate line which lies inferior to the crypts of Morgagni.
Blood supply above the pectinate line (upper 2/3 of anal canal
Superior rectal artery
Blood supply below pectinate line (inferior 1/3 anal canal)
Middle and inferior rectal arteries
Venous drainage above pectinate line (superior 2/3 anal canal)
Superior rectal vein followed by hepatic portal circulation
Venous drainage below pectinate line (inferior 1/3 anal canal)
Middle and inferior rectal veins followed by vena cava circulation
Innervation above the pectinate line (superior 1/3 anal canal)
inferior mesenteric plexus, pelvic splanchnic nerves, inferior hypogastric plexus
Innervation below the pectinate line (inferior 1/3 anal canal)
Pudendal nerve
Epithelium in the colorectal zone of the anal canal
Simple columnar epithelium
Epithelium in the transitional zone of the anal canal
Simple columnar and stratified squamous epithelium
Epithelium in the anoderm zone of the anal canal
Stratified squamous non-keratinised epithelium
Epithelium in the cutaneous / squamous zone of the anal canal
stratified squamous keratinised epithelium
Function of anal canal
Fecal continence and defecation
Function of colon
Absorb fluid and electrolytes from chyme, mixing chyme, peristalsis, mass movements
Illness caused by the colon failing to reabsorb water
Diarrhoea
What are intestinal / bacterial flora?
Resident microbial populations in the large intestine
Functions of intestinal flora in the large intestine
Ferment dietary fibre (cellulose) and lipid, produce gases (flatus), synthesis vitamins (vit B complex and vit K)
Which vitamins are synthesised in the large intestine by intestinal flora?
Vitamin B complex and vitamin K
Function of vitamin B complex
Impact energy levels, brain function, cell metabolism, RBC growth, nerve function, preventing infections, cell health
Function of Vitamin K
Important for blood clotting and wound healing. It is fat-soluble.
Two aspects of motility function of large intestine
Mixing/squeezing chyme via circular muscles contraction to break down large faecal material for defaecation. Propulsive movements
Two aspects of propulsive movements of the large intestine
Peristalsis and mass movements
Function of peristalsis in large intestine
propels semi-solid faecal matter towards rectum. Is short range in the transverse and descending colon.
What are mass movements?
Infrequent, more sustained contractions usually after meals (gastro-colic reflex). Moves chyme/faeces from proximal colon to distal colon
What triggers the desire to defaecate?
Rectal distension
What is defaecation?
An involuntary reflex that ejects faecal matter from the rectum via the anal canal.
What is an ileostomy?
The ileum (proximal end of small intestine) is brought out through the abdominal wall. The opening is called a stoma and the faecal matter is collected in a bag.
Name of procedure in which the proximal end of the colon is diverted through an opening in the abdominal wall to an external bag
Colostomy
Why would an ileostomy or colostomy be carried out?
As a temporary or permanent measure after bowel cancer, to relieve inflammation or due to inflammatory bowel disease e.g. ulcerative colitis.
What is diarrhoea?
Excessively watery stools 3 or more times a day
Causes of diarrhoea
Mainly GIT infections. Results in failure of fluid absorption and reduced colonic motility
Results of diarrhoea
Dehydration, electrolyte imbalance (need electrolyte replacement - cause of 1 in 9 children deaths worldwide)
What is constipation?
Not defaecating at least 3 times a week. Hard, dry faeces that is difficult to pass.
Cause of constipation
Increased fluid absorption, GIT spasm, drug side effect e.g. opiates (morphine), lack of dietary fibre.
