8 Large Intestine Flashcards
Q: What does the large intestine consist of? (6) What’s the main part? How long is it? wide? Compared to small intestine?
A: (end of ileum)
- caecum
- ileocecal valve/ junction/ sphincter
- colon *** (a, d, t, s)
- appendix
- rectum
- anal canal
150 cm long and 6 cm wide
(shorter and wider)
Q: What is the caecum? size? where in conjunction to other GI tract parts?
A: blind pouch (larger in herbivores as more important for them- grass heavy diet)
where the small intestine meets the large
immediately distal to the ileocaecal valve
Q: What is the appendix? role? Current opinion? Problems with it result in?
A: thin, finger like extension of the caecum
limited physiological role in humans
Current opinion is that it provides a safe refuge for gut bacteria
Appendicitis is a common problem treated with surgical removal of the appendix; patients undergoing this procedure go on to live perfectly normal lives.
Q: What is the ileocecal valve? role? Normal state? (2) positive?
When does its state change?
A: muscular sphincter
separates the distal ileum (end of SI) from the caecum (the first part of the large intestine)
It is tonically active and constricted
-prevents the microbiota (gut bacteria) from migrating into the ileum
Only relaxes to allow passage of the fluid chyme into the large intestine
Q: What are the main functions of the colon? (4)
A: -re/absorption of water
- re/absorption of electrolytes/ions
- elimination of waste
- microenvironment for gut bacteria
Q: Describe the blood flow to the large intestine. (2) What does this reflect? Problem?
A: cecum, a colon and first 2/3 of t colon
=> receives blood from middle colic artery
final third of the transverse colon, descending colon, sigmoid colon and rectum
=> receives blood from inferior mesenteric artery
embryological division between the midgut and hindgut
region between the 2 is sensitive to ischaemia
Q: How in the innervation of the large intestine distributed? (4) Reflects?
A: superior mesenteric plexus provides sympathetic innervation to the cecum, appendix, ascending and transverse colon (near to the left colic flexure)
inferior mesenteric plexus innervates the colon from the left colic flexure to the rectum
inferior hypogastric plexus also innervates the rectum
possibly- embryological division between the midgut and hindgut
Q: Describe the different parts of the colon in terms of location. (4)
A: ascending colon starts at the ileocaecal valve -> runs up the right-hand side of the body to the hepatic flexure (a flexure is a ‘bend’, and this one is near the liver)
transverse colon starts at the hepatic flexure and runs across the abdomen to the splenic flexure (‘bend’ near the spleen)
descending colon starts at the splenic flexure and runs inferiorly to the first bend of the sigmoid colon (this is a less defined junction than the aforementioned flexures)
sigmoid colon is an S-shaped part of bowel that starts at the descending colon and runs until the rectum
Q: What is the rectum? role? Histology compared to colon? (2)
A: dilated portion of the colon that can act as a storage site for faeces
It has a similar histological structure to the colon, however it has
- transverse rectal folds in the submucosa
- and no taeniae coli in the mucularis externa (muscle layer of gut wall)
Q: What do transverse rectal folds form? function?
A: transverse rectal folds form convenient ‘shelves’ for faeces to occupy until a convenient time to defaecate (so that faeces = not constantly pushing against anal sphincter)
Q: What makes up the anal canal? (2) Control? Function? Nerves?
A: surrounded by two anal sphincters
- internal (circular smooth muscle NOT under conscious control// under central control)
- external (circular muscle under conscious control- gives us control over defaecation)
controls the movement of things out of the GI tract
External anal sphincter is controlled by pudendal nerves
Q: What are 4 unique features of the large intestines?
A: Appendices epiploicae
Longitudinal muscle
Circular muscle
Nodules of lymphoid tissue
Q: What are appendices epiploicae? function?
A: fatty tags that arise from the serosa, and do not seem to have a physiologically meaningful function
-suggested to have protective function against intra-abdominal infections
Q: Describe the longitudinal muscle of the large intestine. (4) Why is it structured this way? What structure do they create?
A: colon has three bands of longitudinal muscle = taeniae coli (doesn’t have continuous layer like rest of GI tract)
roughly equally spaced around the circumference
bands are relatively thicker than typical longitudinal muscle layers
are actually shorter than the length of the colon, which causes the colon to form regular ‘pouches’ called hastra
Large intestine motility is different from small intestine, so need the taeniae coli
Q: Describe the circular muscle of the large intestine. In tandem with longitudinal?
A: segmentally thickened
bundles of muscle from the taeniae coli penetrate the circular muscle at irregular intervals to keep them together
Q: What are haustra? What causes them?
A: regularly occurring ‘pouched’ segments
taenia coli (longitudinal muscle) = shorter than the length of the colon, which causes the colon to gather together and form haustra
Q: Compare lymphoid tissue found in the small intestine and large.
A: nodules of lymphoid tissue are common in the walls of the distal small intestine (peyers patches)
large intestine as solitary nodules
Q: What is reabsorbed in the large intestine? Which part specifically does this? why?
How is the resorption function of the large intestine achieved? (4)
A: electrolytes/ions and water- more in proximal colon where the chyme is more fluid-like (As the contents move along and have water reabsorbed, the contents become dehydrated)
- Sodium and chloride are absorbed by exchange mechanisms and ion channels
- -> drive osmotic gradient
- Water follows by osmosis
- Potassium moves passively into the lumen via gap junctions -> lost in faeces
Q: How much can the large intestine absorb a day? usually? why? When do you get diarrhoea?
A: has the capacity to absorb 4500 mL per day, but usually only reabsorbs 1500 mL
vast majority is absorbed by the small intestine
If the water volume entering the colon exceeds 4500 mL, then diarrhoea results (too much to reabsorb)
Q: Describe the general gut tube plan.
A: middle->
epithelium= 1 cell thick and contains diff specialised cells ->
lamina propia (with epithelium and basement membrane makes mucosa) ->
2 thin layers of muscle= muscularis mucosa= can throw internal gut surface into folds ->
submucosa= has nerve plexus and blood vessels travelling through ->
inner circular muscle layer ->
outer longitudinal muscle layer= gathered into 3 bands that run along length of gut ->
serosa= allows gut to move over itself
Q: Describe the mucosal of the large intestine.
A: (abundant crypts) smooth and does not have many villi, which means it has a considerably small surface area than the small intestine (because the small intestine is where nutrient absorption really takes place) as no glucose or amino acid absorption in the colon
Enterocytes (most abundant cell)
- short irregular microvilli
- reabsorption of salt (i.e. electrolytes instead of nutrients)
- movement of ions creates osmotic gradient to absorb water passively
abundant goblet cells:
-dominate crypts
stem cells:
- found at bottom of invaginations called ‘colonic crypts’
- similar to the small intestine. Cells migrate up the crypts and into the lumen, and are sloughed off after a few days
- no paneth cells: SI needs them to prevent pathogens that have been ingested causing problems-> LI have a bacterial population (commensal) you need for normal gut function
- enteroendocrine cells are rarer than in SI (LI has simpler role)
- glycocalyx does not contain digestive brush border enzymes that the small intestinal glycocalyx does as digestion has been completed by this point
Q: Describe the goblet cells of the large intestine. Where are they? Role? (2) abundance? Stimulation?
A: Colonic crypts are dominated by goblet cells (less along the surface)
- The mucus ‘covers’ bacteria and particulate matter, to protect the luminal surface from infect and/or abrasion -> less likely to escape and cause infections
- secrete mucous to facilitate the passage of increasingly dry luminal contents (as more and more water is reabsorbed)
Because of this, their abundance increases markedly along the length // more in LI than SI per sq cm as food there is liquid and easier to transport
ACh from local nerves (enteric NS and PNS) stimulates goblet cells to secrete // sympathetic activity tends to inhibit function and it causes the f/l/ight response
Q: What is glycocalyx? role? how? name?
A: rich carbohydrate layer on apical membrane
protects digestive lumen but allows absorption
traps layer of water and mucous known as unstirred layer = regulates rate of absorption from intestinal lumen
Q: Describe the muscle layers of the large intestine. (4) How do movements compare to the SI?
A: (Like the small intestine, muscularis externa consists of an inner circular and outer longitudinal layer)
Circular muscles segmentally thickened= typically found between haustra
Longitudinal layer concentrated in three bands- taenia coli
Between the taenia, longinitudinal layer is thin
Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals and because it’s shorter than circular muscle layer, ovoid segments called haustra form which can contract individually
Movements of large intestine more complicated than small intestine
