Large Bowel Flashcards
1
Q
What internal body parts make up large bowel
A
- Colon
- Caecum- what is this?Blind pouch just distal to ileocecal valve- larger in herbivores
- Appendix- what is this?Thin, finger like extension of caecum- not physiologically relevant in humans
- Rectum- describe it (how is it different to the colon (2 ways))
- Dilated distal portion of alimentary canal (full tube of mouth to anus)
- Histology similar to colon but has transverse rectal folds in submucosa and no taenia coli in its muscularis externa, and also no appendices epiploicae
- Anal canal- describe it
- Terminal portion of large bowel
- Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters
- 1.5m long and 6 cm diameter
2
Q
What sections can colon be divided into
A
- Ascending colon- where is this?
- Right side of abdomen
- Runs from caecum to hepatic flexure (turn of colon by liver)
- Transverse colon- where is this?
- Runs from hepatic flexure to splenic flexure (turn of colon by spleen)
- Hangs off the stomach, attached by a wide tissue band called greater omentum (posterior side, mesocolon)
- Descending colon- where is this?
- Runs from splenic flexure to sigmoid colon
- Sigmoid colon- where is this?
- Runs from descending colon to rectum
3
Q
What are the 2 functions of large bowel
A
-
Reabsorption of electrolytes and water- describe what moves and how
- Happens more in proximal colon
- Na+ and Cl- absorbed by exchange mechanisms and ion channels
- Water follows by osmosis- large intestine can reabsorb 4.5l water (usually 1.5l)- above this diarrhoea occurs
- K+ moves passively into lumen
- Elimination of undigested food and waste
4
Q
Describe the blood supply to the colon
A
- Proximal transverse colon supplied by middle colic artery (branch of superior mesenteric artery)
- Distal third of transverse colon is perfused by inferior mesenteric artery
- This reflects embryological division between the midgut and hindgut
- Region between 2 is sensitive is ischaemia as blood supply is not as extensive (transverse colon is sensitive to ischaemia)
5
Q
Describe the walls of the colon
A
- Peritoneum carries fatty tags (appendices epiploicae) which has unknown function- suggested to protect against intra-abdominal infections
- Muscle coat has 3 thick longitudinal bands (taeniae coli)- necessary for large bowel motility
- Nodules of lymphoid tissue are common as solitary nodules
6
Q
What are haustra, and why do they form in the large bowel?
A
- The taenia coli are shorter in large bowel than in small bowel
- This causes formation of pouched ovoid segments- haustra
7
Q
What are the 4 layers of the walls of the large bowel?
A
- Mucosa
- Submucosa (Meissner’s plexus)
- Muscularis (Auerbach’s plexus/Myenteric plexus)
- Serosa
8
Q
Describe the mucosal structure (which cells are abundant, which cell type isn’t in the large bowel mucosae but is present in the small bowel
A
- Enterocytes and goblet cells are abundant
- Abundant crypts with stem cells
- Mucosa appears smooth in large bowel as it has no villi- enterocytes have microvilli which are primarily concerned with resorption of salts
- Water is absorbed as it passively follows electrolytes, leaving more solid gut contents
- No Paneth cells and enteroendocrine cells are rarer than in small bowel
9
Q
What is the difference between the glycocalyx of the small bowel and the glycocalyx of the large bowel?
A
Glycocalyx (of the large bowel) doesn’t contain digestive enzymes
10
Q
- Describe the function and distribution of goblet cells in the large bowel, + state what stimulates goblet cell secretion
-
A
- Higher no. than in small bowel
- More prevalent in crypts than along the surface, no. increases distally towards rectum
- Apical ends are packed with mucus-filled secretion granules awaiting release- mucus facilitates passage of solid colon contents and covers bacteria + particles
- ACh (parasympathetic and enteric NS) stimulates goblet cell secretion
11
Q
What are the muscles layers of the muscularis externa
A
- Inner circular muscle layer which are segmentally thickened
- Outer longitudinal muscle layer concentrated in 3 bands- taenia coli- between which the longitudinal layer is thin
- Bundles of muscles from teniae coli penetrate the circular layer at irregular intervals
12
Q
What muscles form haustra?
A
- Longitudinal muscle layer as it’s shorter than circular muscle layer
- Haustra can contract individually
- Haustra are seen along colon apart from rectum and anal canal which are substantial and continuous
13
Q
What are the 2 types of contractions that happen in large bowel?
A
- Colonic contractions- describe
- Kneading process
- Minimally propulsive at 5-10cm/hr at most
- Promotes absorption of electrolytes and water
- In proximal colon, antipropulsive patterns dominate to retain chyme
- Localised segmental contractions- describe
- Happen in transverse and descending colon- called Haustral contractions
- Cause back and forth mixing
14
Q
When do these propulsive movements happen
A
- Short propulsive movements every 30 mins
- Increase in frequency following a meal
15
Q
- Describe what mass movements are
-
A
- Happen 1-3 times daily and resemble a peristaltic wave
- Can propel contents 1/3-3/4 of length of large intestine in few seconds
- Food that contains fibre (indigestible material) promotes rapid transport through colon
