The large bowel (40)

Question 1

What is the main function of the large bowel?

Answer 1

reabsorption of electrolytes and water AND elimination of undigested food and waste

Question 2

What is the structure of the large bowel?

Answer 2

1.5m long, 6cm diameter
ileocaecal valve, caecum (appendix attached), ascending colon, transverse colon, descending colon, sigmoid colon, rectum, anus

Question 3

Where is the ascending colon located?

Answer 3

on the right side of the abdomen, runs from caecum to hepatic flexure (the turning of the colon by the liver)

Question 4

Where is the transverse colon located?

Answer 4

runs from hepatic flexure to the splenic flexure (the turn of the colon by the spleen)
- hangs off stomach, attached by a wide band of tissue, the lesser omentum

Question 5

Where is the descending colon located?

Answer 5

runs from the splenic flexure to the sigmoid colon

Question 6

Where is the sigmoid colon located?

Answer 6

s-shaped, runs from descending colon to the rectum

Question 7

What is the blood supply to the large bowel?

Answer 7

mainly supplied by superior mesenteric artery and inferior mesenteric artery

Question 8

What are the appendices epiploicae?

Answer 8

• fatty tags of visceral peritoneum that hang from teniae coli (thick, longitudinal bands of muscle on colon, necessary for large intestine motility)
• suggested to have protective function against intra-abdominal infections

Question 9

Why are haustra present in the large bowel?

Answer 9

bc teniae coli are shorter than in the small intestine, so pouched ovoid segments are formed

Question 10

What is the rectum?

Answer 10

• dilated distal portion of alimentary canal

- similar histology to colon, but no teniae coli, and has transverse rectal folds in submucosa

Question 11

What is the anal canal?

Answer 11

• the terminal portion of bowel

- surrounded by internal circular muscle and external striated muscle and anal sphincters

Question 12

What are the layers in the large bowel?

Answer 12

• mucosa: epithelium, lamina propria, muscularis mucosae
• submucosa
• muscularis: circular and longitudinal
• serosa: connective tissue and epithelium

Question 13

What cells are present in the mucosa of the large bowel?

Answer 13

• goblet cells for mucus–> sliding for hard faeces to move
• enterocytes
• no paneth cells or enteroendocrine cells
• no villi, but microvilli for water absorption
• stem cells in crypts
• glycocalyx (unstirred layer) for protecting surface of bowel

Question 14

What is the difference in the presence of goblet cells in the large bowel vs small bowel?

Answer 14

• higher # of goblet cells in large bowel
• more prevalent in crypts than along surface
• no. inc. distally towards rectum
• apical ends packed w/ mucus-filled secretion granules –> to facilitate passage of increasingly solid colonic contents and to cover bacteria
  N.B. ACh stimulates goblet cell secretion

Question 15

What is the structure of the muscle layers in the large bowel?

Answer 15

• muscularis externa consists of an inner circular and outer longitudinal layer like the small bowel
• circular muscles segmentally thickened
• longitudinal layers concentrated in 3 bands- teniae coli (between these bands, layer is thin)
• teniae coli penetrate circular layer at irregular intervals

Question 16

What is the motility in the large bowel like during the day?

Answer 16

• segmenting contractions- kneading
• minimally propulsive- 5-10cm/hr at most –> promotes absorption of electrolytes and water
• anti-propulsive patterns in proximal colon to retain chyme
• localised segmental contractions of circular muscle cause mixing in transverse and descending colon
• short, propulsive movements every 30 mins

Question 17

What is mass movement in the large bowel?

Answer 17

• resembles peristaltic wave
• 1-3 times a day
• propels contents up to 3/4 length of large intestine in a few seconds
• fibre (indigestible material) promotes rapid transport through colon

Question 18

What is the nervous supply to the large bowel?

Answer 18

• parasympathetic: vagus nerve innervates ascending colon and most of transverse colon…pelvic nerves innervate more distal parts
• sympathetic (stops motility): lower thoracic and upper lumbar spinal cord
• external anal sphincter controlled by somatic motor fibres in pudendal nerves (voluntarily controlled)
• afferent sensory neurons detect pressure, esp. w/in rectum
• enteric nervous system important (Hirschsprung’s disease)
• myenteric plexus ganglia below teniae coli

Question 19

What is the function of the interstitial cells of cajal?

Answer 19

act as pacemaker of GI smooth muscle

Question 20

What is Hirschsprung’s disease?

Answer 20

• absence of ganglion cells in bowel–> causes the muscles in the bowels to lose their ability to move stool through the intestine (peristalsis)
• presents w/ constipation in infants

Question 21

Where is stool stored until convenient to void?

Answer 21

rectum- filled w/ faeces by mass movement in the sigmoid colon

Question 22

What controls the defecation reflex?

Answer 22

primarily the sacral spinal cord (both reflex and voluntary- external sphincter)

Question 23

How does defecation occur?

Answer 23

• faeces in the rectum expand the rectal walls
• stretch/pressure receptors send signals via myenteric plexus–> initiates peristaltic waves in descending, sigmoid colon and rectum
• internal anal sphincter controlled by parasympathetic fibres which relax involuntarily
• external anal sphincter= skeletal muscle controlled by a somatic nerve supply–> allows conscious control of defecation
• if urge is resisted, the sensation subsides

Question 24

What is the function of the “social part” of the rectum?

Answer 24

• to distinguish between solid, liquid and gas

- to know what can be passed appropriately in what circumstance

Question 25

What gives the odour of faeces?

Answer 25

bacterial fermentation

Question 26

What solids are present in faeces?

Answer 26

- cellulose - bacteria - cell debris - bile pigments - salts

Question 27

What are the roles of intestinal flora?

Answer 27

- synthesise and excrete vitamins e.g. Vit K - prevent colonisation by pathogens by competing for attachment sites or essential nutrients - antagonise other bacteria through the production of substances that inhibit/kill non-indigenous species - stimulate production of cross-reactive antibodies (that will prevent infection/invasion) - stimulate development of certain tissues: caecum and lymphatic tissues - digest fibre (BAD) - produce short chain FAs that regulate gut hormone release - be absorbed and used as an energy source

Question 28

What types of normal flora are usually found in the bowel?

Answer 28

- Bacteroides (gram -ve, anaerobic, non-spore forming bacteria, implicated in colitis and colon cancer) - Bifidobacteria (gram +ve, non-spore forming, lactic acid bacteria, prevents colonisation by potential pathogens)