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Z YEAR 1 BRS Gastroentology > Large Bowel > Flashcards

Large Bowel Flashcards

1
Q

What does the large bowel consist of?

A
  • colon
  • caecum
  • appendix
  • rectum
  • anal canal.
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2
Q

What is the caecum?

A

a blind pouch just distal to the ileocecal valve- larger in herbivores

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3
Q

What is the appendix?

A

a thin, finger-like extension of the caecum - not physiologically relevant in humans

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4
Q

What are the principal functions of the large bowel?

A
  1. the reabsorption of electrolytes & water

2. elimination of undigested food and waste

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5
Q

What is the size of the large bowel?

A

1.5m long, 6cm diameter

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6
Q

How does the ascending colon run?

A

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

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7
Q

How does the traverse colon run?

A

from the hepatic flexure to the splenic flexure (the turn of the colon by the spleen). Hangs off the stomach, attached by a wide band of tissue called the lesser omentum (posterior side, mesocolon)

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8
Q

How does the descending colon run?

A

from the splenic flexure to the sigmoid colon

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9
Q

How does the sigmoid colon run?

A

Sigmoid colon (s-shaped) colon runs from descending colon to the rectum

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10
Q

What is the proximal transverses colon supplied by?

A

The proximal transverse colon is supplied with blood by the middle colic artery (branch of the superior mesenteric artery

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11
Q

What is the distal third of transverse colon perfused by?

A

Distal third of transverse colon is perfused by the inferior mesenteric artery

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12
Q

What does the colon blood supply reflect?

A
  • Reflects embryological division between the midgut and hindgut.
  • Region between the two is sensitive to ischemia.
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13
Q

What does the peritoneum carry?

A
  • The peritoneum carries fatty tags (appendices epiploicae)

- The muscle coat has 3 thick longitudinal bands (taeniae coli); the gut wall is pouched in appearance (haustra)

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14
Q

What is the purpose of the appendices?

A

Structural or functional purpose of appendices epiploica unknown- suggested to have a protective function against intra-abdominal infections

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15
Q

What is the taenia coli necessary fo?

A

Large intestine motility

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16
Q

Where are nodules of lymphoid tissue?

A

Nodules of lymphoid tissue are common in the walls of the distal small intestine (Peyer’s patches) and large intestine (solitary nodules)

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17
Q

What are Haustra?

A
  • Taenia Coli shorter than small intestine

* Cause the formation of pouched ovoid segments called haustra (singular haustrum).

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18
Q

What is the function of the colon?

A

(Re) absorption
•Colon absorbs electrolytes & water.
•More in proximal colon.
•Na+ and Cl- absorbed by exchange mechanisms and ion channels.
•Water follows by osmosis.
•K+ moves passively into lumen.
•Large intestine can reabsorb approx 4.5 litres water (usually 1.5 litres). Above this threshold diarrhoea.

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19
Q

Describe the rectum

A
  • Dilated distal portion of the alimentary canal.
  • Histology similar to the colon, but distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa.
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20
Q

Describe the anal canal

A

•Terminal portion is anal canal. Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters.

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21
Q

What is the mucosal structure of the large bowel?

A

Like the small intestine
•Enterocytes and goblet cells are abundant.
•Abundant crypts
•Stem cells are found in the crypts.

22
Q

What is the mucosal organisation like?

A
  • Mucosa appears smooth at the gross level because it has no villi (smaller surface area than small bowel).
  • Enterocytes have short, irregular microvilli - primarily concerned with resorption of salts.
  • (Water is absorbed as it passively follows the electrolytes, resulting in more solid gut contents)
  • Crypts dominated by goblet cells.
23
Q

What are the goblet cells like in the large bowel?

A
  • Higher no. of goblet cells than small bowel.
  • More prevalent in the crypts than along the surface, no. increases distally towards rectum.
  • Apical ends are packed with mucus-filled secretion granules awaiting release.
24
Q

Why is mucus needed?

A
  • Mucus - facilitates the passage of the increasingly solid colonic contents and covers bacteria & particulate matter.
  • Acetylcholine (parasympathetic and enteric nervous system) stimulates Goblet Cell secretion.
25
Q

What is the mucosal organisation like in the large bowel?

A

•Villi are absent from large bowel.
•Enterocytes are still the dominant cells facing the gut lumen, but the mucus-secreting goblet cells (stained red) dominate the crypts.
•As in the small bowel, new cells arise from crypt stem cells.
•Crypts dominated by goblet cells.
•No Paneth cells & enteroendocrine cells are rarer than in small bowel
-Glycocalyx does not contain digestive enzymes

26
Q

Describe microvilli

A
  • Microvilli (~0.5-1.5mm high) make up the “brush border”.
  • Several thousand microvilli per cell
  • Surface of microvilli covered with glycocalyx
27
Q

Describe Glycolyax

A
  • rich carbohydrate layer on apical membrane
  • serves as protection from digestional lumen yet allows for absorption.
  • traps a layer of water & mucous known as “unstirred layer”
  • regulates rate of absorption from intestinal lumen
28
Q

What are the muscle layers in the large bowel?

A
  1. Like the small bowel, muscularis externa consists of an inner circular and outer longitudinal layer.
  2. Circular muscles segmentally thickened.
  3. Longitudinal layer concentrated in three bands- taenia coli
  4. Between the taenia, longitudinal layer is thin
  5. Bundles of muscle from the teniae coli penetrate the circular layer at irregular intervals.
29
Q

Describe muscle in the large bowel

A
  • Shorter than circular muscle layer, ovoid segments called haustra- can contract individually.
  • Apart from rectum and anal canal- substantial and continuous.
  • Movement of large bowel more complicated than small intestine
30
Q

What is the motility of the large bowel?

A
  • Colonic contractions- kneading process- minimally propulsive- 5-10cm/hr at most.
  • Promotes absorption of electrolytes and water.
  • Short propulsive movements every 30 mins.
  • Increase in frequency following a meal.
31
Q

What is motility like in the proximal colon?

A

In the proximal colon also ‘antipropulsive’ patterns dominate to retain chyme

32
Q

What is motility like in the transverse and descending colon?

A

In transverse and descending colon, localised segmental contractions of circular muscle called Haustral contractions cause back and forth mixing.

33
Q

What is mass movement like in the large bowel?

A
  • 1-3 times daily- mass movement- resembles 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).
34
Q

What is the parasympathetic control in the large bowel?

A
  • ascending colon and most of transverse colon innervated by vagus nerve
  • more distal innervated by pelvic nerves
35
Q

What is the sympathetic control in the large bowel?

A

lower thoracic and upper lumbar spinal cord (t8-L2)

36
Q

What is the external anal sphincter controlled by?

A

somatic motor fibres in the pudendal nerves (S2, S3 S4)

37
Q

Which neurones detect pressure?

A

Afferent sensory neurons detect pressure

38
Q

How is the enteric nervous system important in the large bowel?

A

Enteric nervous system also important- Hirschsprung’s disease (no enteric intramural ganglia)

39
Q

Where is the myenteric plexus ganglia concentrated?

A

below taenia coli

40
Q

What is the hormone control in the large bowel?

A
  • Presence of food in stomach can stimulate mass movement- hormonal? Neural?
  • Hormonal/paracrine control. e.g. aldosterone promotes sodium and water absorption (synthesis of Na+ ion channel, Na+/K+ pump)
41
Q

What happens during defecation?

A
  1. Rectum filled with faeces by mass movement in the sigmoid colon.
  2. Stores stool until convenient to void.
  3. Defecation reflex controlled primarily by the sacral spinal cord- both reflex and voluntary actions.
  4. Reflex to sudden distension of walls of rectum.
42
Q

What do pressure receptors do in defecation?

A
  • Pressure receptors send signals via myenteric plexus to initiate peristaltic waves in descending, sigmoid colon and rectum. Internal anal sphincter inhibited.
  • Weak intrinsic signal augmented by autonomic reflex.
43
Q

What is under voluntary control?

A
  • External anal sphincter under voluntary control.

* Urge resisted, sensation subsides

44
Q

Describe the rectum

A
  • Last few centimeters of the rectum known as the “social part” of the rectum
  • Can distinguish between solid, liquid and gas.
  • That perceptual ability is important in knowing what can be passed appropriately in what circumstance.
45
Q

How much faeces is produced?

A

•150g/day adult

46
Q

What makes up faeces?

A
  • Two thirds water.
  • Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+).
  • Bile pigments give colour.
  • Bacterial fermentation gives odour.
47
Q

What is flora in the large bowel?

A
  • All mammals have symbiotic relationships with their gut microbial community (microbiome)
  • Stomach and small bowel have few bacteria- protected.
  • Large bowel contains many
  • Essential to normal function.
  • Diverse, highly metabolically active community.
48
Q

How big is the microbiome?

A

•The microbiome in an average adult human comprises approximately 1.5 kg of live bacteria, with the active biomass equivalent to a major human organ.

49
Q

What is the role of intestinal flora?

A
  1. Synthesise and excrete vitamins e.g. Vitamin K- germ-free animals can have clotting problems.
  2. Prevent colonisation by pathogens by competing for attachment sites or for essential nutrients.
  3. Antagonise other bacteria through the production of substances which inhibit or kill non-indigenous species.
  4. Fibre (indigestible carbohydrate) can be broken down by colonic bacteria
50
Q

What can intestinal flora stimulate and produce?

A
  1. Stimulate the production of cross-reactive antibodies. Antibodies produced against components of the normal flora can crossreact with certain related pathogens, and thereby prevent infection or invasion.
  2. Stimulate the development of certain tissues, including caecum and lymphatic tissues
  3. Produces short chain fatty acids which can regulate gut hormone release, or be absorbed to be used as an energy source or to influence functions such as food intake or insulin sensitivity directly
51
Q

What are the types of normal flora?

A
  • Most prevalent bacteria are the Bacteroides- Gram-negative, anaerobic, non-spore forming bacteria. Implicated in the initiation colitis and colon cancer.
  • Bifidobacteria are Gram-positive, non-sporeforming, lactic acid bacteria. Have been described as “friendly” bacteria. Thought to prevent colonization by potential pathogens
52
Q

What are the links between gut bacteria and?

A
  1. Drug metabolism
  2. Insulin resistance
  3. Bile acid metabolism
  4. Lipid metabolism
  5. Obesity

Z YEAR 1 BRS Gastroentology (11 decks)

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