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Gastroenterology > Large Bowel > Flashcards

Large Bowel Flashcards

1
Q

Large bowel - Anatomy and function

What does the large bowel consist of?

What are the principle functions of the large bowel?

How long is the large bowel? How wide in diameter?

A

Large bowel consists of: colon, caecum, appendix, rectum and anal canal

Caecum - blind pouch distal to ileocecal valve

Appendix - extension of caecum, finger like projection

Principal functions of the large bowel:

  • reabsorption of electrolytes & water
  • elimination of undigested food & waste

1.5 m long. 6 cm diameter

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2
Q

Detail where each part of the colon runs to and from:

  • Ascending colon
  • Transverse colon
  • Descending colon
  • sigmoid colon
A

Ascending colon: right side of abdomen. Caecum —> hepatic flexure (turn of colon by the liver)

Transverse colon: hepatic flexure -> splenic flexure (turn of colon by spleen). Hangs off stomach attached by greater omentum (wide band of tissue)

Descending colon: splenic flexure -> sigmoid colon

Sigmoid colon (S shaped) - descending colon -> rectum

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3
Q

Colon - blood supply

What supplies the proximal transverse colon with blood?

What supplies the distal third of the transverse colon with blood?

What is a religion sensitive to ischemia?

A

Proximal transverse colon - supplied by middle colic artery (branch of superior mesenteric artery)

Distal third of transverse colon - perfused by inferior mesenteric artery

Region of embryological division between midgut & hind gut - sensitive to ischemia

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4
Q

Colon:

What is the functional purpose of of the appendices epiploica?

What is the function of taenia coli?

Where are nodules of lymphoid tissue commonly found?

A

Appendices epiploica (fatty tag of peritoneum) function unknown by might have protective function against intra abdominal infections

Taenia coli (longitudinal bands on muscle coat of peritoneum) - needed for large intestine motility

Nodules of lymphoid tissue common in walls of distal small intestine & large intestine

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5
Q

Colon - function

The colon re absorbs what?

Where does the most absorption occur?

How does it absorb each of the above^?

Above what threshold of water absorbed does diarrhoea occur?

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 absorb 4.5 L water (usually 1.5 L) above this threshold -> diarrhoea
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6
Q

Rectum:

What portion of the alimentary canal is the rectum?

What distinguishes the histology of the rectum from the colon?

A

Dilated distal portion of alimentary canal

Histology similar to colon. But distinguished by transverse rectal folds in its submucosa + absence of taenia coli in its muscularis externa.

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7
Q

Anal canal:

What portion of the rectum is the anal canal?

What type of anal sphincters in the anal canal surrounded by?

A

Terminal portion of rectum is the anal canal

Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters

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8
Q

Large bowel - mucosal structure

What types of cells are found in the large bowel?

A

Similar to small intestine

  • enterocytes + goblet cells abundant
  • Abundant crypts
  • stem cells found in the crypts
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9
Q

Large bowel - mucosal organisation

How does mucosa appear at the gross level and why?

What type of villi do enterocytes have? Why?

What type of cells dominate crypts?

A

Mucosa appears smooth at gross level -> bc no villi (hence smaller SA than small bowel)

Enterocytes have short, irregular microvilli - mainly concerned w resorption of salts

(Water is absorbed as it passively follows the electrolytes, leading to more solid gut contents)

Crypts dominated by goblet cells

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10
Q

Large bowel - Goblet cells

Amount of goblet cells in the large bowel compared to the small bowel?

Where are goblet cells the most prevalent?

What are apical ends packed with?

What is the role of mucus?

Acetylcholine stimulates what?

A

Higher number of goblet cells than small bowel

More prevalent in crypts than along surface, no. Increases distally towards rectum

Apical ends packed w mucus filled secretion granules

Mucus - facilitates the passage of the increasingly solid colonic contents & covers bacteria & particulate matter

Acetylcholine (parasympathetic & enteric nervous system) - stimulates goblet cell secretion

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11
Q

Large bowel - mucosal organisation

What is absent form the large bowel?

Where are enterocytes dominant and where are mucus secreting goblet cells dominant?

Where do new cells arise from?

What type of cells aren’t found in large bowel crypts?

Compare the prevalence of enteroendocrine cells in the small bowel vs in the large bowel?

What does the glycocalyx not contain?

A

Villi are absent from the large bowel

Enterocytes - dominant cells facing gut lumen
Mucus secreting goblet cells (stained red) - dominate the crypts

New cells arise from crypt stem cells

No paneth cells in crypts
Enteroendocrine cells rarer than in small bowel

Glycocalyx doesn’t contain digestive enzymes

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12
Q

Large bowel - muscle layers

What does the muscularis externa consist of?

How are each of these layers organised?

What are the taenia coli?

A

Muscularis externa consists of: inner circular & outer longitudinal layer (like small bowel)

Circular muscles segmentally thickened

Longitudinal layer concentrated in three bands - taenia coli

Between the taenia, longitudinal layer is thin

Bundles of muscle from the teniae coli penetrate the circular layer at regular intervals

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13
Q

Large bowel - muscle layers:

What are the haustra? What can they do?

Apart from where are the haustra continuous and substantial?

A

Haustra: shorter than circular muscle layer. Ovoid segments that can contract individually

Apart from rectum & canal - substantial & continuous

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14
Q

Large bowel - motility

Describe colonic contractions and their rate. What does this type of contraction promote?

What type of patterns dominate in the proximal colon? What, what does this retain?

Describe contraction in the transverse and descending colon, what is the function of this type of contraction?

What happens every 30 mins and increases in frequency following a meal?

A

Colonic contractions - kneading process - minimally propulsive 5-10cm/hr max
-> promotes absorption of electrolytes and water

In proximal colon: antipropulsive patterns dominate to retain chyme

In transverse & descending colon: localised segmental contractions of haustra, cause back and forth mixing

Short propulsive movements every 30 mins

Increase in frequency after a meal

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15
Q

Large bowel - mass movement

How many times daily is there mass movement in the large bowel? What type of wave does it resemble?

How far can food be propelled down the length of large intestine in a few seconds?

What type of food promotes rapid transport through the colon?

A

1-3 times daily mass movement - resembles peristaltic wave

Can propel contents 1/3 - 3/4 of length of large intestine in a few seconds

Food containing fibre (indigestible) promotes rapid transport through colon

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16
Q

Large bowel: control

Parasympathetic: the vagus nerve innervates what? What do the pelvic nerves innervate?

What is controlled by sympathetic innervation?

What are external anal sphincters controlled by?

What do afferent sensory neurones detect?

A

Parasympathetic: ascending colon & most of transverse colon innervated by vagus nerve. More distal innervated by pelvic nerves

Sympathetic: lower thoracic and upper lumbar spinal cord

External anal sphincter controlled by somatic motor fibres in the pudenal nerves

Afferents sensory neurones detect pressure

17
Q

Large bowel - control

What is absent in Hirschsprung’s disease?

Where is the myenteric plexus ganglia concentrated?

The presence of what in the stomach stimulates mass movement?

Detail elements of hormonal/paracrine control

A

Enteric nervous system is important. No enteric intramural ganglia in Hirschsprung’s disease

Myenteric plexus ganglia concentrated below taenia coli

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)

18
Q

Large bowel - defecation

How does the rectum fill with faeces?

When does the rectum store faeces until?

What is the defecation reflex primarily controlled by?

A

Rectum filled w faeces by mass movement in the sigmoid colon

Stores stool until convenient to void

Defecation reflex controlled primarily by sacral spinal cord - both reflex and voluntary actions

19
Q

Large bowel - defecation

Fill in the gaps:

_______ to sudden _______ of walls of rectum

Pressure receptors send signals via ________ ________ to intiate ________ waves in descending, sigmoid colon and rectum.
What is inhibited?

The weak intrinsic signal is augmented by?

What is under voluntary control?

A

Reflex to sudden distension of walls of rectum

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

External anal sphincter under voluntary control

20
Q

Large bowel - rectum

What are the last few centimetres of the rectum known as? Why is this part important?

A

Last few centimetres of the rectum = social part of rectum

Social part - can distinguish between solid, liquid and gas. Perceptual ability is important in knowing what can be passed appropriately in what circumstance?

21
Q

Large bowel - faeces

What proportion of faeces is water?

What makes up the solid component?

What gives faeces its colour?

What gives faeces its odour?

A

150g/day adult

2/3rds water

Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+)

Bile pigments give colour

Bacterial fermentation gives odour

22
Q

Large bowel flora:

What the significance of the microbiome in the large bowel?

A

Mammals have symbiotic relationship w their gut microbial community (microbiome)

Stomach and small bowel have few bacteria protected
Large bowel contains many - essential to normal function

23
Q

Large bowel flora:

How much live bacteria does the microbiome in an average adult human comprise of? How much is the active biomass equivalent to?

A

Diverse, high metabolically active community

Microbiome in avg adult human - 1.5 kg bacteria. Active biomass equivalent to major human organ

24
Q

Roles of intestinal flora:

Clues:

  • vitamins
  • pathogens
  • other bacteria
A

Secrete and excrete vitamins

Prevent colonisation by pathogens by competing for attachment sites or for essential nutrients

Antagonise other bacteria through the production of substances which inhibit or kill non indigenous species

25
Q

Roles of intestinal flora:

What does intestinal flora stimulate the production of that prevents infection/invasion?

Intestinal flora stimulates the development of which tissues?

A

Stimulate production of cross reactive antibodies -> antibodies against components of normal flora can cross react w certain related pathogens —> prevents infection/invasion

Stimulates the development of certain tissues - inc caecum & lymphatic tissues

26
Q

Roles of intestinal flora:

What can be broken down by colonic bacteria?

What are the uses of short chain fatty acids produced?

A

Fibre (indigestible carbohydrate) can be broken down by colonic bacteria

produces short chain fatty acids -> regulate gut hormone release / absorbed to be used as an energy source / influence functions eg food intake/insulin sensitivity directly

27
Q

Types of normal flora:

What type of bacteria is implicated in initiation colitis and colon cancer?

What are bifidobacteria? What is their role?

A

Most prevalent bacteria are: bacterioides-gram negative, anaerobic, non-spore forming bacteria. Implicated in the initiation colitis & colon cancer

Bifidobacteria = gram positive, non spore forming, lactic acid bacteria. Friendly bacteria, thought to prevent colonisation by potential pathogens

28
Q

What areas are being researched in relation to intestinal flora?

The link between gut bacteria and?

A

Drug metabolism

Insulin resistance

Bile acid metabolism

Lipid metabolism

Obesity

Gastroenterology (5 decks)

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