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Year 1 - BRS > 11.5 - Large Bowel > Flashcards

11.5 - Large Bowel Flashcards

1
Q

What internal body parts make up the large bowel?

A
  • colon
  • caecum
  • appendix
  • rectum
  • anal canal
  • 1.5m, 6cm diameter
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2
Q

What is the caecum?

A

A blind pouch just distal to the ileocaecal valve - larger in herbivores

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3
Q

What is the appendix?

A

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

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4
Q

What is 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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5
Q

What is the anal canal?

A
  • terminal portion of large bowel
  • surrounded by internal (circular muscle) and external (striated muscle) anal sphincters
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6
Q

What are the principal functions of the large bowel?

A
  • reabsorption of electrolytes and water
  • elimination of undigested food and waste
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7
Q

What sections can the colon be divided into?

A
  • ascending colon - right side of abdomen, runs from caecum to the hepatic flexure (the turn of colon by liver)
  • transverse colon - runs from hepatic flexure to the splenic flexure (turn of colon by spleen), hangs off the stomach, attached by a wide band of tissue called the greater omentum (posterior side, mesocolon)
  • descending colon - runs from splenic flexure to sigmoid colon
  • sigmoid colon - s-shaped, runs from descending colon to rectum
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8
Q

Describe the blood supply to the colon.

A
  • the proximal transverse colon is supplied with blood by the middle colic artery (branch of superior mesenteric artery)
  • distal third of transverse colon is perfused by the inferior mesenteric artery
  • reflects embryological divisions between the midgut and hindgut
  • region between the two is sensitive to ischaemia
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9
Q

Describe the walls of the colon.

A
  • the peritoneum carries fatty tags (appendices epiploicae) - unknown function, suggested to have a protective function against intra-abdominal infections
  • the muscle coat has three thick longitudinal bands (taeniae coli) - necessary for large intestine motility
  • nodules of lymphoid tissue are common as solitary nodules in the large intestine ( / Peyer’s patches in distal small intestine)
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10
Q

What are haustra?

A
  • taenia coli shorter in large bowel than in small bowel
  • causes formation of pouched ovoid segments called haustra (haustrum singular) - gives gut wall pouched appearance
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11
Q

How does reabsorption occur in the colon?

A
  • colon absorbs electrolytes and 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 approx 4.5L water (usually 1.5L) - above this diarrhoea occurs
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12
Q

What are the four layers of the walls of the large bowel?

A
  • mucosa
  • submucosa
  • muscularis
  • serosa
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13
Q

Describe the mucosal structure of the large bowel.

A
  • enterocytes and goblet cells are abundant
  • abundant crypts with stem cells
  • mucosa appears smooth at the gross level because it has no villi (smaller SA than small bowel)
  • enterocytes have short, irregular microvilli - primarily concerned with resorption of salts
  • (water is reabsorbed as it passively follows electrolytes, resulting in more solid gut contents)
  • crypts are dominated by goblet cells (although enterocytes still dominant in gut lumen)
  • no Paneth cells and enteroendocrine cells are rarer than in small bowel
  • glycocalyx does not contain digestive enzymes
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14
Q

Describe the function and distribution of goblet cells in the large bowel?

A
  • higher number than in small bowel
  • more prevalent in the crypts than along the surface, number increases distally towards rectum
  • apical ends are packed with mucus-filled secretion granules awaiting release
  • mucus - facilitates passage of increasingly solid colonic contents and covers bacteria and particular matter
  • acetylcholine (parasympathetic and enteric NS) stimulates goblet cell secretion
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15
Q

Describe the microvilli.

A
  • 0.5-1.5um high
  • make up the brush border
  • several thousand microvilli per cell
  • surface of microvilli covered with glycocalyx
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16
Q

What is glycocalyx?

A
  • rich carbohydrate layer on apical membrane
  • serves as protection from digestional lumen yet allows for absorption
  • traps a layer of water and mucus known as ‘unstirred layer’
  • regulates rate of absorption from intestinal lumen
17
Q

What are the muscle layers of the muscularis externa in the large bowel?

A
  • muscularis externa consists of an inner circular layer and an outer longitudinal layer
  • circular muscles segmentally thickened
  • longitudinal layer concentrated in three bands - taenia coli (–> bulges out)
  • between the taenia, longitudinal layer is thin
  • bundles of muscle from the taeniae coli penetrate the circular layer at irregular intervals
18
Q

What muscles form haustra?

A
  • longitudinal muscle layer as it is shorter than circular muscle layer
  • ovoid segments called haustra that can contract individually
  • haustra seen along colon apart from rectum and anal canal which are substantial and continuous
19
Q

Describe colonic contractions.

A
  • 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
20
Q

Describe localised segmental contractions.

A
  • happen in transverse and descending colon
  • localised segmental contractions of circular muscle called Haustral contractions cause back and forth mixing
  • short propulsive movements every 30 minutes
  • increase in frequency following a meal
21
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
22
Q

What parts of the large bowel does the parasympathetic NS control?

A
  • ascending colon and most of transverse colon innervated by vagus nerve
  • more distal colon innervated by pelvic nerves
23
Q

Where does sympathetic control come from?

A

Lower thoracic and upper lumbar spinal cord

24
Q

What controls the external anal sphincter?

A

Somatic motor fibres in pudendal nerves

25
Q

What do afferent sensory neurones detect?

A

Pressure

26
Q

What other systems control the large bowel?

A
  • enteric NS also important - myenteric plexus ganglia are concentrated below taenia coli
  • hormonal/paracrine control e.g. aldosterone promotes sodium and water absorption (synthesis of Na+ ion channel, Na+/K+ pump)
  • presence of food in stomach can stimulate mass movement - hormonal? neural?
27
Q

What is Hirschsprung’s disease?

A

When there is no enteric intramural ganglia meaning large bowel muscles lose ability to move stool through colon –> severe constipation

28
Q

Describe the steps to defecation.

A
  • rectum filled with faeces by mass movement in sigmoid colon
  • stores stool until convenient to void
  • defecation reflex controlled primarily by the sacral spinal chord - both reflex and voluntary actions
  • the reflex is 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 –> if we resist the urge to defecate the sensation subsides
29
Q

What is special about the rectum for passing material through it?

A
  • last few cm of 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
30
Q

Describe the composition of faeces.

A
  • 150g/day produced as an adult
  • 2/3 water
  • solids: cellulose, bacteria, cell debris, bile pigments, salts (K+)
  • bile pigments give colour
  • bacterial fermentation gives odour
31
Q

Where is the flora usually?

A
  • in large bowel - stomach and small bowel have few bacteria and usually protected by high acid content of stomach
  • all mammals have symbiotic relationship with their gut microbiome
32
Q

How much microbiome do we have?

A
  • diverse, highly metabolically active community
  • 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
33
Q

What are the roles of intestinal flora?

A
  • synthesise and excrete vitamins e.g. vitamin K (germ-free animals can have clotting problems)
  • prevent colonisation by pathogens by competing for attachment sites / essential nutrients
  • antagonise other bacteria through the production of substances which inhibit/kill non-indigenous species
  • stimulate production of cross-reactive antibodies - antibodies produced against components of the normal flora can crossreact with certain related pathogens, and thereby prevent infection/invasion
  • stimulate the development of certain tissues, including caecum and lymphatic tissues
34
Q

What do the gut bacteria do to fibre?

A
  • fibre (indigestible carbohydrate) can be broken down by colonic bacteria
  • produces short chain fatty acids which can:
  • regulate gut hormone release
  • be absorbed to be used as an energy source
  • influence functions like food intake/insulin sensitivity directly
35
Q

What are the main types of flora bacteria?

A
  • most prevalent are Bacteroides - gram -ve, anaerobic, non-spore forming bacteria that are implicated in the initiation of colitis and colon cancer
  • Bifidobacteria are gram +ve, non-spore forming, lactic acid bacteria that are ‘friendly’ and thought to prevent colonisation by potential pathogens
36
Q

What does the gut microbiome link to in terms of metabolism?

A
  • drug metabolism
  • insulin resistance
  • bile acid metabolism
  • lipid metabolism
  • obesity
37
Q

What is a faecal matter transplantation used for?

A

As a source of introducing a microbiome from healthy individuals to replenish microbiome of unhealthy individuals

Year 1 - BRS (73 decks)

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