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large bowel Flashcards

1
Q
  • What is the principle function of the large bowel?

- Dimensions of the large bowel?

A

Reabsorption of electrolytes and water, and the elimination of undigested food and waste.

    1.5m long and 6cm in diameter.
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2
Q
  • Describe the anatomical position of the transverse colon.
A

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

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3
Q
  • What supplies blood to the proximal transverse colon?

- What is the distal third of the transverse colon perfused by and what does this reflect?

A

Middle colic artery

Inferior mesenteric artery Embryological division between the midgut and the hindgut - region between the two is sensitive to ischaemia
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4
Q
  • What part of the colon runs from the descending colon to the rectum?
A

Sigmoid (s-shaped) colon

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5
Q

what is the anatomical position of the ascending colon?

what is the anatomical position of the descending colon?

A

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

from splenic fixure to the sigmoidal colon

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6
Q
  • What is the functional purpose of the appendices epiploica?
  • What are taenia coli?
A

Unknown, but suggested to have a protective function against intra-abdominal infections.

3 separate longitudinal bands of smooth muscle on the outside of the ascending, transverse, descending and sigmoid colons.
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7
Q
  • What nodules of lymphoid tissue are common in the walls of the distal small intestine and large intestine respectively?
  • What are the small ovoid segments that give the gut wall a pouched appearance?
A

Distal small intestine - Peyer’s Patches
Large intestine - Solitary nodules

Haustra
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8
Q
  • What does the colon (re)absorb?

- What happens if a person’s large intestine reabsorbs more than 4.5L of water?

A

Water (by osmosis) and electrolytes including Na+ and Cl- by exchange mechanisms and ions channels
(K+ moves passively into the lumen)

Diarrhoea
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9
Q
  • In terms of histology, how is the rectum different to the colon?
A

Distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa

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10
Q
  • What is the anal canal surrounded by?
A

Internal (circular muscle) and external (striated muscle) anal sphincters

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11
Q
  • What are the 4 main layers of the mucosal structure of the large bowel from deep to superficial?
A

Mucosa

Submucosa

Muscularis 

Serosa
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12
Q
  • What similarities are there between the small and large intestine in terms of their mucosa?
A

Enterocytes and goblet cells are abundant

Abundant crypts 

Stem cells are found in the crypts
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13
Q
  • Why does mucosa of the large bowel appear smooth at the gross level?
  • Do large bowel enterocytes have microvilli?
A

No villi (therefore smaller surface area than small bowel)

Short, irregular microvilli are present (primarily concerned with resorption of salts)
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14
Q
  • What type of cell are the crypts in the large bowel dominated by?
  • What happens to the number of goblet cells present in the crypts of the large bowel as you move towards the rectum (distally)?
A

Goblet cells

Increase

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15
Q
  • What stimulates mucous secretion by goblet cells?

- What is the purpose of mucous being secreted by goblet cells in the large bowel?

A

Acetylcholine (ParaNS and enteric nervous system)

Facilitates the passage of the increasingly solid colonic contents and covers bacteria and particulate matter.
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16
Q
  • Are villi present on the large bowel epithelia?

- What make up the brush border in the large bowel?

A

No
(enteroendocrine cells present, but rarer than in small bowel)

Microvilli (~0.5-1.5 micrometres high) 
Surface covered with glycocalyx (but no digestive enzymes)
17
Q
  • Explain the muscular arrangement of the large bowel.
A

Muscular externa consisting of inner circular and outer longitudinal layer.
Circular muscles segmentally thickened.
Longitudinal layer concentrated in 3 bands - taenia coli.

Bundles of muscle from the taenia coli penetrate the circular layer at irregular intervals.

18
Q
  • Explain the movements of the large bowel.
A

Colonic contractions promote absorption of electrolytes and water

In the proximal colon, antipropulsive patterns dominate to retain chyme

In the transverse and descending colon, there are localised segmental contractions of circular muscle called Haustral contractions causing back and forth mixing

Short propulsive movements every 30 minutes
19
Q
  • What causes an increase in frequency of bowel movements?
  • What does mass movement mean?
  • How often does mass movement occur daily?
A

having a meal

High-amplitude propagated contraction similar to a peristaltic wave

1-3 times

20
Q
  • What proportion of the contents of the large bowel can mass movement propel in just a few seconds?
  • What innervates the ascending colon and most of the transverse colon and what type of control is this?
A

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

Vagus nerve, more distal large bowel innervated by pelvic nerves 

Type of control: Parasympathetic
21
Q
  • Where does the sympathetic control of the large bowel stem from?
  • What controls the external anal sphincter?
A

Lower thoracic and upper lumbar spinal cord

Somatic fibres in the pudendal nerves
22
Q
  • What process fills the rectum with faeces?

- What is the defecation reflex controlled primarily by?

A

Mass movement in the sigmoid colon
(store stool until convenient to void)

Sacral spinal cord - parasympathetic reflex
23
Q
  • Outline the events leading to defection
A

Reflex to sudden distention of walls of rectum.

Pressure receptors send signals via myenteric plexus to initiate peristaltic waves in descending colon, sigmoid colon and rectum.

Internal anal sphincter contraction inhibited.

Weak intrinsic signal augmented by autonomic reflex.

External anal sphincter under voluntary control. Urge resisted until sensation subsides.

24
Q
  • What is different about the last few centimetres of the rectum?
  • Roughly how much faeces does an adult defecate daily?
A

Can distinguish between solid, liquid and gas
This ability is important in knowing what can be passed appropriately and in what circumstance

150g
25
Q
  • Outline the composition of faeces

- What gives the odour of faeces?

A

2/3 Water
Solids: Cellulose, bacteria, cell debris, bile pigments (give colour), salts e.g. K+

Bacterial fermentation
26
Q
  • Compare the number of bacteria (flora) between the large bowel and the small bowel and stomach
  • What mass of live bacteria is present as a part of the gut microbial community (microbiome) in humans?
A

Large bowel contains many more than the small bowel and stomach

1.5 Kg
27
Q
  • List all 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 via competitive inhibition

Antagonise other bacteria by producing inhibitory substances that kill foreign species

Stimulate production of cross-reactive Abs (Abs produced against normal flora can cross-react with related pathogens and prevent infection or invasion)

Stimulate development of certain tissues, including caecum and lymphatic tissues

Fibre (indigestible carbohydrate) can be broken down by colonic bacteria
28
Q

what do the FAs that intestinal flora produce do?

A

Regulating hormone release
Or absorbed and used as energy source
or to influence functions such as food intake or insulin sensitivity

29
Q
  • What are the main types of normal flora and which is more prevalent?
A

Bacteroides (more prevalent) - Gram -ve, anaerobic non-spore forming bacteria (Implicated in the initiation of colitis and colon cancer)

Bifidobacteria - Gram +ve, non-spore forming, lactic acid bacteria → Thought to prevent colonisation by potential pathogens
30
Q
  • What links have been found to gut bacteria in recent research?
A

Drug metabolism

Insulin resistance 

Bile acid metabolism 

Lipid metabolism 

Obesity
31
Q
  • What is the purpose of a stool transplantation?
A

Source of introducing microbiome from healthy individuals to replenish microbiome

BRS (41 decks)

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