8.5: Large bowel Flashcards
What is the principle function of the large bowel?
Reabsorption of electrolytes and water, and the elimination of undigested food and waste.
How long and wide is the large bowel?
1.5m long and 6cm diameter
Describe the anatomical position of the transverse colon
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).
What supplies blood to the proximal transverse colon?
Middle colic artery
What is the distal third of the transverse colon perfused by and what does this reflect?
Inferior mesenteric artery
Embryological division between the midgut and the hindgut - region between the two is sensitive to ischaemia
What part of the colon runs from the descending colon to the rectum?
Sigmoid colon
What is the functional purpose of the appendix
Unknown, suggested to have a protective function against intra-abdominal infections
What are taenia coli?
3 separate longitudinal bands of smooth muscle on the outside of the ascending, transverse, descending and sigmoid colons.
What nodules of lymphoid tissue are common in the walls of the distal small intestine and large intestine respectively?
Distal small intestine - peyers patches
Large intestine - solitary nodules
What are the small ovoid segments that give the gut wall a pouched appearance?
Haustra
What does the colon reabsorb?
Water (by osmosis) and electrolytes including Na+ and Cl- by exchange mechanisms and ion Channels
What happens if a person’s large intestine reabsorbs more than 4.5L of water?
Diarrhoea
Histologically, how is the rectum different to the colon?
Distinguished by transverse rectal folds in its submucosa and the absence of taenia coli in its muscularis externa
What is the anal canal surrounded by?
Internal (circular muscle) and external (striated muscle) anal sphincters
What are the 4 main layers of the mucosal structure of the large bowel from deep to superficial?
Submucosa
Mucosa
Muscularis
Serosa
What similarities are there between the small and large intestine in terms of their mucosa?
Enterocytes and goblet cells are abundant
Abundant crypts
Stem cells are found in the crypts
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Why does mucosa of the large bowel appear smooth at the gross level?
No villi (therefore smaller surface area than small bowel)
Do large bowel enterocytes have microvilli?
Short, irregular microvilli are present (primarily concerned with resorption of salts)
What type of cell are the crypts in the large bowel dominated by?
Goblet cells
What happens to the number of goblet cells present in the crypts of the large bowel as you move towards the rectum (distally)?
It increases
What stimulates mucous secretion by goblet cells?
Acetylcholine (ParaNS and enteric nervous system)
What is the purpose of mucous being secreted by goblet cells in the large bowel?
Facilitates the passage of the increasingly solid colonic contents and covers bacteria and particulate matter.
What cells are present in the large bowel epithelium
Enteroendocrine cells
What makes up the brush border in the large bowel?
Microvilli (~0.5-1.5 micrometres high)
Surface covered with glycocalyx
Are paneth cells found in the large bowel?
No
Explain the muscular arrangement of the large bowel.
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.
Explain the movements of the large bowel.
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
What effect does having a meal have on bowel movements
Increases frequency of bowel movement
What does mass movement mean
High-amplitude propagated contraction similar to peristaltic wave
How often does mass movement occur daily
1-3 times
What proportion of the contents of the large bowel can mass movement propel in just a few seconds?
1/3 - 3/4 of length of large intestine in a few seconds
What innervates the ascending colon and most of the transverse colon and what type of control is this?
Vagus nerve, more distal large bowel innervates by pelvic nerves
Parasympathetic control
Where does the sympathetic control of the large bowel stem from?
Lower thoracic and upper lumbar spinal cord
What controls the external anal sphincter
Somatic fibres in the pudendal nerves
What do afferent sensory neurones detect in the large bowel?
Pressure due to increased faecal matter
What condition would a person have if they had no enteric intramural ganglia?
Hirschsprung’s Disease
Where are myenteric plexus ganglia more concentrated?
Below taenia coli
What process fills the rectum with faeces?
Mass movement in the sigmoid colon
What is the defecation reflex controlled primarily by?
Sacral spinal cord - parasympathetic reflex
Outline the events leading to defection
Reflex to sudden distention 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. Urge resisted until sensation subsides.
What is different about the last few centimetres of the rectum?
Can distinguish between solid liquid and gas
This ability is important in knowing what can be passed appropriately and in what circumstance
Roughly how much faeces does an adult defecate daily?
150g
Outline the composition of faeces
2/3 Water
Solids: Cellulose, bacteria, cell debris, bile pigments (give colour), salts e.g. K+
What gives the odour of faeces?
Bacterial fermentation
Compare the number of bacteria (flora) between the large bowel and the small bowel and stomach
Large bowel contains many more than the small bowel and stomach
What mass of live bacteria is present as a part of the gut microbial community (microbiome) in humans?
1.5 kg
List the 7 roles of intestinal flora
-Synthesise and excrete vitamins
-Prevent colonisation by pathogens via competitive inhibition
-Antagonise other bacteria by producing inhibitory substance to kill them
-Stimulate production of cross-reactive Abs
-Stimulate development of certain tissues, including caecum and lymphatic tissues
-Fibre can be broken down by colonic bacteria
-Produce short chain FAs regulating hormone release or be absorbed and used as energy source to influence functions such as food intake or insulin sensitivity directly
What are the main types of normal flora and which is more prevalent?
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
What links have been found to gut bacteria in recent research?
- Drug metabolismInsulin resistanceBile acid metabolismLipid metabolismObesity
What is the purpose of a stool transplantation?
Source of introducing micro biome from healthy individuals to replenish micro biome
