Large Bowel Flashcards
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?
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
Detail where each part of the colon runs to and from:
- Ascending colon
- Transverse colon
- Descending colon
- sigmoid colon
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
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?
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
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?
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
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?
(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
Rectum:
What portion of the alimentary canal is the rectum?
What distinguishes the histology of the rectum from the colon?
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.
Anal canal:
What portion of the rectum is the anal canal?
What type of anal sphincters in the anal canal surrounded by?
Terminal portion of rectum is the anal canal
Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters
Large bowel - mucosal structure
What types of cells are found in the large bowel?
Similar to small intestine
- enterocytes + goblet cells abundant
- Abundant crypts
- stem cells found in the crypts
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?
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
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?
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
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?
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
Large bowel - muscle layers
What does the muscularis externa consist of?
How are each of these layers organised?
What are the taenia coli?
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
Large bowel - muscle layers:
What are the haustra? What can they do?
Apart from where are the haustra continuous and substantial?
Haustra: shorter than circular muscle layer. Ovoid segments that can contract individually
Apart from rectum & canal - substantial & continuous
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?
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
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?
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
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?
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
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
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)
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?
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
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?
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
Large bowel - rectum
What are the last few centimetres of the rectum known as? Why is this part important?
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?
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?
150g/day adult
2/3rds water
Solids: cellulose, bacteria, cell debris, bile pigments, salts (K+)
Bile pigments give colour
Bacterial fermentation gives odour
Large bowel flora:
What the significance of the microbiome in the large bowel?
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
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?
Diverse, high metabolically active community
Microbiome in avg adult human - 1.5 kg bacteria. Active biomass equivalent to major human organ
Roles of intestinal flora:
Clues:
- vitamins
- pathogens
- other bacteria
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
