11.5 - Large Bowel Flashcards
What internal body parts make up the large bowel?
- colon
- caecum
- appendix
- rectum
- anal canal
- 1.5m, 6cm diameter
What is the caecum?
A blind pouch just distal to the ileocaecal valve - larger in herbivores
What is the appendix?
Thin, finger-like extension of the caecum - not physiologically relevant in humans
What is the rectum?
- 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
What is the anal canal?
- terminal portion of large bowel
- surrounded by internal (circular muscle) and external (striated muscle) anal sphincters
What are the principal functions of the large bowel?
- reabsorption of electrolytes and water
- elimination of undigested food and waste
What sections can the colon be divided into?
- 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
Describe the blood supply to the colon.
- 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
Describe the walls of the colon.
- 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)
What are haustra?
- 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
How does reabsorption occur in the colon?
- 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
What are the four layers of the walls of the large bowel?
- mucosa
- submucosa
- muscularis
- serosa
Describe the mucosal structure of the large bowel.
- 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
Describe the function and distribution of goblet cells in the large bowel?
- 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
Describe the microvilli.
- 0.5-1.5um high
- make up the brush border
- several thousand microvilli per cell
- surface of microvilli covered with glycocalyx
What is glycocalyx?
- 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
What are the muscle layers of the muscularis externa in the large bowel?
- 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
What muscles form haustra?
- 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
Describe colonic contractions.
- 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
Describe localised segmental contractions.
- 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
Describe what mass movements are.
- 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
What parts of the large bowel does the parasympathetic NS control?
- ascending colon and most of transverse colon innervated by vagus nerve
- more distal colon innervated by pelvic nerves
Where does sympathetic control come from?
Lower thoracic and upper lumbar spinal cord
What controls the external anal sphincter?
Somatic motor fibres in pudendal nerves
What do afferent sensory neurones detect?
Pressure
What other systems control the large bowel?
- 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?
What is Hirschsprung’s disease?
When there is no enteric intramural ganglia meaning large bowel muscles lose ability to move stool through colon –> severe constipation
Describe the steps to defecation.
- 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
What is special about the rectum for passing material through it?
- 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
Describe the composition of faeces.
- 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
Where is the flora usually?
- 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
How much microbiome do we have?
- 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
What are the roles of intestinal flora?
- 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
What do the gut bacteria do to fibre?
- 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
What are the main types of flora bacteria?
- 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
What does the gut microbiome link to in terms of metabolism?
- drug metabolism
- insulin resistance
- bile acid metabolism
- lipid metabolism
- obesity
What is a faecal matter transplantation used for?
As a source of introducing a microbiome from healthy individuals to replenish microbiome of unhealthy individuals
