Large Bowel Flashcards
What internal body parts make up large bowel
- Colon
- Caecum- what is this?Blind pouch just distal to ileocecal valve- larger in herbivores
- Appendix- what is this?Thin, finger like extension of caecum- not physiologically relevant in humans
- Rectum- describe it (how is it different to the colon (2 ways))
- Dilated distal portion of alimentary canal (full tube of mouth to anus)
- Histology similar to colon but has transverse rectal folds in submucosa and no taenia coli in its muscularis externa, and also no appendices epiploicae
- Anal canal- describe it
- Terminal portion of large bowel
- Surrounded by internal (circular muscle) and external (striated muscle) anal sphincters
- 1.5m long and 6 cm diameter
What sections can colon be divided into
- Ascending colon- where is this?
- Right side of abdomen
- Runs from caecum to hepatic flexure (turn of colon by liver)
- Transverse colon- where is this?
- Runs from hepatic flexure to splenic flexure (turn of colon by spleen)
- Hangs off the stomach, attached by a wide tissue band called greater omentum (posterior side, mesocolon)
- Descending colon- where is this?
- Runs from splenic flexure to sigmoid colon
- Sigmoid colon- where is this?
- Runs from descending colon to rectum
What are the 2 functions of large bowel
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Reabsorption of electrolytes and water- describe what moves and how
- Happens more in proximal colon
- Na+ and Cl- absorbed by exchange mechanisms and ion channels
- Water follows by osmosis- large intestine can reabsorb 4.5l water (usually 1.5l)- above this diarrhoea occurs
- K+ moves passively into lumen
- Elimination of undigested food and waste
Describe the blood supply to the colon
- Proximal transverse colon supplied by middle colic artery (branch of superior mesenteric artery)
- Distal third of transverse colon is perfused by inferior mesenteric artery
- This reflects embryological division between the midgut and hindgut
- Region between 2 is sensitive is ischaemia as blood supply is not as extensive (transverse colon is sensitive to ischaemia)
Describe the walls of the colon
- Peritoneum carries fatty tags (appendices epiploicae) which has unknown function- suggested to protect against intra-abdominal infections
- Muscle coat has 3 thick longitudinal bands (taeniae coli)- necessary for large bowel motility
- Nodules of lymphoid tissue are common as solitary nodules
What are haustra, and why do they form in the large bowel?
- The taenia coli are shorter in large bowel than in small bowel
- This causes formation of pouched ovoid segments- haustra
What are the 4 layers of the walls of the large bowel?
- Mucosa
- Submucosa (Meissner’s plexus)
- Muscularis (Auerbach’s plexus/Myenteric plexus)
- Serosa
Describe the mucosal structure (which cells are abundant, which cell type isn’t in the large bowel mucosae but is present in the small bowel
- Enterocytes and goblet cells are abundant
- Abundant crypts with stem cells
- Mucosa appears smooth in large bowel as it has no villi- enterocytes have microvilli which are primarily concerned with resorption of salts
- Water is absorbed as it passively follows electrolytes, leaving more solid gut contents
- No Paneth cells and enteroendocrine cells are rarer than in small bowel
What is the difference between the glycocalyx of the small bowel and the glycocalyx of the large bowel?
Glycocalyx (of the large bowel) doesn’t contain digestive enzymes
- Describe the function and distribution of goblet cells in the large bowel, + state what stimulates goblet cell secretion
-
- Higher no. than in small bowel
- More prevalent in crypts than along the surface, no. increases distally towards rectum
- Apical ends are packed with mucus-filled secretion granules awaiting release- mucus facilitates passage of solid colon contents and covers bacteria + particles
- ACh (parasympathetic and enteric NS) stimulates goblet cell secretion
What are the muscles layers of the muscularis externa
- Inner circular muscle layer which are segmentally thickened
- Outer longitudinal muscle layer concentrated in 3 bands- taenia coli- between which the longitudinal layer is thin
- Bundles of muscles from teniae coli penetrate the circular layer at irregular intervals
What muscles form haustra?
- Longitudinal muscle layer as it’s shorter than circular muscle layer
- Haustra can contract individually
- Haustra are seen along colon apart from rectum and anal canal which are substantial and continuous
What are the 2 types of contractions that happen in large bowel?
- Colonic contractions- describe
- 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
- Localised segmental contractions- describe
- Happen in transverse and descending colon- called Haustral contractions
- Cause back and forth mixing
When do these propulsive movements happen
- Short propulsive movements every 30 mins
- 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- myenteric plexus ganglia are concentrated below taenia coli
- Hormonal/paracrine control e.g. aldosterone promotes Na+ and water absorption (synthesis of Na+ ion channel, Na+/K+ pump)
What is Hirschsprung’s disease?
When there is no enteric intramural ganglia meaning large bowel muscles lose ability to move stool through the colon
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 sacral spinal cord- 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 sphincter inhibited
- Weak intrinsic signal augmented by autonomic reflex
- External anal sphincter under voluntary control, so we 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 called the ‘social part’
- Can distinguish between solid, liquid, gas
- This ability is important in knowing what can be passed appropriately in what circumstance
Describe faeces- what does it contain?
- We produce 150g/day 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?
It’s diverse and highly metabolically active community
What are the roles of the intestinal flora? (5)
- Synthesise and excrete vitamins e.g. Vitamin K → germ-free animals can have clotting problems
- Prevent colonisation by pathogens by competing for attachment sites or for essential nutrients
- Antagonise other bacteria through production of substances which inhibit or kill non-indigenous species
- Stimulate production of cross-reactive antibodies which can prevent infection and invasion
- Stimulate development of certain tissues inc. caecum and lymphatic tissues
What do gut bacteria do to fibre?
- Break it down (it’s an indigestible carb)
- This 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 or insulin sensitivity directly
What are the main types of flora bacteria? (2)
- Most prevalent are Bacteroides → gram -ve, anaerobic, non-spore forming that are implicated in initiation of colitis and colon cancer
- Bifidobacteria are gram +ve, non-spore forming, lactic acid bacteria that are ‘friendly’ and prevent colonisation by potential pathogens
What things does the gut microbiome have links to in terms of metabolism? (5)
- Drug metabolism
- Insulin resistance
- Bile acid metabolism
- Lipid metabolism
- Obesity
What is faecal matter transplantation used for?
Can be used as a source of introducing a microbiome from healthy individuals to replenish the microbiome of unhealthy individuals
