Physiology and pharmacology of the large intestine Flashcards
How long is the large intestin?
Approx. 1.7m long, 6cm diameter
What is the longitudinal smooth muslce layer in the caecum and colon divided into?
The taeniae coli
Smooth muslce thickened at the internal anal sphincter
Skeletal muscle at the external anal sphincter
What creates haustra?
The activity of the taeniae coli and circular muscle layers in colon causes sac like bulges (haustra)
What material does the caecum recieve?
1-2 L or indigestible residues, unabsorbed biliary components from the terminal ileum
What permits entry of material from the terminal ileum to the caecum?
The gastroileal reflex in response to gastrin and CCK thorough the ileocaecal valve
What is the function of the ileocaecal valve?
Maintains positive resting pressure
Relaxes in response to distension of the duodenum
Contracts in response to distenstion of ascending colon
Controlled by vagus nerve, symp nerves, enteric neurones and hormonal signals
What is the structure of the appendix?
Bilnd-ended tube with extensiev lymphoid tissue connected to the distal caecum via the appendiceal orifice
What does the colon absorb?
Sodium, chloride and water to condence ileocaecal material to solid or semi-solid stool
Short chain fatty acids
What does the colon excrete?
Potassium, bicarbonate and mucus
What is contaiend within faeces?
Water, cellulose, bacteria, bilirubin, small amount of salt
What does the mucosa of the colon contain?
No villus but has colonic folds, crypts and microvilli
What do colonocytes do?
They are surface epithelial cells that mediate electrolyte absorption which by osmosis drives absorption of water
What do crypt cells do?
They mediate ion secretion
What do goblet cells secrete?
Copious mucus containing gylcosaminoglycans which are hydrated to form a slippery surface gel
Trefoil proteins involved in host defence
What enhances sodium absorption and potassium excretion in the colon?
Aldoseterone
What is haustration?
Non-propulsive segmentation
Saccules caused by alternating contraction of circular muscle
Where does haustration occur?
In the proximal colon causing orad movement
What is the function of haustration?
Mixes contents and allows time for fluid and electrolyte reabsorption
What allows for the mass movement within the colon?
Peristaltic propulsive movements
What does defaecation result in?
Periodic egestion
What is mass movement?
Simaltaneous contraction of large sections of the circular muscle of the ascending and transverse colon (haustra disappea) which drives faeces into the distal region
What triggers mass movement?
Meals via the gastrocolic response which involves gastrin and extrinsic nerve plexuses
What triggers the defaecation reflex?
Stretch via the propelling of faeces into the rectum
What do commensals do in the colon?
They increase intestinal immunity by competitino with pathogenic microbes
Promote motility and help to maintain mucosal integrity
Synthesis vitamin K and free fatty acids that are absorbed
Activate some drugs (used in IBD)
Where does gas arise from the in colon?
Swallowed air that enters the colon. Bacteria in colon attack forms of carbohydrate not ingestiblae to humans. Gas not absorbed in large intestine is expelled through the anus
What is constipation?
The presence of hard, dried faeces within the colon (results from delay in defaecatino and enhanced absorption of water)
What can cause constipation?
Ignoring the urge to defaecate
Decreased colonic motility (improper diet, drugs, metabolic disorders, age)
Obstruction of faecal movement
Paralytic ileus following abdo surgery
Impairment of motility/defaecatino reflux (hirschprung disease )
What are symptoms of constipation?
Abdominal discomfort, headache, loss of apppetite, general malaise caused by prolonged sitention of the large intestin
What is appendicoliths?
Hardened, calcified, faecal matter wihin the appendix
What are laxatives used for?
Treat constipation
What are purgatives used for?
Agents that cause purgin, or cleansing of the bowels by promoting evacuation
When should laxative or purgatives NOT be used?
When there is a physical obstructino to the bowel
What is the mechanaism of action of laxatives?
Increase peristalsis/ soften faeces causing, or assisting evacuatino
When can laxative/purgatives be used?
When straining is damaging to health (patients with angina)
Painful defaecation (haemorrhoids)
Clear the bowel before surgery of colonoscopy
Treat drug induced constipation
What do bulk laxatives do?
They are indigestible polysaccharide polymers (methycellulose) that improve stool consistency but are slow acting
What are osmotic laxatives?
Poor absorbed solutes (magnesium sulphate or hydroxide) that are rapidi acting
What are faecal softners?
Detergent-like action (docusate sodium)
What are stimulant purgatives?
Bisacodyl, sodium picosulphate
Increased water and electrolyte secretion and increase peristalsis
What does IBS present as?
Bouts of diarrhoea, constipation or abdominal pain
How is IBS treated?
Adjustment of diet, anti-diarrhoeals, anti-spasmodics or laxatives as required
What can IBD affect?
Entire gut (Crohn’s disease) or colon (UC)
When are glucotricoids used in IBD?
For acute attacks bur prolonged used limited by adrenal suppresion
What are aminosalicylates used for?
Useful for UC for the maintenance and mild disease. Sulfasalzine releases 5-ASA by colonic bacteria
Mesalazine releases 5-ASA in colon
