Physiology and Pharmacology of the Large Intestine Flashcards
When does stuff enter the caecum?
After the gastroileal reflex (in response to gastrin and CCK).
What signals is the ileocaecal valve controlled by?
The vagus nerve, sympathetic nerves, enteric neurones and hormonal signals.
What is the opening to the appendix called?
The appendiceal orifice.
What is absorbed in the large intestine?
Sodium, chloride, water, short chain fatty acids (from carbohydrate fermented by colonic flora).
What is secreted in the large intestine?
Potassium, bicarbonate and mucus.
What are the main functions of the ascending and transverse colon?
Fluid reabsorption and bacterial fermentation.
What are the main functions of the descending and sigmoid colon?
Final drying and storage.
What are present in the colon to increase surface area?
Colonic folds, crypts and microvilli (no villi).
What cells mediate electrolyte absorption?
Colonocytes.
What cells mediate ion secretion?
Crypt cells.
What do goblet cells do in the large intestine?
Secrete lots of mucus containing glycosaminoglycans (hydrated to form a slippery gel), trefoil proteins involved in host defence.
What is enhanced by aldosterone?
Sodium absorption and potassium secretion.
What are the 3 patterns of motility in the large intestine?
Haustration (non-propulsive segmentation), peristaltic propulsive movements (mass movement), defecation (periodic egestion).
What are haustra?
Saccules cause by alternating contraction of the circular muscle.
What is the main difference between segmentation and haustration?
Haustration has a much lower frequency.
What are the functions of haustration and what is it generated by?
It causes oral movement, allows time for fluid and electrolyte reabsorption, mixes content. It is generated by slow wave activity.
What is mass movement?
Simultaneous contraction of large sections of the circular muscle of the ascending and transverse colon (haustra disappear) which drives faeces into distal regions.
How often does mass movement occur and what is it triggered by?
About 1 to 3 times daily, triggered by a meal via the gastrocolic response.
What mediators does the gastrocolic reponse involve?
Gastrin and extrinsic nerve plexuses.
What happens when mass movement occurs in the distal colon?
It propels faeces into the rectum triggering the defecation reflex in response to stretch.
What happens when rectal stretch receptors are activated?
Causes contraction of smooth muscle of sigmoid colon and rectum - internal anal sphincter relaxes.
Once the sigmoid colon and rectum have contracted, what are the 2 options?
- Relaxation of skeletal muscle of external anal sphincter.
- Contraction of skeletal muscle of external anal sphincter.
What is defection assisted by?
Straightening of the anorectal angle (squatting), abdominal skeletal muscle contraction and expiration against a closed glottis.
What do gut bacteria synthesise?
Vitamin K2 and free fatty acids (from carbohydrate).
What are all the causes of conspitation?
Ignoring/suppressing the urge to defecate, decreased colonic motility, obstruction of faecal movement, paralytic ileus following abdominal surgery, impairment of motility/defecation reflex (Hirshprung disease).
What symptoms can constipation cause?
Abdominal discomfort, headache, loss of appetite and general malaise.
What are the symptoms of constipation caused by and what are they not caused by?
Cause by prolonged distention of the large intestine, not toxins absorbed from retained faecal matter.
Name a metabolic disease causing decreased motility.
Hypokalaemia.
When should laxatives or purgatives not be used?
When there is physical obstruction to the bowel.
What do laxatives do?
Increase peristalsis and/or soften faeces causing, or assisting evacuation.
What can cause laxative dependency?
Using laxatives too regularly where you develop an atonic colon.
What are clinical indications of laxatives/purgatives?
When straining is potentially damaging to health e.g. angina or defecation is painful e.g. haemorrhoids, to clear the bowel before surgery or endoscopy, to tread drug-induced constipation or constipation in bedridden or elderly patients.
What are the 4 classes of laxative?
Bulk, osmotic, stimulant, faecal softeners.
What are bulk laxatives and are they fast or slow acting?
Indigestible polysaccharide polymers. Slow acting.
Give an example of a bulk laxative.
Methylcellulose.
What are osmotic laxatives and are they fast or slow acting?
Poorly absorbed solutes, fast acting.
Give 3 examples of osmotic laxatives and how they are taken.
Magnesium sulphate or hydroxide orally, sodium citrate rectally, lactulose orally.
What is the function of stimulant purgatives?
They cause increased water and electrolyte secretion and increased peristalsis.
Give 3 examples of stimulant purgatives.
Bisacodyl, sodium picosulphate, senna.
What is a common side effect of stimulant purgatives?
Abdominal cramps.
How do faecal softeners work?
They have a detergent-like action.
Give 3 examples of faecal softeners and how they are taken.
Docusate sodium orally, arachis oil as an enema.
What is the treatment for IBS?
Adjustment of diet, anti-diarrhoeals, anti-spasmodics or laxatives as required.
