Physiology + Pharmacology of the large intestine Flashcards
What makes up the large intestine?
Caecum and appendix Colon - ascending - transverse - descending - sigmoid Rectum Anal canal and anus
What is the longitudinal smooth muscle layer in caecum and colon divided into?
Three strands. - the taeniae coli
Where is smooth muscle thickened?
internal anal sphincter - surrounded by the skeletal muscle of the external anal sphincter
What forms the haustra?
Activity of the taeniae coli and circular muscle layers in colon
- moves due to contraction
What is entry of substances into the caecum permitted by?
gastroileal reflex in response to gastrin and CCK through the ‘one-way’ ileocaecal valve
What is the role of the ileocaecal valve?
maintaining a positive resting pressure
relaxing in response to distension of the duodenum
contracting in response to distension of the ascending colon
being under the control the vagus nerve, sympathetic nerves, enteric neurones and hormonal signals
How is the appendix attached to the distal caecum?
the appendiceal orifice
What may block the the appendiceal orifice to cause appendicitis?
faecalith- a hard mass of fecal matter
Name the functions of the colon?
Absorption: (net) of Na+, Cl- and H2O to condense ileocaecal material to solid, or semi-solid, stool
Absorption: of short chain fatty acids:
Carbohydrate not absorbed by the small intestine is fermented by colonic flora to short chain fatty acids
Secretion: (net) of K+, HCO3- and mucus
Reservoir: Storage of colonic contents
Periodic elimination of faeces
Where is mucus produced?
goblet cells in the large intestine
How is the surface area of the colon increased?
colonic folds, crypts and microvilli
What mediates electrolyte absorption which, by osmosis, drives absorption of H2O?
surface epithelial cells - colonocytes
What do crypt cells mediate?
ion secretion
What do goblet cells secrete?
copious mucus containing glycosaminoglycans – hydrated to form a slippery surface gel
trefoil proteins involved in host defence
What are Na+ absorption and K+ secretion are enhanced by?
aldosterone
Describe Haustration?
haustra are saccules caused by alternating contraction of the circular muscle – similar to segmentation in function, but much lower frequency (minutes) – contributes to long transit time
occurs in proximal colon – causes oral movement (reverse)
generated by slow wave activity
mixes content – allows time for fluid and electrolyte reabsorption
What is mass movement?
simultaneous contraction of large sections (about 20 cm) of the circular muscle of the ascending and transverse colon (haustra disappear)
- drives faeces into distal regions
What triggers mass movement?
triggered by a meal (often breakfast) via the gastrocolic response involving:
gastrin
extrinsic nerve plexuses
Where does mass movement occur and what does it trigger?
distal colon
defaecation reflex in response to stretch
What does passive rectal distension trigger?
smooth muslce of the internal anal spinchter - retrosphincteric reflex
What happens if defaecation is not desired?
Contraction of skeletal muscle of external anal sphincter
Where do intestinal gases arise from?
swallowed air (most ‘burped up’ – eructation), some enters small intestine but is either absorbed, or passed to the colon bacteria in the colon which attack forms of carbohydrate that are indigestible to humans gas that is not absorbed in the large intestine is expelled through the anus - selective expulsion requires abdominal contractions; internal and external sphincters are contracted to form an ‘exit’ too narrow for solid matter to escape
What is the role of commensals in the colon?
increase intestinal immunity by competition with pathogenic microbes
promote motility and help maintain mucosal integrity
synthesise vitamin K2 and free fatty acids (from carbohydrate) that are absorbed
activate some drugs (e.g. used in treatment of IBD)
What is Hirschprung disease?
colon that is devoid of neurons as enteric system fails to grow – normally treated by surgical resection, causes constipation
What is purgation?
the emptying of not just the large intestine but also the small intestine
When should laxatives and purgatives NOT be used?
When there is an obstruction of the bowel
What is the role of laxatives?
increase peristalsis and/or soften faeces causing, or assisting, evacuation
When should purgatives and laxatives be used?
when ‘straining’ is potentially damaging to health (e.g. patients with angina), or when defaecation is painful (e.g. haemorrhoids) predisposing to constipation
to clear the bowel before surgery, or endoscopy
to treat drug-induced constipation, or constipation in bedridden, or elderly patients
