Physiology of Large Intestine Flashcards
what comprises the large intestine
caecum appendix colon rectum anal canal and anus
name the different parts of the colon
ascending
transverse
descending
sigmoid
structure of caecum and colon
longitudinal smooth muscle layer
divided into 3 strands - taeniae coli, but encircles the rectum and anal canal
structure of internal sphincter and external sphincter
thickened smooth muscle surrounded by skeletal muscle of the external sphincter
functions of caecum and appendix
no specialised functions
caecum receives material from terminal ileum
how is material from terminal ileum permitted to the caecum
via the gastroileal reflex in response to gastrin and CCK through one-way ileocaecal valve
what material does the caecum receive from the terminal ileum
indigestible residues
unabsorbed biliary components
unabsorbed fluid
action of the ileocaecal valve - pressure
maintains positive resting pressure
action of the ileocaecal valve - relaxation
relaxes in response to distension of duodenum
action of the ileocaecal valve - contraction
contracts in response to distension of the ascending colon
nervous control of ileocaecal valve
vagus nerve
sympathetic nerves
enteric neurones
hormonal signals
structure of appendix
blind-ended tube with extensive lymphoid tissue connected to distal caecum via appendiceal orifice
what causes appendicitis
appendiceal orifice may become obstructed by faecalith
primary functions of the colon
absorption
secretion
reservoir
periodic elimination of faeces
primary functions of the colon - absorption
absorbs;
sodium, chloride and water
short chain fatty acids
why does the colon absorb sodium, chloride and water
to condense ileocaecal material to solid, or semi-solid, stool
why does colon absorb short chain fatty acids
carbohydrate that is not absorbed by small intestine is fermented by colonic flora to short chain fatty acids
primary functions of the colon - secretion
secretes potassium, bicarbonate and mucus
primary functions of the colon - reservoir
stores colonic contents
primary functions of ascending and transverse colon
fluid reabsorption
bacterial fermentation
primary function of descending colon
final ‘drying’ (desiccation) and storage
structure of mucosa of the colon
lacks villi
possesses colonic folds, crypts and microvilli - increasing surface area
function of surface epithelial cells (colonocytes) on mucosa of colon
mediate electrolyte absorption which, by osmosis, drives absorption of water
function of crypt cells on mucosa of colon
mediate ion secretion
function of goblet cells on mucosa of colon
secrete;
copious mucus containing glycosaminoglycans – hydrated to form a slippery surface gel
trefoil proteins involved in host defence
describe transepithelial movement of electrolytes
complex - involves numerous transporters and ion channels
how is sodium absorption and potassium secretion enhacned
by aldosterone
describe haustration
pattern of motility - non-propulsive segmentation
action of haustration
haustra are saccules caused by alternating contraction of the circular muscle (similar to segmentation - lower in frequency)
contributes to long transit time
how does haustra contribute to long transit time
disappear before and reappear after a mass movement
generated by slow wave activity
mixes content – allows time for fluid and electrolyte reabsorption
describe peristaltic propulsive movements
patterns of motility - mass movement
action of peristaltic propulsive movements
simultaneous contraction of large sections of circular muscle of the asending and transverse colon - driving faeces into distal regions
how is peristaltic propulsive movements triggered
by a meal via the gastrocolic response involving;
gastrin
extrinsic nerve plexuses
how is defaecation reflex triggered
mass movement in the distal colon propels faeces into rectum - triggering reflex in response to rectal stretch
action of commensal bacteria in the large intestine
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. IBD)
how do intestinal gases arise
swallowed air - (may be burped - eructation) enters small intestine and absorbed, or passed to the colon
bacteria in the colon which attack forms of carbohydrate that are indigestible to humans
how is gas not absorbed by the large intestine expelled
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
