small bowel and large intestine Flashcards
large intestine function: recall the function of the large intestine (and compare with the small intestine), and explain the movements of the large intestine and control of defecation
what does the large intestine reabsorb
ions, water
where are most ions and water reabsorbed
proximal colon, where chyme is more fluid
how are Na+ and Cl- absorbed
exchange mechanisms
how is water absorbed
follows ions by osmosis
how is K+ absorbed
moves passively into lumen via gap junctions
result of water entering colon exceeding 4500mL/day
diarrhoea
what 2 cells are abundant in large intestine
enterocytes, goblet cells
where are stem cells located in large intestine, and their fate
at bottom of colonic crypts, which migrate up and differentiate before being sloughed off after a few day (possess some memory)
features of large intestine mucosa
smooth, no villi so small SA vs small intestine as no glucose or amino acid absorption in colon
features of enterocytes in large intestine
short, irregular microvilli for reabsoprtion of salt
function of goblet cells in large intestine
secrete mucous to facilitate passage of increasingly dry luminal contents as more and more water reabsorbed
abudance of goblet cells in large intestine
increases along length; within crypts
function of mucous
cover bacteria and particulate matter to protect laminal surface from infection and/or abrasion
what stimulates goblet cell secretion
ACh from local nerves
why are paneth cells absent in large intestine
commensal bacteria present, and paneth cells may destroy bacteria; don’t need same degree of protection
why are enteroendocrine cells fewer in number in large intestine
regulation of digestion and absorption in small intestine very complicated so more required there
features of glycocalyx in large intestine
doesn’t contain digestive brush border enzymes
what are basic colonic contractions and why are they used
kneading process that provide minimal propulsion (5-10cm/hour), allowing chyme to stay in colon for long time to promote absorption
what contraction does proximal colon elicit and why
antipropulsive to impede propulsion and keep food in region for longer
contractions in transverse and descending colon and why
localised segmental contractions of circular muscle (Haustral contractions), shuffling contents forwards and backwards; short propulsive movements every 1/2 hour (increase after meal)
how often does coordinated mass movement in colon occur
between 1 and 3 times a day
purpose of mass movement
peristaltic wave that propels contents up to 3/4 length of large intestine in a few seconds
what promotes mass movements
indigestible high-fibre food
what are faeces
indigestible waste of chyme that entered large intestine
when is the afferent sensation of needing to defaecate perceived
suddenly following mass movement
when is the internal anal sphincter relaxed
in defecation reflex (spinal reflex via sacral spinal cord - same as voluntary activity)
ejection of faecal waste
distension of rectal wall detected by pressure receptors → signials sent via myenteric plexus → peristaltic waves initiated in descending, sigmoid colon and rectum → internal anal sphincter relaxes
when does sensation subside until
next mass movement
what can last few centimeters of rectum (“social”) distinguish between
gas, liquid and solid (difficult to tell difference between gas and oil)
amount of faeces produced by healthy adult per day
150g (2/3rds is water)
solid components of faeces
cellulose, bacteria (smell), cell debris, bile pigments/salts (give healthy brown colour)
