Large intestine structure and function Flashcards
how does the small intestine join with the large intestine
from the ileum - ileocaecal valve - caecum (blind sac) - vermiform (worm-like) appendix + lymph nodes = colon
what are the four parts of the colon
ascending, transverse, descending, sigmoid
what are the two muscle layers of the colon
circular - complete
longitudinal - incomplete
what are the bands that run the entire length of the colon
teniae coli
what does contraction of the teenier coli produce
pouches (HAUSTRA)
puckered appearance
what is the mucosa of the colon comprised of
simple columnar epithelium - flat
what enables lubrication of the colon for movement of faeces
large, straight crypts lined with large number of goblet cells
what is the rectum
Straight, muscular tube (between end of sigmoid colon and anal canal)
what is the mucosa of the rectum made from
simple columnar epithelium
what is different abut the muscular external of the rectum compared to other areas of the GI tract
very thick
what is the anal canal
the 2-3 cm between distal rectum and anus
where is the muscular thicker in the anal canal than in the rectum
at the internal anal sphincter
what is the external anal sphincter made from
skeletal muscle
what kind of epithelium is found in the anal canal
starts as simple columnar - goes to stratified squamous
what is the role of the colon
Actively transports sodium from lumen into blood - osmotic absorption of water - dehydration of chyme - creates solid faecal pellets
**no important absorption role in humans
what does the long residence time of matter in the colon lead to
bacterial colonisation
what does the colonic microflora help with
bacterial fermentation of undigested carbohydrates:
- short chain fatty acids (energy source)
- vitamin K (blood clotting)
- gas (flatus - nitrogen, CO2, hydrogen, methane, hydrogen sulphide)
what normally keeps the anus closed
internal anal sphincter (smooth muscle under AUTOMATIC control)
external anal sphincter (skeletal muscle under VOLUNTARY control)
what creates the urge to defaecate
following a meal - wave of intense contraction (mass movement contraction) - goes down the colon to the rectum
distension of the rectal wall produces by mass movement of faecal material into rectum - stimulates mechanoreceptors - creates the urge to defaecate
what controls the defection reflex
parasympathetic control via pelvic splanchnic nerves (NOT SYMPATHETIC)
what are the 3 stages of the defection reflex
- contraction of rectum
- relaxation of internal and contraction of external anal sphincters
- increased peristaltic activity in colon - causes increased pressure on external anal sphincter - causes it to relax under VOLUNTARY control
= expulsion of faeces
what control the voluntary delay of defaecation
descending pathways
what leads to constipation
NO absorption of toxins from faecal material following long periods of retentions
what are the symptoms of constipation
decreased frequency headaches nausea loss of appetite abdominal distension (due to distension of rectum)
what is diarrhoea
too frequent passage of faeces which are too liquid
what can cause diarrhoea
pathogenic bacteria protozoans viruses toxins food
**major cause of death in children under 5 in developing countries
give examples of enterotoxigenic bacteria
vibrio cholerae
escherichia coli
how do enterotoxigenic bacteria cause diarrhoea
- Produce protein enterotoxins which maximally turn on intestinal chloride secretion from crypt cells = increase H2O secretion
- Act by elevating intracellular second messengers:
- cAMP
- cGMP
- calcium - H2O secretion swamps absorptive capacity of villus cells profuse watery diarrhoea (25 litres per day for cholera)
why do people with cystic fibrosis not get diarrhoea
they cannot secrete water
what is the treatment for secretory diarrhoea
enterotoxins DONT damage villus cells so give sodium/glucose solution - drive H2O absorption = rehydration
secretion still going = wash away infection
ORS - oral rehydration therapy
