Large Intestine Structure and Function Flashcards
State the structure and functions of the large intestine.
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Describe the structure of the colon
- 1.5-1.8 metres.
- Four parts: ascending, transverse, descending, sigmoid.
- Circular muscle layer complete but longitudinal muscle layer incomplete.
- Three bands (teniae coli) run enter length of colon.
- Contraction of teniae coli –> pouches (haustra).
- Mucosa comprised of simple columnar epithelium (flat, no villi).
- Large straight crypts lined with large number of goblet cells –> lubrication for movement of faeces.
Describe the structure of the rectum.
- Straight, muscular tube (between end of sigmoid colon and anal canal).
- Mucosa: simple columnar epithelium
- Muscularis externa: thicker compared to other regions of GIT.
Describe the structure of the anal canal.
- 2-3cm between distal rectum and anus.
- Muscularis thicker than rectum –> internal anal sphincter.
- External anal sphincter –> skeletal muscle.
- Epithelium: simple columnar –> stratified squamous.
Describe the function of the colon.
- No important nutrient absorption role.
- Actively transports Na from lumen into blood –> osmotic absorption of water –> dehydration of chyme –> solid faecal pellets.
- Long residence time in colon –> bacterial colonisation.
Bacterial fermentation of undigested carbs; - Short chain fatty acids
- Vitamin K (blood clotting)
- Gas (flatus): nitrogen, CO2, hydrogen, methane, hydrogen sulphide
Describe defaecation.
Normally anus closed by;
- Internal anal sphincter (smooth muscle under autonomic control).
- External anal sphincter (skeletal muscle under voluntary control).
Following meal;
- Wave of intense contraction (mass movement contraction) from colon to rectum.
- Distension of rectal wall –> mechanoreceptors –> defaecation reflex –> urge to defaecate.
Describe the defaecation reflex.
Under parasympathetic control (via pelvic splanchnic nerves);
- Contraction of rectum.
- Relaxation of internal sphincter and contraction of external sphincter.
- Increased peristaltic activity in colon.
Increase in pressure on external –> relaxes external –> expulsion of faeces.
Voluntary delay of defaecation can occur due to descending pathways.
Describe constipation and its symptoms.
- No absorption of toxins from faecal material following long periods of retention.
- Frequency of bowel movements vary considerably between individuals.
Symptoms (due to distension of rectum);
- Headaches
- Nausea
- Loss of appetite
- Abdominal distension
Describe diarrhoea and its aetiology.
- Too frequent passage of faeces, which are too liquid.
Causes;
- Pathogenic bacteria
- Protozoans
- Viruses
- Toxins
- Food
- Stress
Describe enterotoxigenic bacteria.
- Produce protein enterotoxins that maximally turn on Cl secretion from crypt cells –> H2O secretion.
- Act by elevating intracellular second messages: cAMP, cGMP, Ca.
- H2O secretion swamps absorptive capacity of villi –> profuse water diarrhoea (25L/day in cholera).
Describe the treatment of secretory diarrhoea.
- Enterotoxins don’t damage villi.
- Give Na/glucose solution –> drives H2O absorption –> rehydration.
- Secretion still going –> washes away infection.
- Oral rehydration therapy (ORS).