Large Intestine Structure and Function Flashcards

1
Q

State the structure and functions of the large intestine.

A

a

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2
Q

Describe the structure of the colon

A
  • 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.
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3
Q

Describe the structure of the rectum.

A
  • Straight, muscular tube (between end of sigmoid colon and anal canal).
  • Mucosa: simple columnar epithelium
  • Muscularis externa: thicker compared to other regions of GIT.
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4
Q

Describe the structure of the anal canal.

A
  • 2-3cm between distal rectum and anus.
  • Muscularis thicker than rectum –> internal anal sphincter.
  • External anal sphincter –> skeletal muscle.
  • Epithelium: simple columnar –> stratified squamous.
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5
Q

Describe the function of the colon.

A
  • 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
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6
Q

Describe defaecation.

A

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.
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7
Q

Describe the defaecation reflex.

A

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.

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8
Q

Describe constipation and its symptoms.

A
  • 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
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9
Q

Describe diarrhoea and its aetiology.

A
  • Too frequent passage of faeces, which are too liquid.

Causes;

  • Pathogenic bacteria
  • Protozoans
  • Viruses
  • Toxins
  • Food
  • Stress
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10
Q

Describe enterotoxigenic bacteria.

A
  • 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).
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11
Q

Describe the treatment of secretory diarrhoea.

A
  • Enterotoxins don’t damage villi.
  • Give Na/glucose solution –> drives H2O absorption –> rehydration.
  • Secretion still going –> washes away infection.
  • Oral rehydration therapy (ORS).
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