Large Intestine Structure and Function

Question 1

State the structure and functions of the large intestine.

Answer 1

a

Question 2

Describe the structure of the colon

Answer 2

• 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.

Question 3

Describe the structure of the rectum.

Answer 3

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

Question 4

Describe the structure of the anal canal.

Answer 4

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

Question 5

Describe the function of the colon.

Answer 5

• 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

Question 6

Describe defaecation.

Answer 6

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.

Question 7

Describe the defaecation reflex.

Answer 7

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.

Question 8

Describe constipation and its symptoms.

Answer 8

• 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

Question 9

Describe diarrhoea and its aetiology.

Answer 9

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

Causes;

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

Question 10

Describe enterotoxigenic bacteria.

Answer 10

• 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).

Question 11

Describe the treatment of secretory diarrhoea.

Answer 11

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