Large Intestine Structure and Function

Question 1

T/F: large intestine has a complete circular muscle layer but incomplete longitudinal muscle layer.

Answer 1

True

Question 2

1. Entire length of colon has 3 bands, what are they called?

2. When the bands contract, what do they form?

Answer 2

1. Teniae coli

2. pouches called haustra -puckered appearance

Question 3

What kind of epithelium makeup the mucosa?

Answer 3

Simple columnar epithelium (flat)

Question 4

Large, straight crypts are lined with which cells & what is their function?

Answer 4

large number of goblet cells: lubrication for movement of faeces.

Question 5

Rectum:

1. What kind of epithelium makeup the mucosa?
2. Compare the muscularis externa thickness of rectum to other regions of alimentary canal.

Answer 5

1. simple columnar epithelium

2. Thicker compared to other regions

Question 6

Anal Canal:

1. What kind of epithelium makeup the mucosa?
2. Internal anal sphincter is made up of what?
3. External anal sphincter is made up of what kind of muscle?

Answer 6

1. Goes from simple columnar to stratified squamous.
2. Thickening of smooth muscle of muscularis externa (thicker than rectum) makes the sphincter.
3. Skeletal muscle

Question 7

What role colon plays in nutrient absorption?

Answer 7

No important role

Question 8

What is the function of colon?

Answer 8

Actively transports Na from lumen into blood. Creates osmotic gradient so water follows leading to dehydration of chyme resulting in solid faecal pellets.

Question 9

Why there is long residence time for chyme in colon?

Answer 9

To promote bacterial colonisation. Colonic microflora = 10^14 bacteria (1kg).

Question 10

What is the role of bacteria in the colon?

Answer 10

Ferments undigested carbs (e.g. cellulose). Products of fermentation include short chain fatty acids (sometimes only energy source in ruminants such as cows), vitamin K (important in blood clotting), & gas (flatus - N2, CO2, H2, CH4, hydrogen sulphide (characteristic aroma)).

Question 11

Normally, anus is closed by what?

Answer 11

internal anal sphincter & external anal sphincter.

Question 12

What controls internal & external anal sphincters?

Answer 12

Internal = smooth muscle under autonomic control. external = skeletal muscle under voluntary control.

Question 13

What produces the urge to defaecate?

Answer 13

Following a meal: wave of intense contraction (mass movement contraction) movies from colon to rectum. This mass movement of faecal material into rectum results in distension of rectal wall which triggers the mechanoreceptors which then triggers defaecation reflex (urge to defaecate).

Question 14

Defaecation reflex:

1. Which nerves controls the reflex?
2. Does sympathetic system influence reflex?
3. Outline how the reflex occurs.
4. Can defaecation be delayed?

Answer 14

1. Under parasympathetic control via pelvis splanchnic nerves.
2. No
3. Contraction of rectum. Relaxation of internal & contraction of external anal sphincters. Increased peristaltic activity in colon increases pressure on external anal sphincter which relaxes under voluntary control which leads to expulsion of faeces.
4. yes, voluntary delay of defaecation due to descending pathways (kick in after you are 2 years of age).

Question 15

Constipation:

1. T/F: No absorption of toxins from faecal material following long periods of retention
2. what are symptoms associated with constipation?

Answer 15

1. True
2. Headaches, nausea, loss of appetite, abdominal distension. These all are caused by distension of rectum (as lots of material there).

Question 16

Define diarrhoea.

Answer 16

Too frequent passage of faeces which are too liquid.

Question 17

What are the causes of diarrhoea?

Answer 17

Pathogenic bacteria, protozoans, viruses, toxins, food, stress?

Question 18

What is the major killer of children <5 in developing countries?

Answer 18

Diarrhoea

Question 19

Enterotoxigenic bacteria:

1. Give some examples that cause diarrhoea.
2. What do these bacteria produce & what is the consequence of such secretion?

Answer 19

1. Vibrio cholerae, Escherichia coli
2. Produce protein enterotoxins (don’t damage villus cells) which maximally turn on intestinal Cl secretion from crypt cells leading to increased H2O secretion. These toxins increase intracellular 2nd messengers such as cAMP (V cholerae), cGMP (E coli) & calcium (C difficle).
   H2O secretion swamps absorptive capacity of villus cells: profuse watery diarrhoea (25L/day for cholera).

Question 20

What is the treatment of secretory diarrhoea?

Answer 20

Na/glucose solution: drives H2O absorption so rehydrates. Secretion still going so that infection is washed away.
Oral rehydration therapy saves 4 million children’s lives per year.