Physiology - Large Intestine Structure and Function

Question 1

What is/ was the function of the veniform appendix?

Answer 1

Contains lymph nodes

Immune function

Question 2

What are the 4 parts of the colon?

Answer 2

Ascending, Transverse, Descending, Sigmoid

Question 3

How do the longitudinal and circular muscles differ in the colon?

Answer 3

The circular muscle layer is complete but the longditudinal muscle is incomplete

Question 4

What are teniae coli?

How do they form haustra?

Answer 4

Bands of longitudinal muscle which line the entire length of the colon.

Contraction of these create pouches called haustra.
This is due to the incomplete arrangement of longitudinal muscle

Question 5

What type of epithelium is found in the mucosa of the large intestine?

Answer 5

Simple columnar epithelium

Question 6

Are crypts present in the colon?

Answer 6

Yes.
These large straight crypts are lined by many goblet cells which secrete mucous.
This lubricates the colon for the movement of faeces

Question 7

Describe the structure of the rectum

Answer 7

The rectum is a straight muscular tube lying between the sigmoid colon and the rectum.
It is lined with simple columnar epithelium.
Its muscularis externa is much thicker than other areas of the alimentary canal

Question 8

Describe the structure of the anal canal

Answer 8

The anal canal is about 2-3cm in length lying between the distal rectum and the anus.

Its muscularis externa is thicker than that of the rectum (internal sphinter)
It also has an external sphincter which is skeletal muscle.

The anal canal shows a change in its mucosal epithelium from simple columnar to stratified squamous.
(Change from inside the body to outside)

Question 9

What is the function of the colon?

Answer 9

No important nutrient absorption role in humans

Actively transports sodium from the lumen to blood->
Osmotic absorption of water->
Dehydration of chyme ->
Solid faecal pellets

Question 10

What similarities are there between water absorption in the colon and water absorption in that gall bladder?

Answer 10

Its the exact same mechanism.

Sodium potasium pump creates a low concentration of sodium inside the cell. (Potassium recycled)
Sodium transported into cell from lumen.
Water moves pancellularly

Question 11

What is colonic microflora?

What does it produce?

Answer 11

Long term residence of faeces in the colon causes bacterial colonisation. This is the colonic microflora.
The bacteria are about 10^14 (weighing about 1kg)

They ferment the undigested carbohydrate that we can’t (cellulose)
This produces:
-Short chain fatty acids (energy source in ruminants)
-Vitamin K (blood clotting)
-Gas (flatus)

Question 12

What is the clinical significance of vitamin K production by colonic bacteria?

Answer 12

Babies are born with a sterile colon. They therefore don’t get any vitamin K until they establish their microflora.
Vitamin K injection given to newborns to help their bleeding risk

Question 13

What chemicals are contained in flatus?

Answer 13

Nitrogen, CO2, hydrogen methane, hyodrogen sulphide

Question 14

Describe how we get the urge to defaecate

Answer 14

Normally, the anus is closed by the internal (autonomic control) and the external (voluntary control) anal sphincters.

Following a meal you get a wave of intense contraction (Mass Movement Contraction) through the colon to the rectum.

Mass movement of faecal material into the rectum causes distension of the rectal walls.

Mechanoreceptors detect this and signal the defeacation reflex causing an urge to defaecate.

Question 15

What is the defaecation reflex?

Answer 15

Under parasympathetic control via pelvic splanchnic nerves.

A. Contraction of Rectum
B. Relaxation of internal and contraction of external sphincters (the poo is docking)
C. Increased peristaltic activity in the colon

Increased pressure on external anal sphincter
When it relaxes under voluntary control there is the expulsion of faeces

Question 16

Are toxins absorbed from faecal material following long periods of retention?

Answer 16

No

Question 17

Do the frequency of bowel movements vary between individuals?

Answer 17

Yes

Question 18

What are the symptoms associated with constipation?

Answer 18

Headaches
Nausea
Loss of appetite
Abdominal distension

Question 19

What is diarrhoea?

Answer 19

Too frequent passage of faeces which are too liquid

Major killer of children under 5 years of agein developing countries.
1500 cases of child diarrhoea
2.5 million deaths/year

Question 20

What are the causes of diarrhoea?

Answer 20

Pathogenic bacteria
Protoxoans
Virus'
Toxins
Food
Stress

Question 21

Give an example of 2 enterotoxigenic bacteria.

Answer 21

Vibrio Cholerae

Escherichia Coli

Question 22

What are enterotoxigenic bacteria and what is their pathology?

Answer 22

Produce protein enterotoxins which maximally turn on intestinal chloride secretion from crypt cells.
This causes huge H2O secretion

They act by elevating intracellular second messengers: cAMP, cGMP, Calcium

H2O secretion is massive compared to villus absorptive capacity. This results in profuse watery diarrhoea
(25 litres per day for cholera)

Question 23

How do you treat secretory diarrhoea?

Answer 23

Enterotoxins dont damage villus cells

Give a sodium/glucose solution to drive water absorption and cause rehydration.
Secretion will still occur to wash infection away

This is called oral rehydration therapy and saves 2 million children lives per year