Large Intestine Structure and Function

Question 1

Divisions of the colon?

Answer 1

Caecum and appendix 
Ascending colon
Transverse colon
Descending colon
Sigmoid colon

Question 2

How is the muscularis externa of the large intestine different from the rest of the gut tube?

Answer 2

The longitudinal muscle layer is incomplete, forms shorter sheets - called teniae coli

Question 3

What are haustra?

Answer 3

Pouches of intestine that bulge out when the teniae coli contract - puffer jacket appearance

Question 4

How is the large intestines surface different from the small intestines?

Answer 4

It is flat, no villi projecting outwards

There are large crypts on the surface, lined with goblet cells to produce mucous for lubrication

Question 5

What is the rectum? Describe its wall

Answer 5

Straight, muscular tube between sigmoid colon and anal canal

Has simple columnar epithelium lining its wall and a thick muscularis externa

Question 6

Where is the anal canal? Describe its wall

Answer 6

Between the rectum and the anus
Has an even thicker muscularis than the rectum, epithelium changes from simple columnar to stratified squamous as you move towards the skin

Question 7

Describe the 2 sphincters of the anal canal

Answer 7

Internal anal sphincter - smooth muscle, non-voluntary

External anal sphincter - skeletal muscle, voluntary

Question 8

How does the colon absorb water?

Answer 8

Same method as the gall bladder does, draw sodium into the body and water follows
dehydrates chyme to make faecal matter and reabsorbs water

Question 9

Longer time spent by faecal matter in colon allows for what?

Answer 9

Bacterial colonization

About 1 Kg of bacterial flora

Question 10

How does having bacteria in the gut help us?

Answer 10

They have cellulase and can break down carbohydrates that we cannot
When they break down cellulose they leave behind short chain fatty acids which are absorbed in the colon
They also produce vitamin K for us (blood clotting)

Question 11

A while after a meal when the body is ready to defaecate, what happens in the colon?

Answer 11

A wave of intense contraction called the Mass Movement Contraction, moves faecal matter all the way along colon into the rectum - peristaltic movement

Question 12

What does the distension of the rectal wall cause?

Answer 12

Defaecation reflex - parasympathetic response via pelvic splanchnic nerves

Question 13

What is the defaecation reflex?

Answer 13

Distension of rectum causes: urge to defaecate:
Contraction of rectum
Relaxation of internal anal sphincter
Contraction of external anal sphincter
Increased peristaltic activity in colon until external anal sphincter voluntarily relaxed

Question 14

What are some symptoms associated with constipation? What are they due to?

Answer 14

Due to distension of the rectum

Nausea
Headaches
Loss of appetite
Distension of abdominal wall

Question 15

What is Diarrhoea? What are some causes of Diarrhoea?

Answer 15

Too frequent passage of faeces that are too liquid

Bacteria
Protozoans 
Viruses 
Toxins 
Foods

Question 16

Is Diarrhoea dangerous?

Answer 16

Yes, major killer of children under the age of 5 in developing countries

Question 17

What type of bacteria are responsible for a lot of severe Diarrhoea?

Answer 17

Enterotoxigenic bacteria

E. Coli

Question 18

How do Enterotoxigenic bacteria cause increased fluid in faecal matter?

Answer 18

Cause crypt cells to maximally secrete water by stimulating Cl secretion - act by elevating cellular messengers (cAMP, Ca, cGMP)
Excess H2O swamps absorptive capacity of villi

(they don’t damage villus cells though)

Question 19

How can you increase water reabsorption in a diarrhoea patient?

Answer 19

Give them sugar and sodium to activate the SGTL1 glucose import mechanism, which will result in more water absorption - “oral rehydration therapy”
Still need to let diarrhoea run though, will still be extra secretion but need to wash infection away