Large Intestine Structure And Function

Question 1

What are the 4 parts of the colon

Answer 1

Ascending, transverse, descending, sigmoid

Question 2

How does the ileum join the large intestine

Answer 2

Ileocaecal valve

Question 3

What is the purpose of ileocaecal valve

Answer 3

Makes sure material moves from small to large intestine and not other direction

Question 4

What is the caecum

Answer 4

First part of large intestine (blind ended sac)
More significant in herbivores

Question 5

Where does the appendix attach? What’s it’s role?

Answer 5

Hangs off the caecum
Has lymph nodes important for gut health (often become infected)

Question 6

What is unique about the longitudinal muscle layer of the colon, what’s the consequence of this

Answer 6

Circular muscle layer complete but Incomplete longitudinal muscle layer, instead has 3 bands (teniae coli) which span length of colon
Consequence is that bits of gut poke between gaps of teniae coli (haustra - which gives puffer jacket appearance)

Question 7

Why is the surface mucosa of the large intestine entirely flat

Answer 7

No villi in colon
Helps generate the faecal stool

Question 8

What is unique about the crypts in large intestine, what are there role

Answer 8

Crypts are much deeper than in small intestine (don’t come above surface)
Lined with goblet cells to lubricate the faeces

Question 9

What does the rectum connect, what’s unique about its muscularis externa

Answer 9

Connects sigmoid colon and anal canal
Muscularis externa is very thick to grab faecal material and push it down and out the body

Question 10

What is unique about the internal anal sphincter and external anal sphincter of the anal canal

Answer 10

Internal anal sphincter - muscularis even thicker than rectum + smooth muscle as with rest of gut

External anal sphincter - skeletal muscle therefore under voluntary control

Question 11

What happens to the epithelium in anal canal

Answer 11

Turns from simple columnar to stratified squamous (skin)

Question 12

What is the main role of the colon (how does it do this)

Answer 12

Absorb water to dehydrate the chyme to excrete solid faecal pellets
(Actively transports sodium from lumen into blood - water follows dehydrating the chyme)

Question 13

Why is long residence in the colon Encouraged

Answer 13

Long residence allows time for bacterial colonisation which allows for the bacterial fermentation of indigestible carbohydrate

Question 14

What are the 3 products of bacterial fermentation of carbohydrates in the colon

Answer 14

Short chain fatty acids
Vitamin K (blood clotting) (babies are given vitamin K injection as they don’t have developed colonic microflora)
Gas (by products)

Question 15

What happens following a meal to tell us we need to poo (what reflex)

Answer 15

Intense wave of MMC from colon to rectum moves the material towards the rectum causing distension of rectal wall
This stimulates the mechanoreceptors which switches on the defaecation reflex prompting the urge to defaecate

Question 16

What nerve controls the defaecation reflex

Answer 16

Under parasympathetic control via the pelvic splanchnic nerve (unlike everything else in GI which is controlled by the vagus nerve (except salivation), the vagus nerve doesnt go this far down)

There is also a voluntary delay of defaecation by descending pathways which holds off reflex to allow you to reach toilet

Question 17

What symptoms are associated which constipation (4), what causes this

Answer 17

Headaches
Nausea
Loss of appetite
Abdominal distension

Caused by distension of rectal wall
(There isnt any absorption of toxins from faecal material following long periods of retention)

Question 18

What are the causes of diarrhoea (6)

Answer 18

Pathogenic bacteria
Protozoans
Viruses
Toxins
Food
Stressful circumstances

Question 19

How do enterotoxigenic bacteria cause diarrhoea

Answer 19

Produce protein enterotoxins which maximally turn on intestinal chloride secretion from crypt cells —> massive increase in H2O secretion
This secretion swamps the absorptive capacity of villus cells —> profuse watery diarrhoea (major cause of death in children in developing countries)

Question 20

How is diarrhoea treated

Answer 20

Enterotoxins don’t damage villus cells as they only act to turn on crypt cells
As villus intact sodium + glucose solution is given which will drive water uptake —> rehydration