Large Intestine Structure & Function

Question 1

Order of the large intestine:

Answer 1

Caecum (with vermiform appendix coming off)
Ascending Colon
Transverse Colon
Descending Colon
Sigmoid Colon
Rectum
Anal Canal
Anus

Question 2

What is the caecums function?

Answer 2

Its a blind ended sac acting like a big fermentation sac, much bigger in full herbivores who need bacteria to break down cellulose more

Question 3

Where is the appendix and what does it do?

Answer 3

Most often retrocaecal but varies

It doesn’t have on but is associated with a lot of lymph nodes.

Question 4

How long is the colon?

Answer 4

1.5m

Question 5

Whats different about the muscularis Externa of the large intestine?

Answer 5

The longitudinal muscle layer isn’t complete but instead comes in three strips that run the length of the colon called teniae coli

Question 6

The colon has a quilted or puffa jacket appearance, what produces this?

Answer 6

The 3 teniae coli, when they contract the circular muscle layer is forced out the gaps and produces the haustra or sacculations.

Question 7

Whats different about the mucosa of the large intestine to small?

Answer 7

It has a simple columnar epithelium just like the small intestine but with no villi

Question 8

Whats different about the crypts in the large intestine to the small intestine?

Answer 8

They’re much longer in the colon and lined with goblet cells to lubricate movement of faeces

Question 9

What is the rectum?

Answer 9

A straight muscular tube connecting the sigmoid colon and anal canal.
It has a thicker muscularis externa than most of the canal

Question 10

What is the anal canal?

Answer 10

A short 2-3cm channel.

Its muscularis externa forms the internal anal sphincter (IAS).

Question 11

Where is the external anal sphincter, and what is it made from?

Answer 11

In the anal canal, distal to the internal anal sphincter.

Its made of skeletal muscle and so is voluntarily controlled

Question 12

How does epithelium change leaving the anal canal?

Answer 12

IT changes from simple columnar to stratified squamous like the skin.

Question 13

What is the main function of the colon?

Answer 13

Actively absorbing sodium in order to take up water.

Question 14

A huge amount of bacteria exists in the colon, (colonic microflora) why?

Answer 14

The long residence time of faeces allows extensive bacterial colonisation

Question 15

What are the roles of colonic microflora?

Answer 15

• Breakdown undigested carbohydrates (cellulose) to short chain fatty acids which we use for energy
• Also produce vitamin K which we use for blood clotting

Question 16

What gasses are there in the colon? (Flatus)

Answer 16

Nitrogen
CO2
Hydrogen
Methane
Hydrogen Sulphide

Question 17

What sphincters control defecation and how?

Answer 17

Internal Anal Sphincter - Smooth muscle - Autonomic (part of the anal canal)
External Anal Sphincter - Skeletal Muscle - Voluntary

Question 18

What is Mass Movement Contraction?

Answer 18

Waves of intense peristaltic contraction from colon to rectum that occur after eating and result in:

• Rectal wall distends by mass movement of faeces
• Stimulates mechanoreceptors
• Initiates defaecation reflex (urge to shit)

Basically making room for fresh food

Question 19

What controls the post-meal defecation reflex?

Answer 19

Parasympathetic control via pelvic splanchnic nerves

Question 20

How does defaecation occur?

Answer 20

• Contraction of rectum
• Relaxation of Internal anal sphincter
• Increased peristaltic activity in colon -> increased pressure on External Anal Sphincter (EAS)
• Voluntary Relaxation of EAS
• Faeces Expulsed

Question 21

As you know defaecation can be voluntarily delayed, why cant babies do this?

Answer 21

The descending pathways required for conscious control of defecation dont develop until your around 2. Prior to that potty training wont work because kids cant physically stop

Question 22

What are the symptoms of constipation and why?

Answer 22

The build up of material causes rectum distension which can cause:
= Headaches
= Nausea
= Loss of Appetite
= Abdominal Distension

Question 23

Define Diarrhoea

Answer 23

Unusually frequent and liquid faeces

Question 24

What are some causes of diarrhoea?

Answer 24

• Pathogenic Bacteria e.g. C.Diff/Vibrio Cholerae
• Protozoans
• Viruses
• Toxins (from bad food or bacteria)
• Certain foods cause it in certain people

Question 25

How many people die from diarrhoea?

Answer 25

About 1/4 of <5yr deaths in the developing world are due to diarrhoea

Question 26

Define an enterotoxin?

Answer 26

A toxin produced in or affecting the intestines

Question 27

Name 2 enterotoxigenic bacteria?

Answer 27

Vibrio Cholerae

Escherichia Coli

Question 28

How do enterotoxogenic bacteria cause diarrhoea?

Answer 28

• Produce protein enterotoxins
• Elevates intracellular secondary messengers (e.g. cAMP, calcium etc)
• Maximally activates intestinal Cl- secretion from crypt cells
• You passively secrete loads of water
• Secretion swamps absorptive capacity of villi
• Diarrhoea

Question 29

How do we treat diarrhoea?

Answer 29

Sodium/Glucose solution (Oral Rehydration Salts, ORS)

• Gut absorbs sodium & glucose
• Drives H20 absorption
• Keeps you hydrated
• Secretion is still ongoing and will wash away the infection eventually