Large Intestine Structure and Function

Question 1

Describe the layout of the layers of muscle

Answer 1

Circular muscle complete but longitudinal muscle layer incomplete, with strips of it overlying the circular muscle

Question 2

How is the longitudinal muscle organised in the large intestine?

Answer 2

Three bands of longitudinal strips called teniae coli, which runs he entire length of the colon

Question 3

What three features distinguish the colon from the small intestine?

Answer 3

• Teniae coli
• Haustra (sacculation between teniae coli)
• Omental appendices

Question 4

What is the epithelium of the colon?

Answer 4

Simple columnar - flat, no villi to efficiently move bolus

Question 5

What is the importance of the crypts and goblet cells?

Answer 5

To provide lubrication for movement of faeces

Question 6

What are the feature of the rectum?

Answer 6

• Simple columnar epithelium

* Muscularis externa thicker

Question 7

What are the features of the anal canal?

Answer 7

• Muscularis externa thicker than rectum –> thickening forms internal anal sphincter
• External anal sphincter = skeletal muscle (allows control)
• Junctional epithelium: simple columnar –> stratified squamous keratinised as moves to outer body surface skin

Question 8

What is the role of the colon in terms of absorption?

Answer 8

• No important nutrient absorption role

* Dehydrates chyme

Question 9

How does the colon dehydrate chyme?

Answer 9

Actively transports Na from lumen into blood –> osmotic absorption of water –> dehydration of chyme –> solid faecal matter

Question 10

What can long residence time in colon lead to?

Answer 10

Bacterial colonisation

Question 11

What are the products of fermentation of carbohydrates?

Answer 11

• Fatty acids
• Vitamin K
• Gas: nitrogen, CO2, hydrogen, methane, hydrogen sulphide

Question 12

What is the importance of intestinal bacteria?

Answer 12

Fermentation of carbohydrates

Question 13

What occurs during defection following a meal?

Answer 13

• Wave of intense contraction in colon to rectum
• Distension of rectal wall produced by mass movement of faecal material into rectum –> mechanoreceptors –> defection reflex –> urge to defaecate

Question 14

Describe the defaection reflex?

Answer 14

Under parasympathetic control via pelvic splanchnic nerves:

1. Contraction of rectum
2. Relaxation of internal and contraction of external sphincters
3. Increased peristaltic activity in colon due to contraction –> Increased pressure on external anal sphincter - relaxes under voluntary control –> expulsion of faeces

Question 15

What are the symptoms of constipation?

Answer 15

• Headache
• Nausea
• Loss of appetite
• Abdominal distension

Question 16

Name 5 causes of diarrhoea

Answer 16

• Pathogenic bacteria
• Protozoans
• Viruses
• Toxins
• Food

Question 17

What is enterotoxigenic bacteria?

Answer 17

Cause of bacteria i.e. Vibrio cholera, escherichia coli

Question 18

How does enterotoxigenic bacteria cause diarrhoea?

Answer 18

• Produce protein enterotoxins which maximally turn on intestinal chloride secretion from crypt cells –> Increase H2O secretion
• Elevate cAMP, cGMP, calcium

Question 19

What is the treatment of secretory diarrhoea?

Answer 19

• Give sodium/glucose solution -> drive H2O absorption –> rehydration
• Secretion still going –> wash away infection
• Oral rehydration therapy (ORS)