The large intestine

Question 1

Describe the path/anatomy of the large intestine

Answer 1

Start at caecum, then moves to ascending colon, transverse colon, descending colon and rectum + anal canal

Question 2

What are the principal functions of the colon?

Answer 2

Reabsorption of water and electrolytes

removal of undigested food

Question 3

Describe the anatomical position of the large intestine

Answer 3

Asc- right side
Transverse - hepatic to splenic flexure
Desc splenic flexure to rectum

Question 4

Describe the blood supply of the large intestien

Answer 4

Middle colic - brnach of superior mesentery supplies promixal 2/3
Inferior mesenteric - distal third

Question 5

How do the large and small intestine differ? Give 3 differences

Answer 5

Fatty tags

Thick longitudinal bands of muscle
- for motility
Lymphoid tissue
- Peyer’s v solitary nodules

Question 6

What are haustra?

Answer 6

Pouched ovoid segments formed by taenia coli - muscle

Question 7

Where does most reabsorption occur? How?

Answer 7

More in proximal colon
Na, Cl are taken in via channels, K comes in passively
Water then moves via osmosis

Question 8

How is the rectum distinguished from the colon?

Answer 8

Histologically fairly similar
but no taenia coli
presence of transverse rectal folds
- faecal ‘shelf’

Circulr muscle - internal
Striated muscle - external

Question 9

What are the layers of large intestine?

Answer 9

Serosa
Longitudinal
Circular
Submucosa
Muscularis mucosae - for folding
Lamina propria

Question 10

How is the large intestine simialr to small intestine?

Answer 10

Abundance of enterocytes and goblet cells

Abundant crypts
- full of stem cells

Question 11

How might the mucosa differ?

Answer 11

Absence of villi
- no need to absorb

Irregular microvilli
- absorption is NOT macronutrients, so villi not needed

More goblet cells

• release mucous for passage of solid matter
  
  • stimulated by ACh

No Paneth cells
- unnecessary destroyal of gut flora
Glycocalyx has no digestive enzymes

Question 12

Describe the muscle layers of the large intestine

Answer 12

External muscle - muscularis externa
- inner circular and outer longitudinal

Longitudinal is concentrated in taenia coli
- between them it is thin

Muscle from taenia penetrates circular to anchor it to rest of colon

Continuous for complex movements

Question 13

Why is the large intestine pouched?

Answer 13

Longitudinal muscle shorter than circular

Question 14

HOw is motility maintained in the large intestine? Describe the types

Answer 14

Minimally propulsive

Mixing promoted by HOAUSTRAL contraction
- move between haustra

Propulsive movement occurs every 30 minutes

Mass movement

• peristaltic wave - 1-3 waves daily
  
  • moves 1/3 - 3/4 length of intestine in few seconds

Question 15

How is the large intestine controlled?

Answer 15

Parasympathetic

• ascending and transverse by VAGUS
• distal by PELVIC

Sympathetic

• LOWER THORACIC, UPPER LUMBER spinal cord
  
  • inhibitory function
    
    • motility
    • blood supply

Enteric

• for human disease - IBS, Hirschprung’s - no enteric ganglia
• myenteric plexus present in muscle layers, under taenia coli

Hormonal

• mass movement?
• aldosterone for water and sodium uptake

Question 16

What is the process of defecation?

Answer 16

Rectum filled by matter formed from mass movement

Stored until convenient to empty

Reflex controlled by

• sacral SC
  
  • voluntary and reflex

Causes distension of rectum walls

Sends signal via myeteric plexus to intitiate peristaltic waves and inhibit internal anal sphincter

External sphincter is voluntary

Question 17

What is the importance of the last few cm’s of the rectum

Answer 17

Can distinguish between solid, liquid and gas

Question 18

What are the components of faeces?

Answer 18

150g/day
2/3 water, 1/3 solid
- cellulose
- bacteria - gives odour
- cell debris
- bile pigments - give colour

Question 19

Describe the flora of the large intestine - background, maybe not too important

Answer 19

Symbiotic relationship
- not just parasitic

Diverse
- 1.5kg total microbiome

Question 20

What is the role of the flora in the large intestine?

Answer 20

Synthesise and excrete Vitamins
- Vit K - absence is clotting problem

Prevent pathogenic colonisation by competing for attachment sites and nutrients

Antagonise other bacteria by producing inhibitory substances

Stimulate production of cross-reactive ABs
- made against flora but can react to similar ones that may be pathogenic in nature

Stimulate development of cecum and lymphatic tissues

Break down fibre

Form short chain fatty acids that:

• regulate gut hormone release
• can be absorbed as an energy source
• influence food intake/insulin sensitivity

Question 21

What are the types of gut flora?

Answer 21

Most are gram negative ,anaerobic and non-spore forming
Implicated in colitis and colon cancer

Bifidobacteria

• gram +
• prevent pathogenic colonisation