9. Large intestine

Question 1

What does the large intestine consist of?

Answer 1

• Appendix
• Caecum
• Colon (ascending, transverse, descending, sigmoid)
• Rectum
• Anal canal

Question 2

What are the main functions of the large intestine?

Answer 2

• Reabsorption of water and electrolytes
• Eliminate waste
• Microenvironment for gut bacteria

Question 3

What is the size of the large intestine?

Answer 3

• Length - 150cm

* Width - 6cm

Question 4

Which arteries supply the large intestine?

Answer 4

• Superior mesenteric artery (for ascending and transverse portions)
- middle colic artery
- right colic artery
- ileocolic artery
• Inferior mesenteric artery (for descending, sigmoid and rectum)
- left colic artery
- sigmoid arteries

Question 5

Describe the innervation of the large intestine

Answer 5

• Parasympathetic system (vagus nerve) - ascending and transverse colon
• Pelvic nerves - distal colon
• Sympathetic system arises from lower thoracic and upper lumbar spinal cord

Question 6

What separates the distal ileum from the caecum?

Answer 6

• Ileocecal valve

• muscular sphincter
• tonically active/constricted
• relaxes to allow passage of fluid chyme
• prevents microbiota entering ileum

Question 7

Outline the anatomy of the colon

Answer 7

• Ascending
- starts at ileocecal valve
- bends at hepatic flexure
• Transverse
- from hepatic flexure
- bends at splenic flexure
• Descending
- runs inferiorly from splenic flexure
• Sigmoid
- S-shaped bowel
- from descending colon to the rectum

Question 8

Describe the rectum and anal canal

Answer 8

• Rectum
- dilated portion of the colon
- transverse rectal fold in submucosa - enables holding of faeces
- no taeniae coli in muscularis externa
• Anal canal
- controls movement of excretion
- internal sphincter: smooth muscle, central control
- external sphincter: striated muscle, voluntary control, pudendal nerves

Question 9

What are appendices epipolicae?

Answer 9

• Fatty tags
• Arise from serosa
• No function

Question 10

What are the taeniae coli?

Answer 10

• Continuous longitudinal muscle
• Colon has 3 bands
• Equally spaced around circumference
• Thicker than usual longitudinal muscle of GIT
• Shorter than colon - causes sacculation of colon forming haustra

Question 11

Describe the circular muscles

Answer 11

• Segmentally thickened

* Bundles of muscles from the taeniae coli penetrate the circular muscles at irregular intervals - keeps them together

Question 12

How is the lymphoid tissue found in the large intestine?

Answer 12

Solitary nodules

Question 13

Outline the reabsorption of substances in the large intestine

Answer 13

• Reabsorption of water and ions in the proximal colon
- chyme is mostly fluid
• 1500ml reabsorbed a day
• >4500ml in colon = diarrhoes
• Sodium and chloride absorbed by exchange mechanisms
• Water absorbed by osmosis
• Potassium absorbed passively via gap junctions

Question 14

What are the features of the colon wall and their functions?

Answer 14

• Enterocytes - short, irregular microvilli, intracellular machinery, salt reabsorption
• Goblet cells - secrete mucus - facilitates passage, protection
• Colonic crypts: stem cells at bottom, dominated by goblet cells
• No Paneth cells - due to commensal bacteria
• Less enteroendocrine cells
• Mucosa - smooth, no villi, less reabsorption/none of glucose
• Glycocalyx - meshwork of glycoproteins on microvilli, surface for adsorption/enzymes (no digestive brush border enzymes)

Question 15

Describe and explain the motility in the proximal colon

Answer 15

• Antipropulsive contractions
• Impede propulsion
• Keeps chyme for longer to promote absorption
• Kneading process

Question 16

Describe the motility in the transverse/descending colon

Answer 16

• Localised segmental contractions of circular muscle
• Haustral contractions
• Shuffle content forwards and backwards
• Presents chyme to mucosa for absorption
• Short propulsive movements every 30 minutes

Question 17

What is mass movement?

Answer 17

• Intense, prolonged peristaltic contraction
• 1 to 3 times a day
• Moves content 3/4 of the way
• High fibre foods promote this

Question 18

Summarise defaecation

Answer 18

• Rectum fills with faeces sitting on rectal folds
• Stored until voided
• Distension in rectal wall
- internal anal sphincter relaxed, spinal reflex in sacral spinal cord
- signals sent to myenteric plexus, peristaltic waves in descending/sigmoid colon & rectum
• Resisting the urge - subsiding of sensation until next mass movement
• Last few cm of rectum can distinguish state of faeces
• 150g of faeces produced a day (healthy)

Question 19

What is the net weight of microbiota in the large intestine

Answer 19

1500g

Question 20

What are the most prevalent types of bacteria in the large intestines?

Answer 20

• Bacteroidetes (phylum)

• gram negative
• anaerobic
• non-spore forming

Question 21

How does the microbiota change throughout your lifetime?

Answer 21

• Born sterile
• Peak at 12-18 months
• Declines with age

Question 22

Describe the role of the microbiota in the large intestine

Answer 22

• Symbiotic relationship
• Diverse, highly metabolically active
• Synthesise and excrete Vitamin K (important for coagulation)
• Prevent pathogen colonisation via competition - antagonise unnatural bacteria
• Stimulate development of some tissue
• Stimulate production of antibodies
• Loss of commensal bacteria => predisposed infection