Large Bowel

Question 1

What is the function of the large bowel?

Answer 1

• Reabsorption of electrolytes and water
• Elimination of undigested food and waste

Question 2

What is the structure of the large bowel?

Answer 2

• 1.5m long
• 6cm diameter
  1. Appendix —> end of caecum
  2. Caecum —> connects to ileum
  3. Ascending colon —> up
  4. Hepatic flexure —> connects ascending and
  transverse
  5. Transverse colon —> across
  6. Splenic flexure —> connects transverse and
  descending
  7. Descending colon —> down
  8. Sigmoid colon —> end
  9. Rectum
  10. Anal canal

Question 3

What are the 4 sections of the large bowel?

Answer 3

1. Ascending - caecum to hepatic flexure
2. Transverse - hepatic to splenic flexure
   - connected to greater omentum
3. Descending - splic flexure to sigmoid
4. Sigmoid colon - descending to rectum

Question 4

What is a volvulus?

Answer 4

Twist in colon —> obstruction
- in regions that are freely hanging (alternates)
—> caecum
—> transverse colon
—> sigmoid colon

Question 5

What are the 3 regions of the large bowel?

Answer 5

1. Upper —> intraperitoneal = in peritoneal cavity
2. Middle —> retroperitoneal = behind
3. Lower —> extraperitoneal = outside

Question 6

How is blood supplied to the large bowel?

Answer 6

Superior mesenteric - middle colic artery
—> to 2/3 traverse colon
Inferior mesenteric —> rest of large bowel

• split —> midgut and hindgut during embryological
  development
  —> sensitive to ischaemia

Question 7

How does lymphatic drainage occur in the large bowel?

Answer 7

Lymph vessels run along arteries —> superior mesenteric nodes
+ solitary nodules

Question 8

What are appendices epiploicae?

Answer 8

Fatty tages on the anterior mesenteric border of bowel peritoneum

Question 9

What is the function of the caecum?

Answer 9

Absorption —> electrolytes - Na+, Cl-
- K+ —> passive
—> water - up to 4.5L (more —> diarrhoea)
—> osmosis following ions
- most in proximal ∵ chyme enters as liquid

Question 10

What are haustra?

Answer 10

Small pouches of colon —> segmented/folded
∵ taenia coli shorter than colon —> purse-stringing

Question 11

What is the rectum?

Answer 11

• Dilated distal portion —> stores faeces
• Tranverse rectal folds in submucosa
• No taenia coli in muscularis externa

Question 12

What is the anal canal?

Answer 12

Terminal portion —> channel between rectum and anus
- Folded —> 2 layers
- internal circular muscle —> internal sphincter
- external striated muscle —> external sphincter
- Crohn’s —> fistula between internal and external
sphincter —> watering can peritoneum

Question 13

What are the 4 layers of the large bowel?

Answer 13

1. Mucosa —> innermost
2. Submucosa
3. Muscularis
4. Serosa

Question 14

What is the structure of the large bowel mucosa?

Answer 14

Folded but no villi —> appears smooth:
- Projections
- enterocytes - absorbs salts
- microvilli —> short, irregular
- glycocalyx coat —> no digestive
enzymes
—> sugars trap untirred
layer (protection)
- Crypts
- goblet cells - mucus secretion —> lubrication
- ACh stimulates (PNS)
- many more —> inc along colon
- stem cells - replace dead cells

Question 15

What is the structure of the large bowel muscularis?

Answer 15

• Circular —> inner
  - segmentally thickened
  - hautra can contract individually
• Longitudinal —> outer
  - 3 bands —> teniae coli (penetrate
  circular layer irregularly)

Question 16

How does movement of faeces occur in the large bowel? (2)

Answer 16

Segmental = contraction of each haustrum (circular)
- every 30mins
- propels faeces 5-10 cm/hr propulsion (inc after meal)
- proximal colon —> anti-propulsive to retain chyme
- facilitated by taeniae coli

Mass movement = forceful peristaltic waves
- 1-3 times a day —> triggers defection (sigmoid colon)
- propels faeces 1/3 to 3/4 the length of the large bowel
- fibre promotes rapid transport

• distended colon —> muscle too stretched —> no
  contraction —> no peristalsis —> backflow

Question 17

How does nervous control of large bowel occur? (2)

Answer 17

Extrinsic —> long reflex:
- parasympathetic - vagus (prox) and pelvic (dist) nerves
—> promotes digestion
- sympathetic - lower thoracic and upper lumber spinal
nerves
—> inhibits digestion
- somatic - anal canal - pudenal nerves (70% somatic +
30% autonomic motor fibres)

Intrinsic —> short reflex:
- enteric - submucosal (meissners) plexus
- myenteric plexus (below taenia coli)
- operates itself but also affected by ANS

Question 18

How does defecation occur?

Answer 18

1. Faeces in rectum via sigmoid colon mass movement
   —> rectal wall distension
2. Pressure receptors sense —> signals via myenteric
   plexus + sacral spinal cord
3. Peristaltic waves in descending colon, sigmoid colon,
   rectum + internal anal sphincter inhibited —> subsides
   urge to defecate
4. Voluntary control of external anal sphincter —>
   defecation

Question 19

What is Hirschsprung’s disease?

Answer 19

Nerve cells in the intestines don’t develop normally before an infant is born (congenital) —> problems with passing stool

Question 20

What are the 6 components of faeces?

Answer 20

1. Water —> 2/3
2. Cellulose
3. Bacteria —> odour (fermentation)
4. Cell debris
5. Bile pigments —> colour
6. Salts
   - 150g/day
   - melena = excretion of digested blood when bowel wall
   bleeding/ulcerated

Question 21

What are the 6 functions of the gut microbiome?

Answer 21

Microbiome = diverse, highly metabolically active
community of live bacteria (1.5 kg)
1. Synthesise and excrete vitamins (eg. K)
2. Prevent infection - outcompete and inhibit/kill
pathogenic
- stimulate cross-reactive antibody
production
3. Stimulate tissue development (eg. caecum, lymph)
4. Breakdown fibre
5. Energy supply from short chain fatty acids produced
6. Hormonal regulation via short chain fatty acids

Question 22

What are the 2 types of bacteria in the gut microbiome?

Answer 22

1. Bacteroides —> gram-negative
   - anaerobic and non-sporeforming
   - most prevalent
2. Bifidobacteria —> gram-positive
   - lactic acid and non-sporeforming
   - prevent pathogen colonisation —>
   ‘friendly’