9. Large intestine Flashcards
What does the large intestine consist of?
- Appendix
- Caecum
- Colon (ascending, transverse, descending, sigmoid)
- Rectum
- Anal canal
What are the main functions of the large intestine?
- Reabsorption of water and electrolytes
- Eliminate waste
- Microenvironment for gut bacteria
What is the size of the large intestine?
- Length - 150cm
* Width - 6cm
Which arteries supply the large intestine?
• 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
Describe the innervation of the large intestine
- Parasympathetic system (vagus nerve) - ascending and transverse colon
- Pelvic nerves - distal colon
- Sympathetic system arises from lower thoracic and upper lumbar spinal cord
What separates the distal ileum from the caecum?
• Ileocecal valve
- muscular sphincter
- tonically active/constricted
- relaxes to allow passage of fluid chyme
- prevents microbiota entering ileum
Outline the anatomy of the colon
• 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
Describe the rectum and anal canal
• 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
What are appendices epipolicae?
- Fatty tags
- Arise from serosa
- No function
What are the taeniae coli?
- 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
Describe the circular muscles
- Segmentally thickened
* Bundles of muscles from the taeniae coli penetrate the circular muscles at irregular intervals - keeps them together
How is the lymphoid tissue found in the large intestine?
Solitary nodules
Outline the reabsorption of substances in the large intestine
• 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
What are the features of the colon wall and their functions?
- 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)
Describe and explain the motility in the proximal colon
- Antipropulsive contractions
- Impede propulsion
- Keeps chyme for longer to promote absorption
- Kneading process
Describe the motility in the transverse/descending colon
- 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
What is mass movement?
- Intense, prolonged peristaltic contraction
- 1 to 3 times a day
- Moves content 3/4 of the way
- High fibre foods promote this
Summarise defaecation
• 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)
What is the net weight of microbiota in the large intestine
1500g
What are the most prevalent types of bacteria in the large intestines?
• Bacteroidetes (phylum)
- gram negative
- anaerobic
- non-spore forming
How does the microbiota change throughout your lifetime?
- Born sterile
- Peak at 12-18 months
- Declines with age
Describe the role of the microbiota in the large intestine
- 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
