Large intesine

• Large Intestine


  • 1.5m long
  • Wider in diameter
  • More distensible than small intestine
  • Larger capacity
  • Long flat muscular bands (taenia coli) which draw colon into saccules
• Parts of the Large Intestine


  • Caecum
  • Colon
  • Rectum
• Caecum


  Starts at junction of small intestine at ileo-caecal valve, forms pouch to which appendix attaches
• Colon


  Ascending, transverse and descending portion
• Rectum


  Continues as anal canal opening as anus
• Descending colon empties faeces into rectum (stores stool until defaecation)
• Anus


  Surrounded by fascia and muscles (internal and external anal sphincters)
• Functions of the Large Intestine


  • Receives and stores material from small intestine
  • Water and salts are absorbed
  • Bacteria produce certain B-complex vitamins
  • Forms and propels feces toward the rectum for elimination
• Gut microbiome


  • Metabolism: digests non-digestible CHO (fermentation) → Short chain fatty acids → used as energy by colon cells/ absorbed into blood to increase lipogenesis
  • Gut Morphology: increase mucus, reduce cell permeability, increase vascular flow, increase tissue repair → promote barrier integrity
  • Immune system: influence maturation and development of lymphoid tissue; Increases IgA; influence composition of T cells in lamina propria
  • Nervous system: influence synaptic connectivity
  • Bone Homeostasis: increase uptake of calcium in gut
• Motility in the large intestine


  • Mixing and peristaltic propulsive movements occur and are similar to small intestine, but usually more sluggish
  • Mixing movements facilitate absorption of water and electrolytes
• Peristalsis in the large intestine


  • Less frequent than small intestine
  • Forward movement -> unique waves cause colonic contraction -> mass movement
  • Occurs 3–4 times a day -> associated with eating and distension of the stomach through the gastrocolic reflex
  • Mass movement -> causes sudden distension of the rectum -> triggers defecation
• Factors controlling contractile activity of the colon


  • Intrinsic smooth muscle properties
  • Intrinsic nerves
  • Extrinsic nerves
  • Circulating or locally released chemicals
• Parts of test meal reach different parts of the large intestine at different times

  
• Defecation Reflex


  • Removes undigested feces from the body
  • A spinal reflex triggered by distension of the organ wall
  • The movement of fecal material into the normally empty rectum triggers the reflex
  • Involves sacral spinal cord and can occur independently of higher nervous control
  • Impulses elicited in rectum are transmitted to spinal cord and then back to colon and rectum via sacral parasympathetic fibres (nervi erigentes)
  • Subject to emotional influence
• Other responses also occur during defaecation in response to afferent impulses from rectum to spinal cord and brain
• Faeces


  • Contain inorganic material
  • Bile pigments
  • Undigested plant fibres
  • Bacteria
  • Water
• Constipation


  • Bowel movement occurs once every 2 to 3 days or up to 3 times a day
  • Less than 3 times per week is constipation
  • If faeces in colon too long becomes dry as water absorbed
  • Passing of small hard stools can be difficult
  • Causes: prolonged inhibition of the defaecation reflex, lack of tone in smooth muscle fibres of colon and spasms of muscle layer of large intestine
• Diarrhoea


  • When faeces move faster than usual through the colon: retains liquid
  • Causes are hypersecretion of fluids and abnormally frequent peristaltic waves caused by emotional stress, chemicals and infection
  • Can be fatal
  • Na+, K+ and water loss
  • Causes dehydration, hypovolaemia and shock with cardiovascular collapse
• Pain related to the large intestine


  • Caused by digestive disorder: can be due to strong contraction of muscle, stretching of organ, chemical or mechanical irritation, inflammation of peritoneum or direct irritation of nerves
  • May be referred to a remote site e.g. from peptic ulcer to back upper area
• Appendix


  • An extension of bowel
  • Narrow pouch of tissue made up of lymphoid tissue in submucosa
  • Made of an inner layer of mucosa with submucosa, muscularis, and serosa layers
  • Remnant of the caecum to digest cellulose OR storage area for beneficial bacteria