GI tract

• Gastrointestinal tract


  The digestive system
• Digestion


  Breakdown of large food into molecules

  
• Defence is a function of the gastrointestinal tract
• Gastrointestinal tract


  • Digesting food
  • Absorbing nutrients
  • Eliminating waste

  
• Digestion


  1. Physical breakdown (chewing, peristalsis)

  2. Chemical breakdown (HCl, enzymes)
• Stomach


  Expanded section of the digestive tube, between oesophagus and small intestine. Stores food, churns food and begins digestion, produces acid chyme
• Vagal reflex


  Inhibits smooth muscle tone, allows large volume food storage
• Gastric secretions


  Mucus, lipase, pepsin, intrinsic factor
• Gastric acid secretion


  Proton pump
• Regulation of gastric acid secretion

  Gastrin–ECL–parietal cell axis

  
• Paracrine secretions


  Local hormones, secreted from cells in the mucosa, act locally
• Exocrine secretions


  Secretions from numerous glands with ducts enter the lumen of the gut, involved in digestion, lubrication and protection: mucus (salivary glands), pepsin & HCl (gastric glands), bicarbonate ions & enzymes (pancreas), bile acids (liver)
• Endocrine secretions: secretions called "hormones" synthesised by ductless glands that enter the blood stream, travel to their target tissue where they bind to specific receptors - gastrin (stomach), secretin (duodenal mucosa), pancreozymin-cholecystokinin (duodenal mucosa), insulin (pancreas)
• Absorption


  Nutrients transported across intestinal epithelium into blood or lymph
• Fluid absorption occurs in small intestine and colon
• Disorders of fluid secretion and absorption are important in pathogenesis of constipation and diarrhoea
• Excretion


  Indigestible food residues leave body in faeces, drugs and metabolic products may leave in faeces, saliva, bile, vomit
• Storage


  Proximal stomach, descending colon
• Motility


  Movements of muscular wall, movement from one region to another, storage, mechanical degradation, mixing lumen contents
• The liver is a major metabolic organ involved in carbohydrate, nitrogen and lipoprotein metabolism, as well as bile production and bilirubin excretion
• Defence


  Gut epithelium is an interface with the outside world, intestine is largest mucosal surface and lymphoepithelial organ, breaches in barrier allow "toxins" to enter blood
• Mechanisms of defence


  • Sight, smell and taste
  • Vomit reflex
  • Acid in stomach
  • Natural bacterial flora
  • Intestinal immune system (GALT)
• The gastrointestinal tract has functions of storage & digestion, motility, storage of waste, defence, exocrine secretion, endocrine secretion, paracrine secretion, and excretion
• Mucus

  • Secreted by goblet and mucus neck cell
  • Acts as a lubricant by acting as a barrier that protects the stomach and colon
• Lipase

  • Converts triglycerides to fatty acids and glycerol
• Pepsin

  • Secreted by chief cells
  • Protein digestion
• Intrinsic factor

  • Secreted by parietal cells
• salivary amylase: hydrolyses starch into dextrins and maltose
• Pepsinogen is...

  activated when added to HCl and becomes pepsin. hydrolyses proteins into large peptides
• Liver produces...

  bile. Helps with the digestion of fats through emulsifying
• Lipase

  • secreted by pancreas
  • breaks down lipids into fatty acids and glycerol
• Trypsin

  • secreted by pancreas
  • activated by enterokinase
  • breaks down large peptides into smaller peptides