GI Phys 5

• longest portion of GI tract, 9-12 feet, and coiled

  small intestine
• begins at pyloric sphincter (muscular gateway)


  small intestine

  • duodenum
  • jejunum
  • ileum
• ends at ileocecal sphincter/valve


  small intestine
• large surface area of SI increases capacity of:

  - digestion

  - secretion

  - absorption
• coiled length, circular folds, villi, microvilli (brush border)


  components of large surface area of SI
• Si layers


  mucosa

  • submucosa

  muscularis externa

  • inner circular layer
  • outer longitudinal layer

  serosa
• enteroendocrine cells secrete


  CCK, secretin, GIP
• where does digestion and absorption actually occur?

  microvilli (plasma membrane folds)

  • this is where nutrients move from the GI lumen into epithelial cells
  • move from outside body to inside the body

  
• secretions into SI lumen:

  1.) 1500 mL/day of water, mucus, lots NaCl

  • NaCl important for 2ndary active transport (i.e. absorption of solutes and nutrients)

  2.) 500 mL/day of bile from liver and gall bladder

  • digest and absorb fats

  3.) 1500 mL/day of pancreatic secretions

  • bicarb to neutralize pH (HCl entering from stomach)
  • enzymes to digest and absorb nutrients
• secretion into SI lumen are all ________

  EXOCRINE
• stores bile

  gallbladder
• collective bile ducts

  common hepatic ducts
• gall + liver duct

  common bile duct
• connects main pancreatic duct to duodenum

  sphincter of oddi
• what two organs connect to small intestine at duodenum as accessory organs?

  liver and pancreas
• liver and pancreas secretions are important for:

  GI functions
• exocrine parts of the pancreas

  acinar cells = secretes enzymes

  duct cells = secretes bicarb
• endocrine parts of the pancreas

  islet cells = insulin & glucagon
• what is the Cl- transporter from the pancreatic duct cell and duct lumen?

  CFTR (cystic fibrosis transmembrane conductance regulator)
• What is the pH of the blood leaving pancreatic capillaries around duct cells?

  acidic (opposite effect of parietal cells)
• • break peptide bonds in proteins to form peptide fragments
  • substrate: proteins


  trypsin, chymotrypsin, elastase

  in pancreas
• • splits off terminal amino acid from the carboxyl end of protein
  • substrate: proteins


  carboxypeptidase

  in pancreas
• • splits off two fatty acids from triglycerides, forming free fatty acids and monoglycerides
  • substrate: triglycerides


  lipase

  in pancreas
• • splits polysaccharides into maltose
  • substrate: polysaccharides


  amylase

  in pancreas
• • split nucleic acids into free nucleotides
  • substrate: nucleic acids


  RNA, DNA
• regulation of pancreatic duct secretion

  cascade of increased acid from stomach

  
• regulation of acinar cell secretion

  cascade of increased fatty acids and amino acids

  
• four roles of CCK

  1.) inhibit gastric emptying

  2.) Increase enzyme secretion from the pancreas

  3.) gallbladder contraction

  4.) relaxation of the sphincter of oddi
• • made from cholesterol
  • emulsify fats
  • evenly distribute fats in watery environments
  • reabsorbed in distal ileum
  • contains bilirubin


  bile salts
• digestion & absorption of carbohydrates in the small intestine


  combination of enzymes in brush border and from pancreas do most of the digesting

  • goal: breakdown macroscopic carbs into absorbable monosaccharides

  
• digestion & absorption of proteins in the small intestine


  combination of peptidases in brush border, from pancreas, and epi cell cytosol

  • goal: breakdown macroscopic proteins and peptides into absorbable amino acids

  
• digestion & absorption of lipids in the small intestine


  ingestion of triglycerides -> breakdown triglycerides -> resynthesize triglycerides

  
• ingestion of triglycerides


  emulsification: bile salts

  • smaller size
  • increased surface area
• breakdown triglycerides


  digestion: pancreatic lipases
• absorbable units: fatty acids & monoglycerides

  diffuse into epithelial cells
• resynthesize triglycerides


  combine with amphipathic proteins

  • form chylomicrons
  • exocytosis into lacteals
  • packaging of proteins
  • not directly to the liver!
• • contained in chylomicrons
  • move into lacteals of GI tract
  • flow into sequentially larger lymph vessels
  • bypass the liver
  • enter veins via the thoracic duct near the heart before being processed


  triglycerides and fat-soluble vitamins
• absorption of hydrophilic substances from the SI

  capillaries from:

  • stomach
  • pancreas
  • SI
  • LI
  • ^ all feed into hepatic portal vein
• goes to liver & branches into a second capillary bed = portal system (for processing)


  hepatic portal vein
• the liver capillaries drain into:

  hepatic veins -> inferior vena cava

  • sends nutrients back to systemic circulation