G.I. Physiology

I.  Anatomy and Major Functions of Digestive System

 A.  Mouth
  1.  Mechanical digestion (chewing)
  2.  Saliva
   -starch digestion by salivary amylase
 
 B.  Pharynx and esophagus
 C.  Stomach
  1.  Mixing
  2.  Secretion of Hcl by parietal cells
   helps kill bacteria
   activates pepsin
  3.  Secretion of pepsinogen by chief cells
   initial protein breakdown by pepsin

  4.  Secretion of hormones
   gastrin
   CCK
   etc.
  5.  Minimal absorption (ASA, EtOH, drugs)

 D.  Small intestine
  1.  Gross anatomy=Duodenum, jejunum, ileum
  2.  Majority of digestive and absorptive processes are     enhanced by high surface area due to:
   circular folds
   villi
   microvilli (made up of brush border cells)
  3.  Major functions
   a. mixing
   b.  addition of pancreatic juices, and bile
   c.  neutralization of acids
   d.  major site of chemical breakdown and absorption    of nutrients

 E.  Large intestine
  1. Gross anatomy=cecum, appendix, colon, rectum
  2.  Major functions
   a.  water absorption
   b.  electrolyte absorption
   c.  vitamin absorption
   d.  defecation

II.  Digestion and Absorption of Nutrients

 A.  Carbohydrate
  1. Types of carbohydrates
   a. Polysaccharides (glucose polymers)
    1.  Starch
    2.  Cellulose
    3.  Glycogen
   b.  Disaccharides
    1.  Sucrose (glucose, fructose)
    2.Lactose (glucose, galactose)
    3.  Maltose (glucose, glucose)
   c.  Monosaccharides
    1.  glucose
    2.  fructose
    3.  galactose
  2.  Starch digestion
   -salivary amylase
   -pacreatic amylase
  3. Enzymes in brush border cells (S.I.)
   -dissacharidases
   -oligosaccharidases
  4.  Absorption in small intestine
   Transport across epithlial cells of small intestine
    -facilitated transport (fructose)
    -secondary active transport (glucose, galactose)
    -diffusion into blood

 B.  Protein Digestion and Absorption
  1.  Stomach
   pepsin--cleaves proteins into peptide fragments
  2.  Small intestine
   a.  Pancreatic enzymes (trypsin, chymotrypsin) cleave    peptides into smaller peptide fragments
   b.  Brush border enzymes (carboxypeptidase,      dipeptidase, aminopeptidase) cleave peptides into     single amino acids
  3.  Absorption
   a.  Amino acids absorbed by facilitated diffusion
   b.  Small % of proteins are absorbed intact
    -greater in infants which allows absorption of      maternal antibodies in milk

 C.  Lipids
  1.  Emulsification
   -bile salts (synthesized in liver and stored in gall     bladder, then secreted into S.I.) act as detergent to     break up fat into small droplets
  2.  Breakdown of triglycerides (triacylglycerol) into free    fatty acid (2) and monoglyceride by pancreatic lipase
  3.  Transport of fatty acids and monoglyceride into     epithelial cell, reformation of triglyceride
  4.  Packaging of triglyceride into chylomicron and     exocytosis into lymphatic system
  5.  Drainage into systemic veins

 D.  Water
  1.  Absorption in SI (80%) by osmosis
  2.  Absorption in large intestine by osmosis
 E.  Electrolytes
  1.  Active transport of Na+ in SI
  2.  Iron
  3.  Calcium (depends on Vitamin D)
  4.  Potassium
  5.  Bicarbonate
 F.  Vitamins
  1.  A, D, E, K are fat soluble and are absorbed like fat
  2.  Water soluble vitamins (C, B vitamins, etc) are absorbed   by diffusion or facilitated transport
   Vitamin B12 must first bind to instrinsic factor for     absorption

III.  Regulation of Digestive Processes
 A.  Overview
  Neural control by autonomic nervous system
  Neural control by enteric nervous system
  Endocrine control by GI hormones
 B.  Regulation of Gastric Secretion
  1.  Reflex (cephalic) phase
   Sensory input (sight, smell, taste), memory and     thought trigger vagal (parasympathetic activity) to     enteric nervous system
   Enteric neurons stimulate stomach glands to increase    secretion
  2.  Gastric Phase
   Distension stimulates stretch receptors--neural      response for acetylcholine release
   Chemical stimuli (proteins, caffeine, increased pH)     stimulate release  of gastrin from G cells.  Gastrin     stimulates parietal cell release of Hcl
   Other stimuli (histamine)
  3.  Intestinal phase
   Distension, decreased pH, nutrients) in small intestine trigger release of secretin, CCK, VIP and GIP which inhibit gastric secretion
 C.  Regulation of gastric motility and emptying
  1.  Peristalsis of stomach propels food toward pylorus
  2.  Pyloric sphincter opens slightly and allows small particles to pass
  3.  Process continues, squirting out small portions of stomach contents
  4.  As food enters duodenum of SI, the SI release hormones (secretin, CCK, etc) that decrease stomach motility and distension of SI also signals  increase in SNS activity which decreases stomach motility
 D.  Regulation of pancreatic secretion
  1.  Duct cell secretion of bicarbonate
   -H+ in SI causes secretin release
   -Secretin acts on pancreatic duct cells to cause bicarbonate release
   -Neutralization of acidic stomach contents that have entered SI
  2. Exocrine cells secretion of pancreatic enzymes (trypsin, chymotrypsin, carboxypeptidase)
   -Presence of protein and fat in SI stimulates CCK release
   -CCK causes release of pancreatic enzymes
  3.  Relaxation of hepatopancreatic sphincter to allow release into SI

 E.  Regulation of bile secretion
  1.  Gall bladder contraction stimulated by CCK
  2.  Bile production stimulated by secretin