LEC 25: DIGESTIVE PHYSIOLOGY AND KIDNEYS

• General processes of digestion and absorption

  1. Digestion

  2. Absorption
• Digestion


  • Occurs mainly in small intestine
  • Enzymes break down larger molecules into monomers
• Carbohydrate digestion


  1. Carbohydrate digestion begins in mouth as salivary amylase splits starch

  2. In intestine, pancreatic amylase breaks down starch and glycogen

  3. Brush border enzymes break oligosaccharides and disaccharides into monosaccharides

  4. Monosaccharides are co-transported across apical membrane and exit by facilitated diffusion
• Protein digestion


  1. Pepsin secreted by chief cells begins hydrolysis of denatured proteins in stomach

  2. Pepsin in stomach & pancreatic proteases break proteins into smaller fragments and amino acids

  3. Brush border enzymes break oligopeptides and dipeptides into amino acids
• Lipid digestion

  1. Small intestine is primary site for lipid digestion

  2. Bile salts emulsify fat

  3. Pancreatic lipase breaks emulsified fats into monoglycerides and free fatty acids

  4. Monoglycerides and fatty acids combine to form micelles

  5. Monoglycerides and fatty acids are converted back to triglycerides, combined with lecithin, cholesterol, and other lipids, to form chylomicrons
• Nucleic acid digestion


  1. Pancreatic nucleases hydrolyse nucleic acids to nucleotide monomers

  2. Brush border enzymes, nucleosidases, and phosphatases break nucleotides down
• Vitamin absorption


  1. Most absorbed in small intestine

  2. Water soluble vitamins absorbed by diffusion or passive/active transport

  3. Fat-soluble vitamins carried by micelles of lipids and diffuse into absorptive cells
• Absorption


  • Molecules enter epithelial cells across apical membrane
• Carbohydrate absorption


  Monosaccharides absorbed across apical membrane
• Protein absorption


  Amino acids co-transported across apical membrane
• Lipid absorption


  Monoglycerides and fatty acids absorbed into mucosal epithelial cells
• Nucleic acid absorption


  Breakdown products actively transported
• Vitamin absorption


  Large intestine absorbs vitamins K and B7 made available through bacterial metabolism
• Electrolyte absorption


  Actively absorbed along entire length of small intestine, except calcium in duodenum
• Transport to bloodstream

  Nutrients transported across basolateral membrane into bloodstream, travels to tissues and organs
• Enzymes involved in digestion

  • Amylase acts on carbohydrates (starches), breaks down into simple sugars
  • Lipase acts on lipids (fats), breaks down into fatty acids and glycerol
  • Protease acts on proteins, breaks down into amino acids
• End products of digestion

  • Protein digestion: Amino Acids
  • Fat digestion: Fatty acids and glycerol
  • Carbohydrate digestion: simple sugars (e.g glucose)
  • Nucleic acid: nucleotides
• Anatomy of the kidney and its coverings

  • Renal capsule surrounds kidney, helps maintain shape and protection
  • Perirenal fat surrounds renal capsule, provides cushioning and insulation
  • Renal fascia encloses perirenal fat, anchors kidney to surrounding structures
  • Kidney covered by parietal peritoneum
• Kidney functions


  • Regulating total blood volume and solute concentration
  • Regulating ion concentrations in extracellular fluid
  • Ensuring long-term pH balance
  • Excreting metabolic wastes, toxins and drugs
  • Producing erythropoietin and renin, activating vitamin D
• The right kidney is lower than the left due to crowding by the liver
• Adrenal gland sits on top of kidney
• Renal nerve plexus regulates renal blood flow and influences urine-forming role of nephrons