Renal physio 2

• Reabsorption


  Process by which solutes are taken up from the tubular fluid back into the body
• Reabsorptive mechanisms


  • Primary active transport
  • Secondary active transport
  • Carrier-mediated transport
  • Passive diffusion
• Majority of reabsorption occurs in proximal tubule (~60-65%)
• Transcellular pathway

  Carrier-mediated reabsorption

  
• Paracellular pathway


  Passive diffusion reabsorption
• Na+/K+-ATPase


  Drives most solute reabsorption in proximal tubule

  
• Renal threshold


  Concentration in plasma below which none appears in urine, above which progressively larger quantities appear

  
• Tubular maximum (Tm)


  Maximum capacity of kidney to reabsorb a substance; highest rate at which tubules can transfer a substance before it may be excreted

  
• If a substance is filtered beyond its Tm value it will be reabsorbed but will also be excreted in the urine
• Normally glucose is filtered below its Tm, therefore all is reabsorbed but in diabetes mellitus, glucose is filtered more than its Tm, therefore excreted in the urine
• Specific Na+ dependent transporters

  • Glucose - SGLT1, SGLT2
  • Amino acids - EAAT3, SIT1
  • Phosphate - NaPi2a, NaPi2c
  • Sulfate - NaS1
  • Citrate - NaDC1, NaDC3

  
• HCO3- reabsorption


  1. Na+ gradient drives Na+/H+ counter-transport via NHE3

  2. H+ combines with filtered HCO3- to form H2O and CO2, catalyzed by carbonic anhydrase

  3. CO2 enters facilitated by AQP1

  4. Carbonic anhydrase catalyzes hydroxylation of CO2 to form H+ and HCO3-

  5. HCO3- crosses basolateral membrane via NBCe1 and Na+-dependent HCO3-/Cl- exchanger

  6. Majority of H+ transported into tubule fluid via NHE3 and H+-ATPase

  
• Cl- reabsorption is also indirectly powered by the Na+/K+-ATPase pump and occurs via both paracellular and transcellular routes

  
• ~65% of filtered Ca2+ is reabsorbed, ~90% is paracellular due to favourable electrochemical gradient in late proximal tubule and solvent drag
• Majority of K+ reabsorption occurs by passive mechanisms, mainly via the paracellular route
• Peptide reabsorption


  Hydrolyzed at luminal brush border, amino acids reabsorbed by co-transport with Na+
• Low molecular weight protein reabsorption

  Reabsorbed via endocytosis, then degraded by cellular lysosomes to amino acids
• Amino acid transport


  Move from cell to peritubular space by facilitated diffusion
• Descending limb of loop of Henle

  • Low epithelium, few mitochondria and membranous infoldings, high permeability to H2O, little to no permeability to Na+, Cl- and urea
• Ascending limb of loop of Henle

  • Taller epithelium, many mitochondria and basolateral infoldings, active transport of solutes, main site of Na+ reabsorption, ~25% K+ reabsorption, impermeable to water

  
• Distal convoluted tubule


  • Even taller epithelium, dense array of mitochondria, ~80% of water reabsorbed, impermeable to water unless ADH present (late segment), reabsorbs Na+, Cl-, Ca2+

  
• Urine characteristics


  • Colour: Pale yellow - light amber
  • Turbity/cloudiness: None - slightly cloudy (Horses – cloudy (calcium carbonate crystals) and foamy (mucus))
  • pH: 6.3-6.6 (cats), 7.0-7.5 (dogs), 7.5-8.5 (horses)
  • Urine specific gravity: Concentrated if the USG is > 1.030 (dog) or >1.035 (cat), Dilute if the USG is < 1.008
• What is the significance of medullary hypertonicity 


  This helps with reabsorption of water by maintaining concentration gradient due to Nacl ,urea and H2O