Renal physio 2

Subdecks (2)

Cards (47)

  • 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