BI1002 - Urine Formation

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  • Created by: Gemma
  • Created on: 04-02-14 18:47

Step 1 = ULTRAFILTRATION

  • Plasma is forced throught the glomerular capilleries by hydrostatic pressure of blood into Bowman's space
  • Small molecules and ions pass though - proteins and blood cells stay in capilleries
    • Capillery endothelium pores (fenestrae) = ~100 nM
    • Basement membrane = -ve charged (contributes to filtration)
    • Tubular pores = ~8nM
      • Contain cells called podocytes with finger-like projections called pedicells
  • Fluid formed = glomerular flitrate
  • To find the composition of tubular fluid = micropunture method
    • Use a micropipette to get the contents of the tubule - previously done on turtles
  • Autoregulation of renal blood flow (RBF) and glomeruslus filtration rate (GFR)
    • Need to match the transport capacity to the filteted load - glomerulotubular balance
    • Myogenic control = stretch receptors 
      • Increase in pressure = stretch in smooth muscle which causes Ca+ entry and the muscle contracts - reducing blood flow
      • Humoral factors prostaglandins and nitric oxide so act as vasodilator
      • Macula densa also aids in maintains the right rate
    • Metabolic control
      • Metabolites (H+, adenosine) cause vasodilation. But an increase in pressure = a reduction in metabolities so vasocontricition happens 
  • Forces to produce glomerulus filterate:
    • Starling forces (hydrostatic pressure) acts to force plamsa out the capilleries 
    • Proteins cannot fit through pores so exert oncotic pressure oppose the starling forces 
    • Small pressure from capsule that opposes hydrostatic pressure
    • Net filtration pressure = 2.5 kPa
  • Plasma Clearance = the volume of plasma that is cleared of a substance in a given time
    • 125ml filtrate/min = 180l/day BUT only 1.5 L/day
    • Use inulin to determine GFR
      • freely filtered, not reabsorbed, no secreted, not broken down or synthesised
      • rate of excretion = Uc x Vmg.min-1
      • rate of excretion = filtered load = rate of filtration = Pc (plasma) x GFR
      • Uc x V = Pc x GFR therefore GFR = (Uc x V)/Pc
      • GFR measured by the inulin = inulin clearance
    • Inulin = filtration only (125 ml/min GFR)
    • PAH (para-amiohippuric acid) = filtration and secretion (600 ml/min GFR)
    • Glucose = filtration and reabsorption (0 ml/min GFR)

Step 2 = REABSORBTION (PROXIMAL TUBULE)

  • Tubular reabsorbtion = the movement of solutes back into the blood of a peritubular capillery
  • Proximal tubule reabsorbs = 
    • 2/3rds of filtered H2O, Na+, K+, Cl- and HCO3-
    • All filtered glucose and amino acids
    • Reabsorbtion of Na+ is a driving force for reabsorbtion of most solutes
  • Reabsorbtion mechainism for HCO3-
    • Bicarbonate is freely filtered so must be reabsorbed
    • Enzyme carbonic anhydrase:
      • Exists in 2 forms = brush border and intracellular
    • Brush border = Na+-H+ antiporter exchanges Na+ (tubular fluid) for H+ (intracellular)
    • This results in secretion of H+ into the membrane = shift in the equilibrium towards carbonic acid
    • Carbonic acid is converted into CO2 and water by enzyme carbonic anydrase (in the brush border)
      • H2O

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