Counter current mechanism

• Tubular secretion


  Transport of solutes from peritubular capillaries to tubular fluid, active or passive transport, H+ secreted via Na+/H+ antiporter, K+ secreted in DCT and collecting duct, ammonium ions synthesized in renal epithelial cells and diffuse into tubular fluid

  
• Urine formation


  1. Glomerular filtration

  2. Tubular reabsorption

  3. Tubular secretion

  
• Urine excretion


  • Generation of hypertonic medullary interstitium
  • Dilution of tubule fluid by thick ascending limb and distal convoluted tubule
  • Variability in water permeability of collecting duct in response to ADH

  
• Countercurrent mechanism


  1. Amplifies medullary hypertonicity initiated by active reabsorption of salt by thick ascending limb of Henle's loop

  2. Counter current multiplier - Loop of Henle

  3. Counter current exchanger - Vasa recta
• Urine formation


  • Excretion of either concentrated or diluted urine
  • Generation of a hypertonic medullary interstitium - allows excretion of concentrated urine
  • Dilution of the tubule fluid by the thick ascending limb and the distal convoluted tubule - allows excretion of dilute urine
  • Variability in the water permeability of the collecting duct in response to ADH - determines the final urine concentration

  
• Countercurrent mechanism


  1. Amplifies the medullary hypertonicity initiated by the active reabsorption of salt by the thick ascending limb of Henle's loop

  2. Counter current multiplier - Loop of Henle

  3. Counter current exchanger - Vasa recta
• Counter current multiplier - Loop of Henle

  1. H2O diffuses from the lumen of the descending thin limb into the interstitial fluid = tubular fluid increases in osmotic concentration as it descends to the inner most region of the medulla

  2. When the tubular fluid enters the ascending thin limb (permeable to solute and not to H2O), NaCl diffuses out into the inner medullary interstitial fluid and urea diffuses into the tubular fluid

  3. Continued active secretion by the ascending thick limb, concentration of tubular fluid in the descending thin limb and diffusion from the lumen of the ascending thin limb into the medullary interstitial fluid establishes an osmotic gradient

  
• Counter current exchanger - Vasa recta

  1. The vasa recta is permeable to water, salts and urea

  2. Descending limb of the vasa recta: H2O moves, via osmosis, from the plasma of vasa recta to the hyperosmotic peritubular fluid (created by counter current multiplier) and the solutes diffuse from the peritubular fluid into the vasa recta

  3. Ascending limb: Solutes diffuse back into the peritubular fluid and H2O moves, via osmosis, back into the vasa recta

  4. Net result is that the solutes responsible for medullary gradient are mostly retained in the medulla and the vasa recta carry only slightly more solutes than are brought to them

  
• What is the thick ascending loop of henle permeable to 


  It permiable to sodium , sodium moves out
• Thin descending limb loop of henle is permeable to what 


  To water, water is removed