Renal Phys 6

• water processing

  intrinsic
• water regulation

  extrinsic
• coupling of H2O reabsorption to Na+ reabsorption by osmosis


  image:
• paracellular route


  - main water route in proximal tubule

  - 65% reabsorption occurs here and is non-regulated
• transcellular route


  main water route in collecting ducts where it requires aquaporins = regulated by ADH
• reduces urine loss

  ADH
• regulation and function of aquaporins in water reabsorption

  ADH (vasopressin) induces aquaporin (AQP2) insertion into apical/luminal membranes of the collecting duct

  
• Na+ reabsorption is high and constant (not-regulated)


  proximal tubule
• H2O permeability is high and constant (not-regulated)


  proximal tubule
• reabsorption of Na+ and H2O are coupled


  proximal tubule
• H2O reabsorption is high and constant (not-regulated); no Na+ reabsorption


  descending Loop of Henle
• Na+ reabsorption is high and constant (not-regulated); no H2O reabsorption 


  ascending Loop of Henle
• Na+ reabsorption is variable, is regulated by aldosterone, and is NOT directly coupled to water reabsorption


  collecting duct
• this is variable, is regulated by ADH, and requires the renal medullary gradient (which is partly produced by the Loops of Henle of juxtamedullary nephrons)


  water permeability
• the renal medullary gradient

  - only the juxtamedullary nephrons with long Loops of Henle create and maintain this osmolarity gradient

  - ultimately ALL nephrons flow through it and thus rely on it via the common collecting ducts

  
• what two mechanisms create the renal medullary gradient?

  countercurrent multiplier system and urea recycling
• what mechanism maintains the renal medullary gradient?

  vasa recta circulation
• active transport of NaCl (reabsorption) from the ascending limb; water CAN NOT follow


  countercurrent multiplier system of juxtamedullary nephrons
• diffusion of water via osmosis (reabsorption) from the descending limb makes filtrate more concentrated


  countercurrent multiplier system of juxtamedullary nephrons
• the countercurrent multiplier system diagram


  image:

  
• urea recycling traps urea in the inner medulla helping to establish and maintain the renal medullary gradient


  reabsorbed from the inner medulla, secreted into Loop of Henle

  
• percent of urea excreted from urea recycling

  15%
• ____ mOsm is highest possible concentration at bottom of medulla

  1400

  • 700 mOsm comes from NaCl reabsorption
  • 700 mOsm comes from urea recycling

  
• ascending limb is ___________ to water


  impermeable
• recycling of urea from the collecting duct is promoted by:

  ADH (vasopressin)
• what returns reabsorbed water and solutes to the plasma without washing out the gradient in medullary circulation?

  the vasa recta

  
• solutes keep getting exchanged between ascending and descending vasa recta and are trapped in the medulla


  the medullary circulation
• weak or strong blood flow in vasa recta helps to maintain the medullary circulation gradient?


  extremely weak
• in medullary circulation, what structure is most involved in reabsorption?

  peritubular capillaries in the cortex
• because of the hyperosmotic medullary ISF...


  we can produce very small volumes of highly concentrated urine in the presence of ADH

  
• if you're overhydrated, does ADH increase or decrease?

  decrease; stops altogether
• ADH increases _____ reabsorption and ____ reabsorption; net result: even more ___ reabsorption!


  water; urea; H2O
• overhydration


  image:

  
• decreased osmolarity =


  increased solute volume