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Wednesday, April 11, 2012 CC-BY-NC
Renal: Water balance

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1definition

  • osmolarity: total solute concentration of a solution
  • hypoosmotic: total solute concentraiton less than that of normal extracellular fluid (~300mOsm/L)
  • isoosmotic: totaly solute concentration equal to that of normal
  • hyperosmotic: solute concentration si greater than normal.

2renal regulation of water

  • water is readily absorbed (~99%)
    • majority at proximal tubule
    • hormal control (1/3 of absorption) at collecting duct ONLY.
      • (remember for sodium, it's at both collecting duct and distal convoluted tubule.
    • absorption mechanism : follow sodium.
      • movement of sodium increases local osmolarity in the interstitium, so water flows into the interstitium.
      • flow of wateR: vi atubular cells' membrane and tight junction.
  • when water intake is small, kidney reabsorb more , vice versa.
  • regulation at collecting duct is made possible by
    • high osmolarity of the medullary interstitium done by counter current multiplier system
    • permeability (regulated by vasopressin) of CD to water

3countercurrent multiplier system

  • mechanism of how kidney concentrate the urine at medullary collecting ducts.
  • urine concentration depends on the hyperosmolarity of the interstitual fluid.
    • presence of vasopressin promotes water diffusion out of the ducts inot the interstitial fluid.
  • medullary interstitual fuid becomes hyperosmotic through the function fo Henle's loop,
    • ...which includes proximal tubule, descending limb, ascending limb, distal convoluted tubule.
  • countercurrent multiplier system's mechanism:
    • isoosmotic fluid enter proximal tubule,
    • isomotic fluid enters descending limb, which allows water diffusion but does not reabsorb NaCl
      • water flows out of the tubule.
    • as fluid goes down the limb, it becomes more and more hyperosmotic because interstitutual fluid becomes more and more hyperosmotic
      • it's because the nearby ascending limb actively reabsorb sodium into the interestitual fluid.
    • hyperosmotic fluid enters ascending limb, which is impermeable to water but actively reabsorb NaCl.
      • Na concentration goes down as the fluid ascends in the limb.
    • fluid is isoosmotic when it's at distal convoluted tubule.
    • as fluid descends down the collectnig duct, water flows out into the interstitual space
      • because of the ascending limb, deeper in the medulla the more hyperosmotic is the interstitual fluid.
    • eventually, the urine output is very hyperosmotic because water flows out of the collecting doct.
  • btw, in addition to NaCl, urea also contributes to medullay hyperosmolarity.
  • vasa recta : blood vessel in the medulla
    • specially designed to be hairpin so it doesn't carry solute away from the interstitua fluid and destroy the hyperomolarity.
    • same shape as henle loop :hairpin --> structure prevents it from carrying the solutes away.
      • while descending limb of the vessel absorb solute
      • ascending limb spits the solute back to the medullary

4Hormonal control

  • proximal tubule is highly permeable to water.
  • water permeability in CCD and MCD (cortical collecting duct and medullary collecting duct) depends on hormonal control.
    • vasoppressin, which inserts water channels (aquaporins) into the epithelium.

4.1Vasopressin

  • a peptide hormone, aka antidiuretic hormone
  • reduce urine output and increase blood pressure
  • made by some hypothalamic neurons and released from post. pituitary.
  • coupeld to GPCR V1 (smooth muscle) and V2 ( kidney) : basically two different kinds of g-protein coupled receptors.
  • stimulates insertion of aquaporins in the luminal membrane of collecting duct.
    • promote water reabsorption in collecting duct.
    • diabetes insipidus: a condiction in which kidney cannot conserve water.
      • malfunction of vasopressin
      • loss of water in body leads to high glucose.
  • regulation of vasopressin:
    • osmoreceptor control (more important)
      • sense decrease in osmolarity, decrease vasopressin release, more urine (vice versa)
    • baroreceptor control (less sensitive)
      • more about Na regulation than water regulation
      • sense decrease in blood pressure, increase vasopressin release, less urine output (vice versa)

5Thirst

  • promoted by :
    • decrease in plasma volume (sensed by baroreceptor) --> angiotensin --> decrease in Na excretion, thus less water excreted.
    • increase in plasma osmolarity
    • dry mouth or throat
    • sweating : loss of hypoosmotic salt solution
  • inhibited by water intkae by GI tract