14 Na and Water

  • Abnormal water balance as result of abnormal ADH: hypo/hypernatremia (SIADH, diabetes insipidus)
  • Abnormal Na balance result of abnormal RAAS/SNS: hypo/hypervolemia (HF, hemorrhage)

Effective Circulating Volume

  • increase volume, usually increase ECV
  • increase CO, increase ECV
  • resistance: low resistance, low ECV

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  • cirrhosis: low SVR due to vasodilation

ADH

  • only at very low level
  • when activated at low ECV, can increase water reabsorption too much = hyponatremia

Water Balance

  • any water intake must be excreted to balance normal Na

  • starting point same level as stopping point

Na Balance

  • water intake by itself only transiently increase ECV. Very quickly restored by decrease ADH
  • Na intake increase ECV but not restored to normal level

  • abnormal high Na ingestion > increase osm > increase ADH
  • osmolarity fixed, but ECV goes up
  • fix Na concentration but not Na balance
  • how to fix? RAAS, sympathetic

  • decreased SNS/RAAS balances in vs out for Na/H2O but does not drive down ECV volume to original, only maintains it to have balanced Na/water
  • body continue on higher ECV as long as Na consumption high
  • only thing return to low ECV: lower Na consumption
  • significance: HTN pts have high ECV volume (hypervolemia) despite Na/water balanced

  • hypo/hypernatremia: problem of water balance, not Na balance

Pathology

  • GI loss of Na and water
  • very easy to have hyper/hyponatremia

  • result: hypovolemic, hyponatremic

  • Na excretion always reduced
  • if eat a lot of Na, intake more than excretion

  • often drink more free water than needed
  • result: HF pts hypervolemic and hyponatremic

  • result: SIADH pts euvolemic, hyponatremic