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