14 Na And Water

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• 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

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• 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

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• only at very low level

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

Water Balance

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• any water intake must be excreted to balance normal Na

• starting point same level as stopping point

Na Balance

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• 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

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

Pathology

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• GI loss of Na and water

• very easy to have hyper/hyponatremia

• result: hypovolemic, hyponatremic

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• 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

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• result: SIADH pts euvolemic, hyponatremic