What is the pathophysiology of hypovolemic hypotonic hyponatremia?

Updated: Jun 17, 2019
  • Author: Eric E Simon, MD; Chief Editor: Vecihi Batuman, MD, FASN  more...
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This usually indicates concomitant solute depletion, with patients presenting with orthostatic symptoms. The pathophysiology underlying hypovolemic hypotonic hyponatremia is complex and involves the interplay of carotid baroreceptors, the sympathetic nervous system, the renin-angiotensin system, antidiuretic hormone (ADH; vasopressin) secretion, and renal tubular function. In the setting of decreased intravascular volume (eg, severe hemorrhage or severe volume depletion secondary to GI or renal loss, or diuretic use) owing to a decreased stretch on the baroreceptors in the great veins, aortic arch, and carotid bodies, an increased sympathetic tone to maintain systemic blood pressure generally occurs.

This increased sympathetic tone, along with decreased renal perfusion secondary to intravascular volume depletion, results in increased renin and angiotensin excretion. This, in turn, results in increased sodium absorption in the proximal tubules of the kidney and consequent decreased delivery of solutes to distal diluting segments, causing an impairment of renal free water excretion. There also is a concomitant increase in serum ADH production that further impairs free water excretion. Because angiotensin is also a very potent stimulant of thirst, free water intake is increased, and, at the same time, water excretion is limited. Together, these changes lead to hyponatremia.

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