Electrolytes/Acid-Base Balance

Electrolytes/Acid-Base Balance is a topic covered in the Harrison's Manual of Medicine.

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Disturbances of sodium concentration [Na+] result in most cases from abnormalities of H2O homeostasis, which change the relative ratio of Na+ to H2O. Disorders of Na+ balance per se are, in contrast, associated with changes in extracellular fluid volume, either hypo- or hypervolemia. Maintenance of “arterial circulatory integrity” is achieved in large part by changes in urinary sodium excretion and vascular tone, whereas H2O balance is achieved by changes in both H2O intake and urinary H2O excretion (Table 1-1). Confusion can result from the coexistence of defects in both H2O and Na+ balance. For example, a hypovolemic pt may have an appropriately low urinary Na+ due to increased renal tubular reabsorption of filtered NaCl; a concomitant increase in circulating arginine vasopressin (AVP)—part of the defense of effective circulating volume (Table 1-1)—will cause the renal retention of ingested H2O and the development of hyponatremia.

 OsmoregulationVolume Regulation
What is sensedPlasma osmolalityArterial filling
SensorsHypothalamic osmoreceptorsCarotid sinus
  Afferent arteriole
EffectorsAVPSympathetic nervous system
 ThirstRenin-angiotensin-aldosterone system
What is affectedUrine osmolalityUrinary sodium excretion
 H2O intakeVascular tone
Note: See text for details.
Abbreviations: ANP, atrial natriuretic peptide; AVP, arginine vasopressin; BNP, brain natriuretic peptide.
Source: Adapted from Rose BD, Black RM (eds): Manual of Clinical Problems in Nephrology. Boston, Little Brown, 1988; with permission.


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