The Secret Stories of Sodium How Infants, Athletes, Psychotics, And Otherwise Healthy People Die from Sodium Imbalance

INTRODUCTION Every number tells a story. As such, clinical laboratory science empowers modern medicine with a vast yet growing repertoire of analytes, which subsequently guide the diagnosis, treatment and prognosis of most – if not all - medical conditions. The body's integrated response to dynamic perturbations in homeostasis creates an elusive trail of biochemical footprints, each providing clues to where pathophysiology began and how far the body has strayed off course (i.e. severity of the insult and appropriateness of the response). The ubiquitous cation, sodium, is one of the main markers of whole body homeostasis. The sodium concentration ([Na+]) within the extracellular fluid space is essential for maintaining cellular size and adequate tissue perfusion. As such, the inherent regulation of water and sodium balance is strict, precise, and fiercely protected by the body. Therefore, when blood sodium levels venture outside of the heavily guarded trails of normonatremia (normal blood [Na+]), into hyponatremia (low blood [Na+]) or hypernatremia (high blood [Na+]), each deviation foretells tales of health and disease - and in rare cases, of abuse and neglect. Water and sodium within the exterior milieu is essential to life. However, within the interior milieu, it is the balance between the two that is critical for individual human survival. We refer to this balance, between water and sodium within the internal milieu, as “fluid homeostasis.” The physiological mechanisms that govern fluid intake (thirst) and output (urine), as well as sodium intake (sodium palatability) and output (urine) have been well-described.1-3 Of historical note,… ABBREVIATIONS: [Na+] – sodium concentration, ADH - anti-diuretic hormone, AVP – arginine vasopressin, CVO - circumventricular organs, ECF – extracellular fluid, ICF – intracellular fluid, IV – intravenous, K+ - potassium, RAAS - renin-angiotensin-aldosterone system