Extracellular Fluid Volume Deficit
ECFVD is loss of extracellular fluids: interstitial, intravascular, gastric etc.
Sodium is a major extracellular electrolyte whose primary function is to regulate body fluid.
Osmolarity is the osmotic pull exerted by all particles per unit of solution: Hyper-osmolar, Hypo-osmolar, Iso-osmolar fluid volume deficits.