ECF Volume and Osmolarity Regulation

  • A major physiological concept to appreciate is that there is no definite relationship between the volume and osmolarity of the extracellular fluid (ECF). In other words the ECF volume can contract or expand in conditions where the ECF osmolarity is hyperosmotic, isoosmotic, or hypoosmotic as summarized in Body Fluid Shifts. Given this independence between the ECF volume and osmolarity it is not surprising that these two physiological variable are also regulated independently via distinct physiological mechanisms.