Overview Introduction Body fluid compartments total body water (TBW) is composed of intracellular fluid (ICF) ~2/3 of TBW extracellular fluid (ECF) (~1/3 of TBW) which is composed of interstitial fluid ~ 3/4 of ECF interstitial fluid is fluid in which cells are bathed plasma ~1/4 of ECF plasma volume is measured by radiolabeled albumin solute composition of fluid compartments intracellular fluid potassium is abundant in the ICF mnemonic: HIKIN = HIgh K+ INtracellularly extracellular fluid sodium and chloride is abundant in the ECF ECF is measured by inulin "60-40-20 Rule" 60% of body weight is total body water 40% of body weight, or 2/3 of total body water, is intracellular fluid (ICF) 20% of body weight, or 1/3 of total body water, is extracellular fluid (ECF) Osmosis describes the movement of water through a semipermeable membrane water moves from an area of high water concentration (or low solute concentration) to an area of low water concentration (or high solue concentration) units osmolarity (mOsm/L) solute concentration per liter of solvent (e.g., water) osmolality (mOsm/kg) solute concentration per kilogram of solvent effective osmole a solute that does not cross the cell membrane (e.g., protein) this creates an osmotic force that "pulls" water into its compartment Osmolarity and Volume Changes Under Certain Conditions Conditions Extracellular Fluid Intracellular Fluid Blood Osmolarity Isotonic fluid loss ↓ No change No change Hypotonic fluid loss ↓ ↓ ↑ Isotonic fluid gain ↑ No change No change Hypotonic fluid gain ↑ ↑ ↓ Hypertonic fluid gain ↑ ↓