Overview Introduction Body fluid compartments total body water (TBW) is composed of intracellular fluid (ICF) login to view 1 more bullet extracellular fluid (ECF) (~1/3 of TBW) which is composed of login to view 6 more bullets solute composition of fluid compartments intracellular fluid login to view 2 more bullets extracellular fluid login to view 2 more bullets "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) login to view 1 more bullet osmolality (mOsm/kg) login to view 1 more bullet effective osmole a solute that does not cross the cell membrane (e.g., protein) login to view 1 more bullet 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 ↑ ↓