Two-thirds of total body water is held within cells as Intracellular Fluid (ICF). The intracellular fluid (ICF) compartment is the system that includes all fluid enclosed in cells by their plasma membranes. Extracellular fluid (ECF) or extracellular fluid volume (ECFV) usually denotes all body fluid outside the cells. Also found in the ICF are cellular building blocks like sugars, proteins, carbohydrates, and lipids. If a disturbance were to cause a decrease in ECF osmolarity, then water will move from the ECF into the ICF to attain an osmolar equilibrium;.