Interstitial fluid is a type of extracellular fluid (ECF). ECF is the fluid that is found outside of cells, and it makes up about 20% of the total body weight. Interstitial fluid is the ECF that is found in the spaces between cells, and it makes up about 75% of the ECF. Other types of ECF include plasma, which is the ECF that is found in blood vessels, and lymph, which is the ECF that is found in lymphatic vessels.