Interstitial fluid, also known as intercellular fluid and tissue fluid, is fluid between the cells of multicellular organisms. It is an important element of the body’s extracellular fluid (ECF). Formed by filtration through capillaries, it constitutes 16 percent of adult humans’ total body weight.

ECF constitutes the internal environment of the body. It provides a relatively constant environment for cells and transports materials to and from them. It contains about 37 percent of the total water in the adult human body, although its volume is proportionately larger in infants and children. In humans, ECF must have a concentration of sodium chloride (NaCl), or salt, of about 0.9 percent to maintain homeostasis.

Along with interstitial fluid, ECF includes blood plasma, a protein-rich fluid that carries red and white blood cells; lymph, a fluid found in lymphatic vessels; and specialized, transcellular fluids that include synovial fluid in joint cavities, cerebrospinal fluid, aqueous and vitreous humor in the eye, serous fluid in body cavities and fluid secreted by glands. Although these different fluids make up ECF, they are chemically very similar to one another. The composition of interstitial fluid closely resembles that of blood plasma — although plasma contains far more proteins — and is nearly identical to the composition of lymph fluid. Sodium and chloride are the two most abundant ions found in interstitial fluid, although it also contains calcium, magnesium, potassium, bicarbonate, phosphate, sulfate, carbonic acid, some nonelectrolytes and very small amounts of proteins.

According to Starling’s law of the capillaries, various pressures regulate the exchange of water between blood plasma and interstitial fluid. Blood hydrostatic pressure forces water out of the capillaries into interstitial fluid, and blood colloid osmotic pressure pulls water back into the capillaries. On the other hand, interstitial fluid hydrostatic pressure pushes water out of interstitial fluid into the capillaries, and interstitial fluid colloid osmotic pressure draws it back in. These exchanges occur through intercellular clefts in the walls of capillaries, known as capillary epithelium.

An edema — the presence of abnormally large amounts of fluid in the intercellular tissue spaces of the body — can occur when the relationship between blood plasma and interstitial fluid is disturbed. This disturbance can result from many factors, including retention of electrolytes, especially sodium; an increase in capillary blood pressure; and a decrease in the concentration of plasma proteins, often as a result of increased capillary permeability because of infection, burns or shock.