The cells of large, mobile animals are supported by extracellular fluid. All requirements of cells—oxygen, fuel, nutrients, essential molecules—come from that fluid, and the waste products of cell metabolism go into it. Circulatory systems have muscular chambers, or hearts, that move the extracellular fluid through the body. In open circulatory systems, extracellular fluid is the same as the fluid in the circulatory system and is called hemolymph. This fluid leaves the vessels of the circulatory system, percolates between cells and through tissues, and then flows back into the heart or vessels of the circulatory system to be pumped out again. In contrast, closed circulatory systems completely contain the circulating fluid (blood) in a continuous system of vessels. Blood cells and large molecules stay within the system, but water and low-molecular-weight solutes leak out of the smallest vessels, the capillaries, which are highly permeable.

In animals with a closed circulatory system, extracellular fluid refers to both the fluid in the circulatory system and the fluid outside it. The fluid in the circulatory system is the blood plasma; the *extracellular fluid outside the circulatory system is the interstitial fluid. A 70-kilogram person has a total extracellular fluid volume of about 14 liters. Less than a quarter of it—about 3 liters—is the blood plasma.