Extracellular fluid is actively transported-- after all, it's where all gas exchange happens, and if you get a hypoxic spot in your body then those cells are going to die. The wikipedia article claims, without citation: "The extracellular fluid is constantly "stirred" by the circulatory system, which ensures that the watery environment which bathes the body's cells is virtually identical throughout the body. This means that nutrients can be secreted into the ECF in one place (e.g. the gut, liver, or fat cells) and will, within about a minute, be evenly distributed throughout the body."