Individual cells get their nutrients from the surrounding extracellular fluid (ECF) and dump their waste products into it. A human is about 60 percent water. Two-thirds of that water is intracellular fluid and one-third is the ECF. About 20 percent of the ECF (about 3 liters) is the blood plasma that circulates in our blood vessels. The remaining 80 percent (about 11 liters) is the interstitial fluid that bathes every cell of the body (Figure 39.4). Water and small molecules freely exchange between the interstitial fluid and the blood plasma. Thus nutrients move into the interstitial fluid from the blood plasma, and wastes move into the blood plasma from the interstitial fluid. As long as the composition and physical conditions of the interstitial fluid are held within certain limits, cells are protected from the changes and harsh conditions of the external environment. A stable internal environment makes it possible for an animal to occupy habitats that would kill its cells if they were directly exposed to the external conditions. How is the internal environment kept constant?

The constant composition of the internal environment is always being challenged by the external environment and by the metabolic activities of the cells of the body. Organisms must maintain their internal environment in a state of homeostasis—a narrow range of stable and optimal physical and biochemical conditions. If a physiological system fails to function properly, homeostasis is compromised, and cells are damaged and can die. To avoid the loss of homeostasis, organ systems are controlled and regulated in response to changes in both the external and internal environments. The maintenance of homeostasis is a central theme of physiology.