All animals need nutrients and oxygen and must eliminate carbon dioxide and other waste products of metabolism. single-celled organisms meet all these needs by direct exchange with the external environment. An internal environment makes complex multi-cellular animals possible. Internal environment the physical and chemical characteristics of the extracellular fluids of the body. individual cells get their nutrients from this extracellular fluid and dump their waste into it. a stable internal environment makes it possible for an animal to occupy habitats that would kill its cells if they were exposed to it directly. As multi-cellular organisms evolved, cells became specialized for maintaining specific aspects of the internal environment. Homeostasis the maintenance of a steady state, such as a constant temperature or a stable social structure, by means of physiological or behavioral feedback responses. if a physiological system fails to function properly, homeostasis is compromised, and as a result cells are damaged and can die.