If different specialized cells, tissues, and organs provide for different needs of the multicellular organism, how are these specific services shared by the cells of the whole organism? All of the cells of the body share an internal environment that is made up of extracellular fluids. Cells derive their nutrients from these extracellular fluids, and they deposit their wastes into the extracellular fluid. This internal environment serves the needs of all cells of the body, and therefore its physical and chemical composition must be maintained within a narrow range of physiological conditions that support survival and function. The maintenance of this narrow range of conditions is known as homeostasis. A relatively stable internal environment means that cells can function efficiently even when external conditions could not support the lives of individual cells.

*Homeostasis requires that the activities of the cells and systems of the body be regulated. Regulation requires information—information about internal conditions, external conditions, and what is optimal. Thus organisms must have sensory mechanisms to monitor conditions, effector mechanisms to alter those conditions, and signaling mechanisms to integrate information and enable communication between sensors and effectors. The major informational systems of animals—nervous, hormonal, and immune—use chemical and electrical signals to process information.

The concept of homeostasis also applies to the intracellular environment. Both unicellular and multicellular organisms must regulate the composition of their intracellular environments within a range that allows those cells to survive and function. Individual cells regulate these properties through actions of their membranes and, in the case of eukaryotes, their organelles. Thus self-regulation to maintain a more or less constant internal environment is a general attribute of all life.