Organs perform a certain function.
Most organs have functions in only one organ system. Organ systems are composed of organs, and perform a major function for the organism. Homeostasis is a term coined in 1959 to describe the physical and chemical parameters that an organism must maintain to allow proper functioning of its component cells, tissues, organs, and organ systems. Single-celled organisms are surrounded by their external environment. Most multicellular organisms have most of their cells protected from the external environment, having them surrounded by an aqueous internal environment.