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. This internal environment must be maintained in such a state as to allow maximum efficiency.