What constitutes a system in biological terms?

In biological terms, a system is best defined as a group of organs that work together to perform specific functions necessary for the organism's survival and homeostasis. This reflects the organization of biological structures, where multiple organs collaborate to carry out complex processes. For example, the circulatory system includes the heart, blood vessels, and blood, all functioning together to transport nutrients, oxygen, and waste products throughout the body.

While a collection of tissues or a set of cells might represent lower levels of biological organization, they do not encompass the functional, interconnected nature of a system as required in biological contexts. Similarly, a combination of systems typically refers to interactions at a higher level of organization, such as how the nervous system coordinates with the muscular system, but that is not the foundational idea of what constitutes a system by itself. Thus, identifying a system as a group of organs captures the essence of biological workings effectively.