Answer: Humans—and other complex multicellular organisms—have systems of organs that work together, carrying out processes that keep us alive.
The body has levels of organization that build on each other. Cells make up tissues, tissues make up organs, and organs make up organ systems.
The function of an organ system depends on the integrated activity of its organs. For instance, digestive system organs cooperate to process food.
The survival of the organism depends on the integrated activity of all the organ systems, often coordinated by the endocrine and nervous systems.