BODY STRUCTURE & HOMEOSTASIS: CELLS, TISSUES, ORGANS, & ORGAN SYSTEMS... Please Like, Comment and Share for more Content... Key points 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 i