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.
Cells are the basic units of living things. Tissues are groups of similar cells that are structurally combined to act together to perform a function or provide a structure. Like a muscle. Organs are more complex structures, usually composed of multiple tissue types, that perform complex functions.
