Topic overview

Start with the big picture

Body organization progresses through six levels: chemical, cellular, tissue, organ, system, and organism. Atoms form molecules; molecules make up cells, the smallest living units, whose organelles carry out specialized functions. Similar cells and extracellular matrix form the four basic tissue types: epithelial, connective, muscle, and nervous. At least two tissue types combine to form an organ with a specific task, and functionally related organs make up body systems. The systems work together in one organism, supporting functions such as homeostasis and reproduction. As organization becomes more complex, new properties emerge that cannot be predicted from lower levels alone. This hierarchy also provides a way to consider pathology, which may originate at any level and often becomes evident at the organ or system level.