The human body is composed of cells that typically live in groups of the same or similar cell types called tissue. Tissue forms organs, which are self-contained groups of tissue that perform a specific function. Organs work together to carry out a specific function as an organ system. Together, all of the organ systems make up the organism.
Figure 1: The structure of the human body when viewed from different levels. The chemical level (1) - atoms make up chemicals that form DNA. The cellular level (2) - DNA forms cells. The tissue level (3) - cells form tissue. The organ level (4) - tissue forms organs. The system level (5) - organs form organ systems. The organismal level (6) - organ systems form organisms.