The human body is divided into several levels of organization, from the simplest to the most complex. These levels are chemical, cellular, tissue, organ, organ system, and organism. Anatomy is the study of the structure of the human body. Physiology is the study of its functions. Directional terms are used to describe the location of body parts and regions. These terms always relate to the anatomic position. It is important to understand the basics of the organization of the human body before studying the individual systems.