Anatomy is the study of the structure of body parts and their relationship to each other and to internal organs. Physiology is the study of how body parts work together with systems within the body to affect voluntary and involuntary body functions.
Anatomy is a science encompassing many specialized branches. Two of the basic branches of anatomy are gross anatomy, the study of body parts that are easily visible, such as arms, bones and the liver, and microscopic anatomy or histology, the study of internal and external body tissues.
Anatomy and physiology both deal extensively with body systems, which are organized in a certain hierarchy. The most basic level of physiological and anatomical structure is the chemical level that involves cells, atoms and molecules. Cells are the smallest unit of life, and atoms form molecules through chemical bonds that create the basis for all living activity.
The next structural level is the organ level; an organ is a discrete structure made up of more than one tissue type, with all tissues functioning together to perform related activities. Organ systems come next. These systems consist of multiple internal organs working together for a common purpose such as food digestion, nutrition and reproduction. Last, there is the organism level that is the result of all the simpler levels working together. An organism, such as a human being, can reproduce, respond to environmental changes and process energy signals.