The human body is a complex, highly organized structure made up of unique cells that work together to accomplish the specific functions necessary for sustaining life.
The biology of the human body includes
Physiology (how the body functions)
Anatomy (how the body is structured)
Anatomy is organized by levels, from the smallest components of cells Cells Cells are the basic building blocks of the body. All tissues and organs are made of billions of different cells. Human cells vary in size, but all are quite small. Even the largest, a fertilized... read more 
to tissues and organs Tissues and Organs Tissues are related cells that are joined together. The cells in a tissue are not identical, but they work together to accomplish specific functions. For example, muscle tissue has muscle cells... read more and to organ systems Organ Systems Although each organ has its specific functions, organs also function together in groups, called organ systems (see table ). Doctors categorize disorders and their own medical specialties according... read more .
Gross anatomy is the study of the body's organs as seen with the naked eye during visual inspection and when the body is cut open for examination (dissection).
Cellular anatomy is the study of cells and their components, which can be observed only with the use of special techniques and special instruments such as microscopes.
Molecular anatomy (often called molecular biology) is the study of the smallest components of cells at the biochemical level.
Anatomy and physiology change remarkably between fertilization and birth. After birth, the rate of anatomic and physiologic changes slows, but childhood is still a time of remarkable growth and development ( see Physical Growth of Infants and Children Physical Growth of Infants and Children Physical growth refers to an increase in body size (length or height and weight) and in the size of organs. From birth to about age 1 or 2 years, children grow rapidly. After this rapid infant... read more ). Some anatomic changes occur past adulthood, but the physiologic changes in the body's cells and organs are what contribute most to what we experience as aging ( see Changes in the Body With Aging Changes in the Body With Aging The body changes with aging because changes occur in individual cells and in whole organs. These changes result in changes in function and in appearance. (See also Overview of Aging.) As cells... read more ).
