Physiology is the scientific study of functions and mechanisms in a living system. As a sub-discipline of biology, physiology focuses on how organisms, organ systems, individual organs, cells, and biomolecules carry out the chemical and physical functions in a living system.

According to the classes of organisms, the field can be divided into medical physiology, animal physiology, plant physiology, cell physiology, and comparative physiology. Central to physiological functioning are biophysical and biochemical processes, homeostatic control mechanisms, and communication between cells. Physiology in the 20th century was a mature science; during a century of growth, physiology became the parent of a number of related disciplines, of which biochemistry, biophysics, general physiology, and molecular biology are the most vigorous examples. Physiology, however, retains an important position among the functional sciences that are closely related to the field of medicine