Physiology (; from Ancient Greek φύσις (physis), meaning 'nature, origin', and -λογία (-logia), meaning 'study of') is the scientific study of function in living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system.  Giving the size of the field it is divided into among others: human physiology, plant physiology, cellular physiology, bacterial physiology, viral physiology. 
The study of human physiology dates back to at least 420 BC to the time of Hippocrates, also known as the father of medicine. The critical thinking of Aristotle and his emphasis on the relationship between structure and function marked the beginning of physiology in Ancient Greece, while Claudius Galenus (c. 126–199 AD), known as Galen, was the first to use experiments to probe the functions of the body. Galen was the founder of experimental physiology.
Jean Fernel, a French physician, introduced the term "physiology" in 1525.
In the 19th century, physiological knowledge began to accumulate at a rapid rate, in particular with the 1838 appearance of the Cell theory of Matthias Schleiden and Theodor Schwann. It radically stated that organisms are made up of units called cells. Claude Bernard's (1813–1878) further discoveries ultimately led to his concept of milieu interieur (internal environment), which would later be taken up and championed as "homeostasis" by American physiologist Walter Cannon.[clarification needed]
In the 20th century, biologists also became interested in how organisms other than human beings function, eventually spawning the fields of comparative physiology and ecophysiology. Major figures in these fields include Knut Schmidt-Nielsen and George Bartholomew. Most recently, evolutionary physiology has become a distinct subdiscipline.
Human physiology seeks to understand the mechanisms that work to keep the human body alive and functioning, through scientific enquiry into the natute of mechanical, physical, and biochemical functions of humans, their organs, and the cells of which they are composed. The principal level of focus of physiology is at the level of organs and systems within systems. The endocrine and nervous systems play major roles in the reception and transmission of signals that integrate function in animals. Homeostasis is a major aspect with regard to such interactions within plants as well as animals. The biological basis of the study of physiology, integration refers to the overlap of many functions of the systems of the human body, as well as its accompanied form. It is achieved through communication that occurs in a variety of ways, both electrical and chemical.
Much of the foundation of knowledge in human physiology was provided by animal experimentation. Physiology is the study of function and is closely related to anatomy which is the study of form. Due to the frequent connection between form and function, physiology and anatomy are intrinsically linked and are studied in tandem as part of a medical curriculum.