The term physiology originated from a Greek root physiologikos which means discourse on natural knowledge. Physiology deals with the normal functioning of the body. Unlike metaphysics, science of physiology holds that there is nothing occult or supernatural about life. All the living processes of an organism can be explained, or better to say that, should be explained, with the help of underlying chemical and physical changes and their structural but peculiarities. The progresses of subsidiary subjects, such as physics, chemistry, microscopic anatomy, etc has given us ample opportunity to apply the principles of these branches of science to the understanding of working processes of the living organism.
Although physiology is a part of zoology, is a branch of biology, but due to its vastness physiology is now considered as another bench of biology.
The functional unit of the body is the cell and a group of similar cells constitute a tissue, a group of tissues for May system. Recently, application of electron microscope and histo-chemical techniques has revealed quite a complex structure of the cell. The functions of the body are based on, experimental results and chemical findings. 
Human physiology seeks to understand the mechanism that work to keep the body alive and functioning systematically. The scientific study into the mechanical, physical and biochemical functions of human body, their organs and the cells of which they are composed is the dealing matter of the branch physiology. The principal level of focus of physiology is at the level of organs and systems within systems.
Maximum 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 and structure of the body and its parts.
The functions of the body are based on, experimental results and clinical findings. And so physiology has subdivided into different branches of study.
In the fallowing chapters we shall try to give you a proper idea about the vast subject of physiology. As well as common people the students of biological science and of the medical stream can have some proper study materials in our following chapters.