Physiology is branch of medicine which deals with the study of mechanisms that govern the functions of human body in health. Understanding these mechanisms have helped doctors to understand the disease processes (the pathophysiology of disease) and also have helped scientists is making effective treatment methods viz. drugs, neuro-stimulation, etc. Human body can be considered as a group of organized multi-cellular systems working in harmony towards maintenance of life. Basic unit of life is a cell, that is specialized in human body to sub-serve a particular function in a system basically for maintaining homeostasis. Homeostasis is maintenance of stable internal environment for the cells. From this internal environment the cell receives its nutrients and is able to excrete the waste products. Thus, the cell is able to survive because of a stable internal environment. Years of evolutionary processes have resulted in specialized systems in the body viz. the systems providing n...