Physiology is the study of the functions of the body, its organs and the cells of which they are composed. It is often said that physiology concerns itself with maintaining the status quo or ‘homeostasis’ of bodily processes. However, even normal physiology is not constant, changing with development (childhood, pregnancy and ageing) and environmental stresses (altitude, diving and exercise). Physiology might be better described as maintaining an ‘optimal’ internal environment; many diseases are associated with the disturbance of this optimal environment.