Homeostasis can be defined as the stable condition of an organism and of its internal environment; or as the maintenance or regulation of the stable condition, or its equilibrium; or simply as the balance of bodily functions. The stable condition is the condition of optimal functioning for the organism, and is dependent on many variables, such as body temperature and fluid balance, being kept within certain pre-set limits. Other variables include the pH of extracellular fluid, the concentrations of sodium, potassium and calcium ions, as well as that of the blood sugar level, and these need to be regulated despite changes in the environment, diet, or level of activity. Each of these variables is controlled by one or more regulators or homeostatic mechanisms, which together maintain life - wikipedia 
Homeostasis is brought about by a natural resistance to change in the optimal conditions, and equilibrium is maintained by many regulatory mechanisms. All homeostatic control mechanisms have at least three interdependent components for the variable being regulated: a receptor, a control centre, and an effector. The receptor is the sensing component that monitors and responds to changes in the environment, either external or internal. Receptors include thermoreceptors, and mechanoreceptors. Control centres include the respiratory centre, and the renin-angiotensin system. An effector is the target acted on, to bring about the change back to the normal state. At the cellular level, receptors include nuclear receptors that bring about changes in gene expression through up-regulation or down-regulation, and act in negative feedback mechanisms. An example of this is in the control of bile acids in the liver.
Some centres such as the renin-angiotensin system, control more than one variable. When the receptor senses a stimulus, it reacts by sending an action potential to a control centre. The control centre sets the maintenance range, the acceptable upper and lower limits, for the particular variable such as temperature. The control center responds to the signal by determining an appropriate response and sending signals to an effector which can be one or more muscles, an organ, or a gland. When the signal is received and acted on, negative feedback is fed back to the receptor that stops the need for further signalling.
The concept of the regulation of the internal environment was described by French physiologist Claude Bernard in 1865, and the word homeostasis was coined by Walter Bradford Cannon in 1926. Homeostasis is an almost exclusively biological term, referring to the concepts described by Bernard and Cannon, concerning the constancy of the internal environment in which the cells of the body live and survive. The term cybernetics is applied to technological control systems such as thermostats, which function as homeostatic mechanisms, but is often defined much more broadly than the biological term of homeostasis.