There are many websites dedicated to the subject. Research the homeostatic control including the hypothalamus, ADH, osmoregulation in the kidneys (loop of Henle, concentration of the urine), etc. Also check out our tutorial http://www.biology-online.org/9/15_kidneys_inorganic_ions.htm

Homeostasis is a process whereby the physico-chemical properties of the body internal environment, as well as the stabilty of the body systems are kept constant (tolerable conditions). 😀
The human body is very dynamic, self-regulatory and self-directory, so much so, that when there is a change (hyper- or hypo-), for example: in blood glucose level, body temperature, change in acidity or basicity, etc, etc., of the internal environment, the human body reacts to turn back the lost homeostatic balance. In doing so, different body cells, tissues, organs, and even systems come together to form a temporary anatomical unit that reacts to the change. NOTE: Do check the body glucose metabolism, you will see an example of how a lost homeostatic balance is restored.
The process of homeostasis is not infinite, it has a limit. It is when this limit is exceeded that the condition of disease comes.
The body internal environment is made up of the body fluids, that is the blood, lymph, interstitial fluid (IF) and the cerebrospinal fluid (CSF). I hope you would read my next submission (in 24-hrs time), to read about the mechanism of the process.
ABBAS EL-TA’ALU.

The primary modus operandi for a healthy human organism is the maintainance of homeostasis. For the process to operate, the following requirements are necessary: Set-point, an integrating centre and an effector/s.
SET-POINT: This is detected by peripheral receptors (sensors, sensory nerve endings), which relay the information (eg., a fall in the level of blood glucose- normal value range is 75-110mg/100ml) to an integrating centre.
INTEGRATING CENTRE: It usually recieves information from many different receptors. it is often a particular region of the brain or the spinal cord (eg., the hypothalamus has centres for thist, hunger, appetite, satiation, etc) , but in some cases, it can also be a group of cells in an endocrine gland (eg., the pancreatic islets of Langerhans). The relative strengh of different receptor inputs are weighed (programed) here, which responds by either increasing or decreasing the activity of particular effectors-generally, muscles or secretory glands. The action of the effector is always directed to correct the initial disturbance, so that the initial change and its compensatory reaction result in only slight deviations from the normal value. For example, during fasting or the first stage of starvation, the level of blood glucose falls and because of the programming in the hypothalanus, the islets of Langerhan reorganise and the secretion of the hormone, insulin, is inhibited, while that of the antagonistic hormone, glucagon, is increased. This stimulates the liver to secrete glucose into the blood, helping to prevent blood glucose from continuing to fall. In this way, blood glucose level is maintained within a homeostatic range.
Since disturbances in homeostasis initiate events that lead to changes in the opposite direction, the cause-and-effect sequence is termed as a negative feedback mechanism (or a negative feedback loop). The internal environment is then never absolutely constant. The process of homeostasis is best concieved as a state of dynamic constancy, in which conditions are stabilized above or below the set-point.
Conclussively, I would like to add, that all control systems of the body operate essentially, by negative feedback mechanism. However, there is also a positive feedback mechanism. Though termed as a vicious cycle that does not lead to stability but to instability (action of effectors amplifies those changes that stimulated the effects) and often death, positive feedback mechanism, nevertheless, in rare cases, the human body has learned to use it to its advantage, for example, during blood clotting, childbirth, and in the generation of nerve impulses (signals).

ABBAS EL-TA’ALU,
Dept. of Biochemistry,
V.N. Karazin, Kharkov National University, Ukraine.