Homeostasis

The human body is made up of trillions of cells that all work together for the maintenance of the entire organism. While cells, tissues, and organs may perform very different functions, all the cells in the body are similar in their metabolic needs. Maintaining a constant internal environment by providing the cells with what they need to survive (oxygen, nutrients, and removal of waste) is necessary for the well-being of individual cells and of the entire body. The many processes by which the body controls its internal environment are collectively called homeostasis. The complementary activity of major body systems maintains homeostasis.
Homeostasis is the maintenance of a constant internal environment. It is involved in many bodily functions, such as keeping the body's internal environment constant much like the thermostat of a central heating system. Homeostasis keeps the body's temperature at a certain level and it keeps the pH of the body at a certain level so that enzymes don't denature. Blood glucose is kept constant, CO2 levels and O2 levels are monitored to ensure that enough oxygen and not too much carbon dioxide are in the blood. The overall concentration and volume of blood is also monitored by homeostasis.
Many of the mechanisms involved rely on negative feedback. Negative feedback between the effector and the receptor of a control system reverses any changes to the systems steady state. In body temperature, a movement from the set level (e.g. a rise or fall in body temperature) is detected by receptors in the hypothalamus. These receptors then send information to the control center in the brain which reacts by returning to the original value.