The preservation of stability or constant condition in a biological system by means of automatic mechanisms that work against influences leaning towards disequilibria is Homeostasis. (Homeostasis {hohm-ee-oh-stay"-sis}) Homeostasis is the capacity of the body to preserve relative constancy and work even when severe changes occur in the external environment or in one part of the body. (Homeostasis: Toxiocology Tutor III) This is one of the most basic theories in modern biology and this commenced in the 19th century when the French physiologist Claude Bernard observed the consistency of chemical composition and physical properties of blood and other body fluids. He asserted that this arrangement of the milieu interieur was vital for the life of higher organisms. The 20th century American physiologist Walter B. Cannon invented the word homeostasis and he developed and expanded the concept of self-regulating mechanisms in living systems. (Homeostasis {hohm-ee-oh-stay"-sis}) The word homeostasis comes from the Greek words for same and steady. This term refers to the ways the body reacts to preserve a constant internal environment despite environmental differences and disorders. Both the mind/brain and the body are gifted with a large number of automatic mechanisms of feedback reserve that work against influences tending toward disequilibria. (Walter Cannon (1871-1945): Homeostatis) .

             Homeostatic systems exhibits certain properties: They are ultrastable; Their entire organization internal, structural, and functional, adds to the continuance of equilibrium; They are erratic which means that the end effect of a specific action often has the contrary effect to what was anticipated. (Walter Cannon (1871-1945): Homeostatis) At all levels of organization in living systems like the molecular, cellular, organismic and even populational levels, the homeostatic mechanisms work effectively. In complex organisms like humans, it involves continuous checking and controlling of several factors like the gases oxygen and carbon dioxide, nutrients, hormones, and organic and inorganic substances.