Homeostasis

Homeostasis is the regulation of the internal conditions of a cell or organism to maintain optimum conditions for function in response to internal and external changes. ‘Optimum’ means best for the function concerned. Regulation does not mean that conditions never change: it means that changes are detected and responses bring conditions back towards suitable levels.

Homeostasis means regulating the body's internal conditions within narrow limits despite changes inside and outside the body. 'Constant' does not mean that a condition never changes: it means that changes are controlled so conditions remain within a suitable range.