The condition of physiological homeostasis results from the maintenance of a steady state for the functions of organs and the levels of different blood constituents, despite the constant variability of other factors from without and within tending to upset that steady state. Most organs of the body show variability and adjustment of their functions in the presence of stresses, but the brain cannot itself make such adjustments. Yet any change in the external or internal environment of the body affects the brain, and particularly the cortex, most severely. Homeostatic mechanisms function as defensive mechanisms, protecting the cortex of the brain from harmful alterations in the internal environment.