Equilibrium Homeostasis static equilibrium system in constant environments. The value to the internal variable is constant because the external value is constant. Body temperature of lizards, icefish Steady-state Homeostasis open dynamic equilibrium system value of internal variable is determined by their free and input and the rate of elimination which is concentration dependent. The level of the regulated variable is kept relatively constant despite dramatic changes in the disturbance. Feedback control systems sensory information about a particular variable is used to control the internal level of this variable.
Negative feedback involves the sampling of controlled variables and corrective actions.
Positive feedback involves the sampling of control variables with further action of the same type. Cannot maintain homeostasis because it destabilizes the regulated variable. Multiple control systems important physiological variables are often controlled by many regulatory effectors. May involve multiple set points. Comparative approach determining fundamental design principles of organisms what is most crucial in biochemical design.