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if an environmental variable (such as temperature) or an input or output variable (such as the flow demand on a system) changes and the system can nearly compensate for those changes in some other variable (such as outlet pressure) then the system is said to be regulated or regulated for that variable. If the regulation is obtained by a static compensation in which some linkage or component is introduced that diminishes the sensitivity to change, then this is static regulation (e.g., a spring scale is designed with materials that thermally compensate the spring against temperature change; a dc motor is designed by the choice of its field windings to give a speed regulation against changes in the load put on the motor; a chemical buffer shifts the operating point of chemical equilibrium to hold the pH of a solution constant). In dynamic regulation, two different switch states (an "on" and an "off" state) are arranged so that the system switches from one state to the other when the regulated parameter rises to an upper limit (an on-off thermostat). In feedback regulation (or control as it is technically referred to), an error signal is produced between the existing state of a system and the desired regulated level. This error signal is operationally acted upon, amplified in power, and fed to an actuator to operate a network which can influence the regulated variable so as to reduce the error, e.g., in biology the Na+ angiotension system. The signal is the sodium concentration. When this concentration decreases, aldosterone is liberated from the adrenal cortex. This agent acts on the kidney distal cubules to increase the reabsorption of sodium ions and re-establish the proper concentration of sodium. (Iberall)

(2) a notion valid in the domain of description of heteropoiesis, that reflects the simultaneous observation and description made by the designer (or its equivalent) of interdependent transitions of the system that occur in a specified order and at specified speeds. (Maturana and Varela, 1979)

Any systematic (rule-like or determinate) behavior of one part of a system that tends to restrict the fluctuations in behavior of another part of that system. While both parts must lie in the same feedback loop, regulation involves this basic asymmetry: the regulator detects and responds to discrepancies from some expectation (see criterion, goal), which is of an ordinality higher than the behavior so assessed, and it computes (see computing) the actions appropriate to keep the behavior to be regulated within desirable limits. The regulated part merely responds to the source of fluctuations and to the regulating action- without information about the regulator's expectations. The behavior of the regulator is often hard-wired, as in a thermostat, or rule-like, as in a bureaucracy. Regulation does not imply learning, adaptation (see ultra-stability, control) or intelligence. (Krippendorff)
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