Features of Biological Control

Biological control systems are often regarded as some sloppy variants of the more precise engineering control systems. Classic control theory considers linear, stable and stationary systems [1-3]. To this could be added well defined. Biological systems are nonlinear, often unstable, and never stationary. They work with small feedback gains, typically less than 10 [4—6] they are interwoven, so completely different systems share common routes (hormones, nerves, etc.) and their properties vary from person to person, even in healthy people. 

A classic example is the control of arterioles in the skin [7]. The arterioles participate in blood pressure control, and in case of acute cardiac failure or blood loss, the arterioles contract in order to reserve the cardiac output for the vital organs. But the arterioles also participate in temperature regulation [7]. When they contract, heat loss from the body is minimized, so a patient in shock risks getting hyperthermia, because the blood pressure control overrides the temperature control. Moreover, the same arterioles are fundamental for the regulation of local nutritional flow to the skin and also to the smooth muscle in the vessel walls [7]. So even if skin nutrition is given a low priority compared with blood pressure, the smooth muscle has to get some nutrition to keep up the same blood pressure control. This example shows not only the complexity of biological control, but also how difficult it is to intervene when a system is malfunctioning. 

Many parallels have been drawn between biological and technical control systems. Often the biological system is linearized. This can be justified if the variations are small, but it is typical for biological systems that they are not only nonlinear, but that the nonlinearities are important for proper functioning of the system. Many hormones, as for example insulin, are released in pulses. The amplitude and duration of the pulse and the interval between pulses can vary, so the signal is not just the mean concentration of the hormone, but the duration and the interval may be 

Added to this, the control must be able to work properly under extreme conditions (hard work, fever, injury, stress, etc.), which requires a very robust control. 

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Cascades of regulatory steps are also a signihcant feature of biological control circuits and can introduce significant delays. The interplay between the kinetics of each step, the kinetics of the entire cascade, and the introduced time delays is a topic of current interest [55, 59]. 

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