Fig.4 A simple representation of the human temperature regulation system

Metabolism is the set of chemical reactions that happen in living organisms to maintain life.

Connectedness is a concept from topology. Topology, the discipline that studies the underlying structure of mathematics, offers fascinating reading to aspiring systems theorists. Recommended reading is given in the Bibliography. Clearly, a system is a very general concept; control theory is most interested in certain classes of system and to make progress we delineate the classes. First it is interested in dynamic systems - these arc systems whose behaviour over a time period is of interest. Thus if a system were concerned with population aspects, a similar dynamic system would be concerned with population growth.

Secondly, it is most interested in and most powerful when dealing with linear systems. A linear system is characterized by the property shown in Figure 1.2. The upper part of the figure shows a system's response to some arbitrary stimulus. The lower part shows how, in the presence of linearity, the response to a scaled-up version of the stimulus is simply a scaled-up version of the previous response, with proportionality being preserved.

Finally, it is interested in feedback systems - these are systems where information flows in one or more loops, so that part of the information entering an element may be information that previously left that element (Figure 1.3).

Systems arc often visualized in the form of block diagrams, illustrating the main functions, their supposed interconnection and (possibly) their interconnection to the environment of the system. Thus, a simple representation of the human temperature regulation system might be as shown in Figure 1.4.

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