Communication within biological systems involves either or both of two kinds of channels, each with a different kind of signal. The two types of signals are: (a) nerve impulse, or (b) hormones. These channels appear in control loops that provide external control over many plant processes that characterize the living states. One of these chemical control arrangements, the adrenal glucocorticoid system, has been studied by means of computer simulation in order to assess its role in the regulation of energy fluxes in animals. The dominant time-constant of the controller system ranges from ten minutes to one hour in higher animals and man, and the system follows a circadian rhythm of one cycle per 24 hours at its input. The controller system is nonlinear; its bandpass characteristics are a function of its initial operating level, and cannot be concisely stated. However, the simulation indicates that energy transformations occurring within animals in the frequency domain of one cycle every 24 hours down to one cycle every hour could be entrained or driven by the adrenal glucocorticoid system.

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