Figure 1.

Upper Panel: Schematic representation of the concept of the Internal Milieu (that is, the medium in contact with the cells of a multi-cellular organism), and of the concept of homeostasis. Lower Panel: box representation of some main logical steps involved in the homeostatic process and of the relevance of feed-back signals, that is, of signals which inform the control systems of the 'load error' (discrepancy between the observed value of a physico-chemico parameter and its optimal value) which needs to be compensated for in order to restore optimal parameters. This can be accomplished by changes in the performance of relevant organ systems.