There is a biochemical and anatomical difference. Exhausted would implies that there is no available energy for the task at hand and enervated would suggest that the intended task lack the nervous potential to be actuated. That said, in popular discourse, there is not much difference between the two.
Systems theory has also been developed within sociology. An important figure in the sociological systems perspective as developed from GST is Walter Buckley (who from Bertalanffy's theory). Niklas Luhmann is also predominant in the literatures for sociology and systems theory.
Updated April 26, 2016. The biological perspective is a way of looking at psychological topics by studying the physical basis for animal and human behavior. It is one of the major perspectives in psychology and involves such things as studying the brain, immune system, nervous system, and genetics.