The scheme of evolution required the specialization of parts or organs of the body to undertake discrete functions, eg, the muscles provide locomotion, the gut digests and absorbs nutrition, and the kidneys maintain a constant chemical internal milieu. This specialization, however, could only proceed if a system of communication developed between organs so that their functions could be integrated to benefit the whole organism. This was achieved in two ways—by the transmission of electrical impulses through the nervous system and the transmission of chemical messages through the circulatory system. This review will discuss some of the features of the latter system.
It is evident that these communicating signals must be precisely controlled, and that the controls themselves must be kept under lock and key. For example, the liver must be ready at all times to guard against the dangers of hypoglycemia by maintaining a store of glycogen and the enzymes