BIOREGULATORS. - The individual functions of the organism are not carried out in isolation, but are coordinated with one another so that, under physiological conditions, they begin, cease, are enhanced and are inhibited solely in the interest of the organism as a whole. The result of this coordination is the continuous modulation of the individual functions according to the needs of the organism, which vary from moment to moment with changing internal and environmental conditions (adaptation).
The relations that must necessarily exist among the various organs for their functions to be coordinated in maintaining the organism under normal conditions are brought about by means of so-called bioregulators. There are various types of these. Of the bioregulators, the first to be known was the nervous system. This, by means of the centers and the peripheral nerves, enters into relation with all the other organs and in this way is informed of their functional state and can transmit to them excitatory or inhibitory impulses according to the needs of the organism. Until a few decades ago the nervous system was held to be the only bioregulator in existence. Another type of bioregulator is represented by the ensemble of hormones, that is, by those substances which, produced by certain organs (endocrine glands), pass into the bloodstream and go on to act upon other distant organs. The discovery of hormonal bioregulators is relatively recent. The discovery of the first hormone (secretin) dates back to 1902, and since then the hormones discovered multiplied, and the study of their action today constitutes one of the most interesting chapters of biochemistry and is among the richest in applications for the benefit of practical medicine.
Alongside the two fundamental types of bioregulators just mentioned, the nervous and the hormonal, there exist others, discovered only in recent years and still under study. Such are the bioregulators that act within the organs or indeed within the very cells that produced them (tissue hormones, histohormones), those that regulate fertilization (gamones) and those that regulate the development of the embryo (organizers). In plants too, the presence of bioregulators has recently been demonstrated, of which the best known are the auxins, which regulate the growth of the plant.