BIOREGOLATORI. – The individual functions of the organism do not occur in isolation, but are coordinated with one another so that, under physiological conditions, they begin, cease, are strengthened, and are inhibited solely in the interest of the organism as a whole. The result of this coordination is the continuous adjustment of the individual functions to the needs of the organism, which vary from moment to moment as internal and environmental conditions change (adaptation).
The relationships that must necessarily exist among the various organs so that their functions may be coordinated in maintaining the organism under normal conditions are effected by means of the so-called bioregulators. There are different types. The first bioregulator to be known was the nervous system. Through its centers and peripheral nerves, it establishes relations with all the other organs and is thereby informed of their functional state; it can then transmit to them excitatory or inhibitory impulses according to the needs of the organism. Until a few decades ago, the nervous system was considered the only bioregulator in existence. Another type of bioregulator is represented by the hormones as a whole—that is, by those substances which, produced by certain organs (endocrine glands), pass into the bloodstream and act upon other distant organs. The discovery of hormonal bioregulators is relatively recent. The discovery of the first hormone (secretin) dates to 1902, and since then the hormones discovered have multiplied; the study of their action now constitutes one of the most interesting chapters of biochemistry and is among those richest in applications to the benefit of practical medicine.
Alongside the two fundamental types of bioregulators just mentioned, the nervous and the hormonal, there are others, discovered only in recent years and still under investigation. These include bioregulators that act in the organs or even in the very cells that produced them (tissue hormones, histohormones), those that regulate fertilization (gamones), and those that regulate the development of the embryo (organizers). The presence of bioregulators has recently also been demonstrated in plants; the best known are auxins, which regulate the growth of the plant.