In addition to our nervous system, the endocrinium (system for releasing messenger substances into our bloodstream) has evolved in the course of human evolution as one of the central integration mechanisms that enables communication between cells and organs in the first place.
The released messenger substances/hormones are substances that can already exert extraordinarily large effects in extremely small quantities, as can be seen, for example, from the influence of insulin on blood sugar or the importance of growth hormone or sex hormones. In order for hormones to function at all, they must connect to certain molecules on the cell surface or inside the cell. These receiving sites are called receptors. After successful contact of the hormone with its special receptor, a signal chain is triggered which is ultimately followed by the special reaction of the target cell.