Hormones are the chemical components of an integral system of regulation of body functions. These are substances of different nature that are capable of transmitting signals to cells. The result of these interactions is a change in the direction and intensity of metabolism, the growth and development of the body, the launch of important functions or their suppression and correction.
A hormone is an organic chemical substance, the synthesis of which takes place in the endocrine glands or in the endocrine regions of the glands of mixed secretion. They are released directly into the internal environment, through which they spread and are randomly transferred to target organs. Here they are able to exert a biological effect, which is realized through receptors. Therefore, each hormone has exceptional specificity for a particular receptor. This means that these substances affect one function or process in the body. The classification of hormones by action, tissue affinity and chemical structure shows this more clearly.
Generalunderstanding the meaning of hormones
The modern classification of hormones considers these substances from many points of view. And they are united in one thing: only organic substances are called hormones, the synthesis of which occurs only in the body. Their presence is characteristic of almost all vertebrates, in which the regulation of body functions also represents the combined work of the humoral and nervous systems. Moreover, in phylogenesis, the humoral regulatory system appeared earlier than the nervous system. Even primitive animals had it, although it was responsible for the most basic functions.
Hormones and biologically active substances
It is believed that the very system of biologically active substances (BAS) and their specific receptors is characteristic even of a cell. However, the concepts of "hormone" and "BAS" are not identical. The hormone is called BAS, which is secreted into the internal environment of the body and has an effect on a distant group of cells. BAS, in turn, acts locally. Examples of biologically active substances, which are also called hormone-like substances, are kalons. These substances are secreted by the cell population, where they inhibit reproduction and regulate apoptosis. An example of BAS are also prostaglandins. The modern classification of hormones identifies a special group of eicosanoids for them. They are intended for local regulation of inflammation in tissues and for the implementation of hemostasis processes at the level of arterioles.
Chemical classification of hormones
Hormones by chemicalbuildings are divided into several groups. This also separates them according to the mechanism of action, because these substances have different indicators of tropism for water and lipids. So, the chemical classification of hormones looks like this:
- peptide group (secreted by the pituitary, hypothalamus, pancreas and parathyroid glands);
- steroid group (secreted by the endocrine part of the male gonads and cortical areas of the adrenal glands);
- a group of amino acid derivatives (produced by the thyroid gland and adrenal medulla);
- group of eicosanoids (secreted by cells, synthesized from arachidonic acid).
It is noteworthy that female sex hormones are also included in the steroid group. However, they are by and large not steroids: the influence of hormones of this type is not associated with an anabolic effect. However, their metabolism does not lead to the formation of 17-ketosteroids. Ovarian hormones, although structurally similar to other steroids, are not. Since they are synthesized from cholesterol, they are classified as other steroids to simplify basic chemical classifications.
Classification by place of synthesis
Hormonal substances can also be divided according to the place of synthesis. Some are formed in peripheral tissues, while others are formed in the central nervous system. The method of secretion and excretion of substances depends on this, which determines the peculiarities of the implementation of their effects. The classification of hormones by place looks like this:
- hypothalamic hormones (releasing-factors);
- pituitary (tropic hormones, vasopressin and oxytocin);
- thyroid (calcitonin, tetraiodothyronine and triiodothyronine);
- parathyroid (parathyroid hormone);
- nonadrenal (norepinephrine, epinephrine, aldosterone, cortisol, androgens);
- sexual (estrogens, androgens);
- pancreas (glucagon, insulin);
- tissue (leukotrienes, prostaglandins);
- APUD hormones (motilin, gastrin and others).
The last group of hormonal substances is not fully understood. It is synthesized in the largest group of endocrine glands located in the upper intestines, in the liver and pancreas. Their purpose is to regulate the secretion of exocrine digestive glands and intestinal motility.
Classification of hormones by type of effect
Different hormonal substances have different effects in biological tissues. They are divided into the following groups:
- metabolic regulators (glucagon, triiodothyronine, tetraiodothyronine, cortisol, insulin);
- regulators of the functions of other endocrine glands (releasing factors of the hypothalamus, tropic hormones of the pituitary gland);
- regulators of calcium and phosphorus metabolism (parathyroid hormone, calcitonin and calcitriol);
- regulators of water-s alt balance (vasopressin, aldosterone);
- regulators of reproductive function (sex hormones);
- stress hormones (norepinephrine, adrenaline, cortisol);
- regulators of limits and growth rates, cell division(somatotropin, insulin, tetraiodothyronine);
- regulators of the functions of the central nervous system, limbic system (cortisol, adrenocorticotropic hormone, testosterone).
Secretion and transport of hormones
Secretion of hormones occurs immediately after their synthesis. They enter directly into the blood or tissue fluid. The last place of secretion is typical for eicosanoids: they should not act far from the cell, because they regulate the functions of the entire tissue population. And the hormones of the ovaries, pituitary gland, pancreas and others must be carried with the blood throughout the body in search of target organs that have specific receptors for them. From the blood, they enter the intercellular fluid, where they are sent to the cell of the target organ.
Signal transmission to the receptor
The above classification of hormones reflects the effects of substances on tissues and organs. Although this is possible only after the binding of the chemical to the receptor. The latter are different and are located both on the cell surface and in the cytoplasm, on the nuclear membrane and inside the nucleus. Therefore, according to the method of signal transmission, substances are divided into two types:
- extracellular transmission mechanism;
- intracellular signaling.
This basic classification of hormones allows you to draw conclusions about the speed of signaling. For example, the extracellular mechanism is much faster than the intracellular one. It is characteristic of adrenaline, norepinephrine and other peptide hormones. intracellular mechanismcharacteristic of lipophilic steroids. Moreover, the benefits for the body are achieved faster with the synthesis of peptides. After all, the production of steroid hormones is much slower, and their signal transmission mechanism is also slowed down by the need for protein synthesis and maturation.
Characteristics of signal transmission types
An extracellular mechanism is characteristic of peptide hormones that cannot get beyond the cytoplasmic membrane into the cytoplasm without a specific carrier protein. This is not provided for it, and the signal itself is transmitted through the adenylate cyclase system by changing the conformation of receptor complexes.
The intracellular mechanism is much simpler. It is carried out after the penetration of the lipophilic substance into the cell, where it meets the cytoplasmic receptor. With it, it forms a hormone-receptor complex that penetrates the nucleus and affects specific genes. Their activation leads to the launch of protein synthesis, which is the molecular effect of this hormone. The actual effect is already a protein that regulates a given function after its synthesis and formation.
