Hormones are called the chemical components of a holistic system of regulation of body functions . These are substances of various nature that can transmit signals to cells. The result of these interactions is a change in the directions and intensity of the metabolism, growth and development of the body, the launch of important functions or their inhibition and correction.
A hormone is an organic chemical substance whose synthesis 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 that is realized through receptors. Therefore, each hormone has exceptional specificity for a specific receptor. This means that these substances affect one function or process in the body. The classification of hormones by action, tropism for tissues and chemical structure shows this more clearly.
Understanding the Importance of Hormones
The modern classification of hormones considers these substances from many points of view. And they are united in one: hormones are called only organic substances, 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 one. In primitive animals, it was available, although it was responsible for the most basic functions.
Hormones and biologically active substances
It is believed that the system of biological active substances (BAS) and their specific receptors is even characteristic of the cell. However, the concepts of "hormone" and "BAS" are not identical. A hormone is called a 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 keilons. These substances are secreted by a population of cells where they inhibit reproduction and regulate apoptosis. Prostoglandins are also an example of biologically active substances. The modern classification of hormones gives them a special group of eicosanoids. They are intended for local regulation of inflammation in the tissues and for the implementation of hemostatic processes at the level of arterioles.
Chemical classification of hormones
Chemical hormones 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 gland, hypothalamus, pancreas and parathyroid glands);
- steroid group (secreted by the endocrine part of the male sex glands and cortical areas of the adrenal glands);
- a group of amino acid derivatives (formed by the thyroid gland and the adrenal medulla);
- group of eicosanoids (secreted by cells, synthesized from arachidonic acid).
It is noteworthy that the sex hormones of women are also included in the steroid group. However, they are not, by and large, steroids: the effect of hormones of this type is not associated with the anabolic effect. However, their metabolism does not lead to the formation of 17-ketosteroids. Although ovarian hormones are structurally similar to other steroids, they are not. Since they are synthesized from cholesterol, to simplify the basic chemical classifications, they are assigned to other steroids.
Classification by place of synthesis
Hormonal substances can be divided at the place of synthesis. Some are formed in peripheral tissues, while others are formed in the central nervous system. The method of secretion and release of substances depends on this, which determines the features of the implementation of their effects. The classification of hormones in place looks like this:
- hypothalamic hormones (releasing factors) ;
- pituitary (tropic hormones, vasopressin and oxytocin);
- thyroid (calcitonin, tetraiodothyronine and triiodothyronine);
- parathyroid (parathyroid hormone) ;
- non-adrenal (norepinephrine, adrenaline, aldosterone, cortisol, androgens);
- genital (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 intestine, in the liver and pancreas. Their goal 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, pituitary tropic hormones);
- regulators of calcium and phosphorus metabolism (parathyroid hormone, calcitonin and calcitriol);
- regulators of water-salt balance (vasopressin, aldosterone);
- reproductive function regulators (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).
Hormone secretion and transportation
The secretion of hormones occurs immediately after their synthesis. They enter directly into the bloodstream or into tissue fluid. The last place of secretion is characteristic of 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 distributed with 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.
Signaling to the receptor
The above classification of hormones reflects the effects of substances on tissues and organs. Although this is only possible after binding of the chemical to the receptor. The latter are different and are located both on the surface of the cell, 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 signal transmission.
This basic classification of hormones allows you to draw conclusions about the speed of signal transmission. For example, the extracellular mechanism is much faster than the intracellular. It is characteristic of adrenaline, norepinephrine, and other peptide hormones. The intracellular mechanism is characteristic of lipophilic steroids. Moreover, the benefit for the body is achieved faster in 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.
Characterization of signal transmission types
An extracellular mechanism is characteristic of peptide hormones that cannot enter the cytoplasmic membrane into the cytoplasm without a specific carrier protein. This is not provided for him, 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 a 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.