Endocrine glands-are deprived of the excretory ducts of the gland, which produce hormones (from buckwheat. hormao-encourage, set in motion) and secrete them into the blood or lymph. Blood carries hormones and delivers them to organs, tissues, cells. Hormones regulate metabolism, cell reproduction, the body’s response to constant changes in environmental conditions. The main function of hormones is regulation of homeostasis. Homeostasis-the process of maintaining the internal environment of the body, in which various parameters (eg, blood pressure, body temperature, acid-base balance, etc.) are maintained in equilibrium, despite changes in environmental conditions. Without this, the life of the body is impossible.
Endocrine glands are separated. All of them are divided into two groups: pituitary-dependent, whose function depends on the anterior pituitary, and independent of the pituitary gland. Pituitary glands – thyroid, cortical substance of the pituitary and sexual. Do not depend on the pituitary parathyroid, epiphysis, pancreatic islets, adrenal medulla.
Central nervous system and, first and foremost, the bark of the big hemispheres of the brain regulate the function of endocrine glands. This is done in two ways: direct innervation of the glands and mediated regulation of the hypothalamus pituitary function.
Hypothalamus-the structure of the brain (intermediate brain), which is the highest center of regulation of endocrine functions. It forms a single functional complex with the pituitary gland, in which the hypothalamus itself is a regulator, and the pituitary gland is a performer.
The hypothalamus exercises control over the feeling of thirst, hunger, appetite, body temperature, water balance, sexual function. In the hypothalamus lie the centers that regulate emotions and behavior, sleep and wakefulness, body temperature, etc. Centers of the cerebral cortex, in turn, adjust the reaction of the hypothalamus that arise in response to changes in the internal environment of the body. In recent years, from the hypothalamus, morphine-like enkephalins and endorphins have been isolated. It is believed that they affect the behavior (defensive, food, sexual reactions) and vegetative processes that ensure human survival.
Thus, the hypothalamus regulates all functions of the body, except the heart rate, blood pressure and spontaneous respiratory movements, which are regulated by the medulla oblongata.
The pituitary gland is the most important gland that resembles a pea weighing about 0.5 g in men (0.6–0.7 g in women), hidden under the brain and reliably protected by the wedge-shaped bone of the skull. The tiny pituitary gland is divided into two parts: the anterior – adenohypophysis and the posterior – neurohypophysis. The pituitary gland regulates the activity of many endocrine glands, but also depends on the hypothalamus, which produces substances that affect the cells of the anterior lobe: liberins, stimulating the formation of pituitary hormones that affect the dependent glands, and statins, inhibiting the production of these hormones.
Anterior lobe of the pituitary. One of the most important hormones – growth hormone, or somatotropic hormone (STG), stimulating the growth of bones, muscles, organs, body. The hormone prolactin, or lactotropic hormone (LTG), in men stimulates the growth of testicles, prostate, sperm maturation (in women promotes the formation of milk during breastfeeding). Gonadotropins – follicle stimulating hormone (FSH) and luteinizing hormone (LH) – affect the sex glands: FSH in men stimulates spermatogenesis (in women – follicle maturation in the ovary); LH in men promotes the formation of the male sex hormone testosterone (in women it stimulates ovulation, the formation of the yellow body and the secretion of sex hormones). Thyroid-stimulating hormone (TSH) promotes the formation of thyroid hormones. Adrenocorticotropic hormone (ACTH) acts on the synthesis of glucocorticoid hormones of the adrenal cortex.
Reducing the function of the anterior pituitary depends on the involvement in the process of certain cells that produce hormones. Thus, a decrease in STG production in childhood leads to dwarfism, an increase – to gigantism or acromegaly (excessive growth of certain parts of the body, such as the nose, chin, fingers or toes, an increase in the tongue, lips, some internal organs). Reducing the release of gonadotropins causes serious sexual disorders in men (impotence), in women – termination of menstruation, ovulation. Lack of TSH leads to hypothyroidism, excess-to hyperthyroidism. Violation of the synthesis of ACTH causes Addison’s disease, increased synthesis of ACTH leads to the development of Cushing’s disease.
The posterior pituitary does not synthesize hormones. In the hypothalamus, the hormones oxytocin and vasopressin are formed, which enter the posterior pituitary gland, where they are stored until they are released. In men, oxytocin stimulates the smooth muscles of the VAS deferens and the promotion of sperm (in women, it causes uterine contractions during childbirth and the release of milk during breastfeeding). Both men and women oxytocin increases sensitivity during sexual intercourse and a sense of pleasure, satisfaction, strengthens the attachment of partners to each other. Vasopressin promotes the reverse absorption of water from the primary urine in the kidneys and thereby increases blood pressure. Lack of vasopressin leads to the development of diabetes insipidus, in which a person daily allocates a huge amount of urine (up to 30 liters per day!). With increased function of the posterior lobe increases blood pressure.
The thyroid gland-a large endocrine gland located on the neck in front of the larynx, consists of a large number of follicles (about 30 million). The wall of the follicle is formed by one layer of thyrocyte cells synthesizing the protein thyroglobulin, which enters the cavity of the follicle and is part of the jelly-like colloid containing thyroid hormones (Fig. 1.29). The secretion of thyroid hormones is regulated by the thyroid-stimulating hormone of the pituitary gland. Thyroid hormones (thyroxine and triiodothyronine) provide growth, mental and physical development, regulate the rate of metabolic processes. Iodine is required for normal thyroid function.
Strengthening the function of the thyroid gland leads to hyperthyroidism, or graves ‘ disease, which is manifested by weight loss, fever, acceleration of the pulse, pucheglaziem, increased excitability. Reduced function (hypothyroidism) slows down metabolic processes and disrupts brain activity. In congenital disease, there is cretinism; if the disease develops in adults, there is a mental and physical retardation, reduced sensitivity to the effects of cold, slowing the pulse, a significant increase in weight and coarsening of the skin (myxedema). As a rule, hypothyroidism is associated with a lack of iodine in food and water. In such patients, goiter is formed.
Pay attention! The use of iodized salt for the prevention of hypothyroidism is dangerous, because it is impossible to accurately dose iodine because of the difference in the amount of salt consumed.
In addition to the follicles, which produce thyroxine and triiodothyronine, in the thyroid gland are acropoliscasino cells that produce the hormone thyrocalcitonin, which is involved in the regulation of metabolism of calcium and phosphorus.
Adrenal glands-paired endocrine glands of triangular shape are located above the upper poles of the kidneys. Each adrenal gland consists of two parts: the medulla and cortical substance. The brain substance consists mainly of chromaffin tissue and under the action of the sympathetic nervous system produces adrenaline and norepinephrine. In the cortical substance are clearly visible three zones. Outside is a glomerular zone that produces mineralocorticoid hormone aldosterone, which affects the exchange of water. This is due to the removal of potassium from the kidneys and reverse absorption in the tubules of sodium kidneys. The middle zone (beam) forms glucocorticoids (cortisol), which regulate protein, fat and carbohydrate metabolism, have anti-inflammatory and anti-allergic effects, participate in stress reactions, increase the body’s resistance to infection. In the inner, closest to the brain substance of the mesh zone produced in both sexes sex hormones – male (androgens), stimulating protein metabolism and muscle mass (this action is called anabolic), and female estrogens. Synthesis and isolation of glucocorticoids and androgens is regulated by ACTH, produced by the anterior pituitary gland. The brain substance is formed by chromaffin cells, which by their origin and function are close to sympathetic neurons (see section. “Autonomic nervous system” later in this Chapter). Brain cells produce a small amount of adrenaline and norepinephrine, and only when exposed to strong stimuli secretion increases sharply under the influence of the sympathetic nervous system. These hormones cause narrowing of small blood vessels, which increases blood pressure; increases blood flow through the coronary arteries, accelerates and increases the heart rate; increases the frequency and depth of respiratory movements, increases lung ventilation, dilates the bronchi, as well as relax the smooth muscles of the intestine and weaken the movement of the intestine. In addition, hormones increase the breakdown of glycogen and fat.
Bilateral decrease in the function of the cortical beam zone leads to the development of Addison’s disease (bronze disease) due to glucocorticoid deficiency. It shows weakness, the appearance of the skin dark (bronze) stain, decrease of blood sugar levels. Strengthening the function of the beam zone due to the tumor causes Cushing’s disease. Patients have a moon-shaped face, obesity of the trunk. Strengthening the function of the mesh zone in men leads to premature puberty, reduced function – to impotence (women develop secondary male sexual characteristics, male type of hair).
The sex glands are described in the section. “Male reproductive system” earlier in this Chapter.
Parathyroid. Four small glands, located on the back surface of the thyroid gland, produce parathyroid hormone, which is involved in the exchange of calcium and indirectly phosphorus in the blood. The hormone promotes the leaching of calcium from the bones (demineralization) and its release into the blood. At the same time, excess phosphorus is released from the bones, which is released by the kidneys. At the same time, the hormone prevents the release of calcium by the kidneys and increases its absorption in the intestine. Thyrocalcitonin produced acropolisrpm cells of the thyroid gland, is an antagonist of parathyroid hormone. The level of calcium in the blood is regulated by both hormones. Reducing the content of calcium in the blood is a signal for the synthesis and isolation of parathyroid hormone, which increases the content of calcium in the blood. Conversely, an increase in the content of calcium in the blood is a stimulus to the synthesis and release of thyrocalcitonin, which enhances the fixation of calcium in the bone, reducing its level in the blood. Strengthening the function of the parathyroid glands leads to the destruction of bones and increase its content in the blood and the deposition of calcium in the walls of blood vessels. Reduced function of the parathyroid glands causes calcium deficiency in the blood, which increases the excitability of the nervous system. Convulsions come. This condition is called tetanus and is expressed in spasm and convulsive twitching of muscles, especially the muscles of the face, hands and feet.
Pancreatic islets (islets of Langerhans). I repeat: the pancreas consists of two glands – exocrine (it is described in the section. “Digestive system” earlier in this Chapter) and endocrine (islets of Langerhans), consisting of small clusters of cells scattered across the pancreas that secrete hormones insulin, glucagon and somatostatin. In the human pancreas, the number of Islands from 1 to 2 million Islands contain three types of cells: about 60-80 % are B cells that synthesize insulin; 10-30 % are cells that synthesize glucagon; approximately 10% of cells synthesize somatostatin.
Insulin is of great importance for the regulation of sugar (glucose) in the blood. It lowers blood sugar. Insulin secretion is stimulated by increasing the sugar content in the blood. In addition, insulin stimulates the formation of glycogen, fat and protein. The lack of this hormone in the body leads to the development of diabetes in humans; while in the blood and urine of the patient contains a large amount of sugar. Diabetes is a very common disease. About diabetes, we describe in detail in sec. “Some diseases of the endocrine glands” Chapter 3. Glucagon increases the sugar content in the blood, thus providing an action opposite to the action of insulin. It promotes the breakdown of glycogen and fats. Somatostatin inhibits the secretion of growth hormone by the anterior lobe of the pituitary gland and the secretion of insulin and glucagon.
Epiphysis (pineal body) is very small, no more grain, but its role in the body is great. The epiphysis is a part of the intermediate brain (see section. “Nervous system” later in this Chapter). The function of the epiphysis has a clear daily rhythm: at night the cells synthesize melatonin, during the day – serotonin. This rhythm is associated with illumination, while light inhibits the synthesis of melatonin. The impact is carried out with the participation of the hypothalamus. Currently, it is believed that the epiphysis regulates the function of the sexual glands, primarily puberty, and also serves as a “biological clock” that regulates the rhythm of sleep and wakefulness and other circadian rhythms (the period of which is 20-28 hours). The epiphysis models the activity of the pituitary gland, pancreatic islets, parathyroid glands, adrenal glands, sex glands and thyroid gland. Reduced function of the epiphysis leads to premature puberty.