HUMAN MEDICAL BIOLOGY, ANATOMY, PHYSIOLOGY AND PATHOLOGY - Ya.I. Fedonyuk 2010

ANATOMY, PHYSIOLOGY, PATHOLOGY

CHAPTER 3. ANATOMICAL AND PHYSIOLOGICAL ASPECTS OF BODY FUNCTION AUTO-REGULATION

ENDOCRINE GLANDS AND THEIR PATHOLOGY

GENERAL CONCEPT OF ENDOCRINE GLANDS. HORMONES. HORMONAL REGULATION

Glands of internal secretion, or Endocrine glands, are those that lack excretory ducts and release their secretions (incretions, Hormones) directly into the Blood or Lymph.

The process of synthesis and release of physiologically active substances into the internal environment by endocrine glands is called internal secretion, and these substances are called hormones.

Endocrine glands include the Pituitary Gland, Pineal Gland, Parathyroid glands, Thyroid Gland, Thymus, and Adrenal Glands. The Pancreas and Gonads are mixed glands that produce both hormones and exocrine products.

In addition to endocrine glands, there are paracrine glands and Cells located within various Organs (such as The Stomach, Urethra, and Kidneys) among other Tissues that secrete tissue hormones.

The most widely accepted Classification of endocrine glands is Alyoshin's classification, which divides all endocrine glands into four groups: 1) the adenohypophysis and its dependent endocrine glands (thyroid gland, Testis, Ovaries, and adrenal cortex); 2) glands independent of the adenohypophysis (parathyroid glands, thymus, adrenal medulla, and pancreatic islets); 3) glands of "nervous origin" (Hypothalamus, intestinal and gastric cells); and 4) glands of neuroglial origin (pineal gland, neurohypophysis).

Depending on their origin from various types of epithelium, endocrine organs are divided into five groups (Zavarzin A.A., Shchelkunov S.I.).

1. Glands of endodermal origin that develop from the epithelial lining of the pharyngeal gut—the so-called branchiogenic group. These include the thyroid and parathyroid glands.

2. Glands of endodermal origin derived from the epithelium of the intestinal tube—The Endocrine Part of the Pancreas (pancreatic islets).

3. Glands of mesodermal origin—the interrenal system, the adrenal cortex, and the interstitial Cells of the gonads.

4. Glands of ectodermal origin derived from the anterior neural tube (neurogenic group)—the pituitary gland and the pineal body (pineal gland of the Brain).

5. Glands of ectodermal origin derived from the Sympathetic division of the Autonomic Nervous system. This group includes The adrenal medulla and Paraganglia.

Hormones are physiologically active substances synthesized by specialized cells of endocrine glands. They enter the bloodstream directly and exert their effects exclusively on complex cellular structures such as Cell membranes and enzyme systems.

Based on their chemical Structure, hormones are classified into Steroid Hormones (corticosteroids, androgens, estrogens), protein hormones (Insulin and Glucagon), Tyrosine derivatives (thyroxine, norepinephrine), and fatty acid derivatives (Prostaglandins).

Hormones mediate their effects through receptors in target organs. There are two types of hormonal reception: intracellular and surface membrane-bound. In the first case, hormones easily penetrate The Cell through the membrane and bind to a receptor in the Cytoplasm; subsequently, the hormone-receptor complex is transported into The Nucleus and interacts with Chromatin. This activates RNA Synthesis, modulates DNA and METABOLISM/31.html">Transcription processes, and induces long-term restructuring of cellular metabolism.

The second pathway of reception involves the binding of hormones to cell-surface receptors. This triggers The formation of a hormonal second messenger, cyclic 3,5-adenosine monophosphate, from ATP. The latter acts on cellular Enzymes, thereby altering cell metabolism and permeability.

By influencing Protein Synthesis within cells, hormones regulate physical, sexual, and mental development, Metabolism and Energy production, help maintain Homeostasis (glucose levels, electrolyte balance, osmotic pressure, and blood pressure), and ensure the adaptation of physiological systems. They also influence human behavior and mental state. Hormonal action induces both physiological and morphological changes.

Synergism refers to actions in the same direction. From a cybernetic perspective, this is a manifestation of THE PRINCIPLE OF redundancy, which ensures high reliability in maintaining homeostasis. For example, epinephrine, a hormone of the adrenal medulla, and glucagon, a pancreatic hormone, both activate The breakdown of Liver Glycogen into glucose, resulting in elevated blood sugar levels.

Antagonism refers to actions in opposite directions. An example is insulin, a pancreatic hormone that lowers blood sugar levels, and cortisol (or adrenaline), a hormone of the adrenal cortex that raises blood sugar. Despite their antagonistic effects, these hormones ultimately work together to optimize the carbohydrate Nutrition of tissues.

The Essence of permissive action is that a hormone, while not producing a Physiological Effect of its own, creates the conditions necessary for an organ to respond to the action of a second hormone. For instance, glucocorticoids do not affect vascular tone, yet they create the conditions under which threshold concentrations of adrenaline can elevate blood pressure.

The activity of endocrine glands is regulated by The Nervous System through two main pathways. First, via the hypothalamic-pituitary system: neurosecretory cells of the hypothalamus produce releasing hormones. These hormones reach the anterior pituitary via its portal Venous system and stimulate (liberins) or inhibit (statins) the release of Pituitary Hormones. These hormones then enter the general Circulation, reach The Thyroid Gland and adrenal cortex, and stimulate the secretion of hormones by these target glands.

Secondly, through the efferent influences of the autonomic nervous system, the Central Nervous System regulates the activity of the pancreas and the adrenal medulla.

The regulation of endocrine gland function, like any regulatory process, operates on the principle of self-regulation through feedback loops. This ensures the stabilization of endocrine gland activity. For instance, an increase in blood glucose levels leads to a higher concentration of insulin.