Chapter 1: An overview of endocrine physiology

The function of the endocrine system is to coordinate and integrate cellular activity throughout the body by regulating the function of cells and organs of life and maintaining homeostasis. Homeostasis, or maintaining a stable internal environment, is critical to ensuring cell function.

Endocrine System: Physiological Functions and Components of the Endocrine System

Some of the major functions of the endocrine system include:

:• Adjust water and electrolyte balance; control of circulating volume and blood pressure

• Regulates calcium and phosphate balance to maintain extracellular fluid volume concentrations required for membrane integrity and intracellular signaling

• Regulate energy balance and control the mobilization, use and storage of materials to ensure that the metabolic needs of the cell are met

• Coordinate host hemodynamic and metabolic counterregulatory responses to stress

• Regulates reproduction, development, growth and aging

In the classical description of the endocrine system, a chemical messenger or hormone produced by an organ is released into the circulation to exert an effect on the target organ. Currently, the precise concept of the endocrine system is an integrated network of many different embryonic-derived organs that release hormones ranging from peptides to glycoproteins, exerting their effects in different organs. neighboring or distant cells. The endocrine network of these organs and mediators does not function individually and is tightly integrated with the central and peripheral nervous systems as well as with the immune system, leading to the term currently used. known as the “neuroendocrine” system or the “neuroendocrine-immune system” to describe their correlation. Three basic components make up the core of the endocrine system.

Figure 1-1. Endocrine system. Endocrine organs are located throughout the body, and their function is controlled by hormones delivered through the circulation or produced locally or by direct nerve stimulation. The integration of hormone production from the endocrine organs is regulated by the hypothalamus. ACTH, adrenocortical hormone; CRH, corticotropin-releasing hormone; FSH, follicle stimulating hormone; GHRH, growth hormone-releasing hormone; GnRH, gonadotropin-releasing hormone; LH, luteal-forming hormone; MSH, the hormone that regulates melanin; TRH, thyrotropin-releasing hormone; TSH, thyroid-stimulating hormone; T3, triiodothyronine; T4, thyroxine.

Endocrine glands

Classical endocrine glands are ductless and secrete their chemical products (hormones) into the interstitium from where they reach the circulation. Unlike the cardiovascular, renal, and digestive systems, the endocrine glands are not anatomically connected and are scattered throughout the body (Figure 1–1). Communication between different organs is ensured through the release of hormones or neurotransmitters..

Hormones

Hormones are chemical products, released in very small amounts from cells, that have biological effects on target cells. Hormones may be released from the endocrine glands (ie insulin, cortisol); brain (ie, corticotropin-releasing hormone, oxytocin, and antidiuretic hormone); and other organs such as the heart (atrial diuretic sodium peptide), liver (insulin – growth factor 1), and adipose tissue (leptin).

Target body

The target organ contains cells that express hormone-specific receptors and respond to hormone binding with a demonstrable biological response.

HORMONE AND MECHANISM OF WORK

Based on their chemical structure, hormones can be classified into proteins (or peptides), steroids, and amino acid derivatives (amines). Hormone structure, to a large extent, determines the location of the hormone receptor, with amines and peptide hormones binding to cell surface receptors and steroid hormones being able to cross the plasma membrane and bind to intracellular receptors. An exception to this generalization is thyroid hormone, an amino acid-derived hormone that is transported into cells to bind to its nuclear receptor. The hormone structure also affects the half-life of the hormone. Amines have the shortest half-lives (2–3 minutes), followed by polypeptides (4–40 minutes), steroids and proteins (4–170 minutes), and thyroid hormone (0.75–6.7 days). ).
Protein or Hormone Peptides Protein or peptide hormones make up the majority of hormones. These are molecules with 3 to 200 amino acid residues. Eyes are synthesized as prerohormones and undergo post-translational processing. The eye is stored in secretory granules before being released by exocytosis (Figure 1–2), in a manner reminiscent of how neurotransmitters are released from nerve terminals.
Examples of peptide hormones include insulin, glucagon, and adrenocorticotropic hormone (ACTH). Some of the hormones in this group, such as gonadal hormone, luteinizing hormone, and follicle-stimulating hormone, along with thyroid-stimulating hormone (TSH) and human chorionic gonadotropin, are shown in Figure 1–2. Synthesize peptide hormones. Peptide hormones are synthesized as prerohormones in the ribosome and processed into prohormones in the endoplasmic reticulum (ER). In the Golgi apparatus, hormones or prohormones are encapsulated in secretory vesicles, which are released from the cell in response to the amount of Ca 2+. An increase in cytoplasmic Ca2+ is required for binding of secretory vesicles in the plasma membrane and for exocytosis of vesicular contents. Hormones and post-translational products occurring within the secretory vesicle are released to the extracellular matrix.

Eg: About peptide hormones are adrenocorticotropic hormone (ACTH), insulin, growth hormone, and glucagon. Endocrine cells Interstitium Cytosol Plasma membrane Synthesis Nuclear endoplasmic reticulum Golgi apparatus Secretory vesicle Preprohormone Prohormone Encapsulation Prohormone Storage Hormone Hormone Secretion Ca2 + Hormone (and any “pro” segment) into carbohydrate fraction, leading to them being designated as glycoproteins. Carbohydrate radicals play an important role in determining the biological activities and circulating clearance rates of glycoprotein hormones.

Figure 1–2. Synthesize peptide hormones. Peptide hormones are synthesized as prerohormones in the ribosome and processed into prohormones in the endoplasmic reticulum (ER). In the Golgi apparatus, hormones or prohormones are encapsulated in secretory vesicles, which are released from the cell in response to the amount of Ca 2+. An increase in cytoplasmic Ca2+ is required for binding of secretory vesicles in the plasma membrane and for exocytosis of vesicular contents. Hormones and post-translational products occurring within the secretory vesicle are released to the extracellular matrix. Examples of peptide hormones are adrenocorticotropic hormone (ACTH), insulin, growth hormone, and glucagon.

Steroid Hormones

Steroid hormones are derived from cholesterol and are synthesized in the adrenal cortex, gonads, and placenta. They are lipid soluble, circulate bound to binding proteins in the plasma, and cross the plasma membrane to bind to intracellular or nuclear receptors. Vitamin D and its metabolites are also considered steroid hormones. The synthesis of steroid hormones is described in Chapters 5 and 6.

Amino Acid–Derived Hormones

Amino acid derived hormones are those synthesized from the amino acid tyrosine and include the catecholamines norepinephrine, epinephrine and dopamine; as well as thyroid hormones, which are derived from the combination of two iodized tyrosine amino acids. The synthesis of thyroid hormones and catecholamines is described in Chapters 4 and 6.

Hormone Effects

Depending on where the biological effects of a hormone are produced relative to where the hormone is secreted, its effects can be classified in 1 of 3 ways (Figures 1–3). Endocrine effects when a hormone is released into the circulation and then travels through the bloodstream to exert a biological effect on distant target cells. This effect is endocrine when a hormone released from a cell exerts a biological effect on neighboring cells, usually cells in the same organ or tissue. This effect is self-secreting when a hormone exerts a biological effect on the same cell that released it. Recently, an additional mechanism of hormone action has been proposed in which a hormone is synthesized and acts intracellularly within the same cell. The Th mechanism has been termed endocytosis and has been identified to be involved in the effects of parathyroid hormone-associated peptides in malignant cells and in some effects of androgen-derived estrogens (see Fig. Chapter 9).

Figure 1–3. The mechanism of action of hormones. Depending on where the hormones come into play, they can be classified as endocrine, endocrine, and autoendocrine mediators. The hormones enter the bloodstream and bind to hormone receptors in target cells in distant organs mediating endocrine actions. Hormones bind to the proximal cells and release them to mediate the action of paracrine. Hormones exert their physiological effects by binding to receptors on the same cells that induce them to mediate autocrine effects.