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Endocrine System Overview

• The endocrine system is a system of glands and hormone-secreting cells that regulates body functions.
• Endocrine glands are ductless structures that release hormones directly into the blood.
• Hormones are chemical messengers that regulate cells of the body by binding to receptors.
• Target tissues are tissues that contain specific receptors to which hormones bind.
• Hormone action affects cellular activities through a series of events triggered by receptor-hormone binding.

Biochemical Classification of Hormones

• Hormones are categorized into three biochemical groups: steroids, amines, and proteins/peptides.
• Steroid hormones are derived from cholesterol and are lipid soluble.
• Amines are derived from amino acids.
• Proteins/peptides are composed of amino acids.

Steroid Hormones

• Produced by the adrenal cortex, testes, ovaries, and placenta.
• Synthesized from cholesterol.
• Lipid soluble; therefore, they cross cell membranes readily and bind to intracellular receptors.
• Not typically stored in the endocrine gland.
• Lipid solubility allows for oral administration.

Protein/Peptide Hormones

• Derived from amino acids.
• Preformed and stored in membrane-bound secretory granules.
• Released by exocytosis when needed.
• Water soluble; circulate in the blood predominantly unbound.
• Short half-lives.
• Cannot be administered orally.
• Typically administered by injection.

Amine Hormones

• Include thyroid hormones and catecholamines.
• Thyroid hormones are biologically similar to steroid hormones, primarily insoluble in blood and transported bound to proteins.
• Have longer half-lives.
• Can cross cell membranes and be administered orally.
• Catecholamines are biologically similar to protein/peptide hormones; water soluble and have relatively short half-lives.

Lipid-versus Water-Soluble Hormones

• Hormones are classified based on their solubility; Lipid-soluble ones include steroids and thyroid hormones; Water-soluble ones include peptides and proteins.
• Lipid-soluble hormones have intracellular receptors.
• Water-soluble hormones have receptors on the cell membrane.
• Lipid-soluble hormones are synthesized as needed, stored in vesicles, transported attached to carrier proteins, have a long half-life, and are mostly dissolved in plasma.

Hormone Receptors Locations

• Receptors are located on the cell membrane (for protein hormones), in the cytoplasm (for steroid hormones), or in the nucleus (for thyroid hormones).
• Steroid hormones are lipid soluble and move directly into target cells.

Feedback Control of Hormone Secretion

• The endocrine system is regulated by feedback mechanisms.
• Negative feedback involves stimulus triggers that inhibit further change (e.g., body temperature, sugar metabolism).
• Positive feedback involves stimulus triggers that amplify effects (e.g., labor contractions, lactation).

Transport of Hormones

• Most steroid and thyroid hormones are bound to plasma proteins.
• The liver produces proteins that bind lipid-soluble hormones (e.g., cortisol-binding globulin, thyroid-binding globulin, sex hormone-binding globulin).

Functional Classification of Hormones

• Hormones are classified as either trophic or non-trophic.
• Trophic hormones act on other endocrine glands to stimulate their hormone secretion.
• Examples include thyrotropin (TSH) and adrenocorticotropin (ACTH).
• Non-trophic hormones act on non-endocrine target tissues.
• Examples include parathormone (which acts on bone tissue) and aldosterone (which acts on the kidney).

Mechanisms of Hormones Action

• Hormone action involves changes like opening/closing ion channels, activating/deactivating enzymes, stimulating protein synthesis, inducing secretory activity, and stimulating mitosis.