Endocrine System

Endocrine System: Hormones directly enter the extracellular fluid, then diffuse into the bloodstream, acting on respective target cells via specific circulatory pathways.

Endocrine System = Endocrine Glands + Endocrine Cells

Endocrine Cell Group: Cells within certain organs forming glandular tissues with endocrine functions.

Hormones: Highly efficient physiological substances secreted by the endocrine system.

Endocrine System: Cells or glands with endocrine functions secrete highly efficient physiological substances that regulate organ functions through circulation in the bloodstream or fluid diffusion.

Neuroendocrine: The central nervous system contains neurons with endocrine functions that can produce nerve impulses and synthesize and release hormones.

Neurohormones: Hormones produced by neuroendocrine cells.

Source, Types, Mode of Action, Regulation, and Effect of Hormones

Source: Endocrine cells, endocrine glands

Types: Amines, peptides, proteins, lipids

Modes of Transmission: Endocrine, Paracrine, Autocrine, Intracrine

  • Endocrine: Act on distant target cells through bloodstream transport (blood transport - long-distance).

  • Paracrine: Act on adjacent cells directly through tissue fluid diffusion (fluid diffusion - neighboring).

  • Autocrine: Hormones secreted by endocrine cells diffuse locally and act back on those same cells (self - self).

  • Intracrine: Hormones produced by endocrine cells that act directly by binding to receptors within the same cell.

Steps of Action: Receptor recognition, signal transduction, cellular response, effect termination

Mechanisms of Action:

  1. Hormonal action mediated by cell membrane receptors - Second Messenger Theory
    Hormone + Receptor → G protein-coupled receptor → Adenylyl cyclase → cAMP (second messenger) → Protein kinase A

  2. Hormonal action mediated by intracellular receptors - Gene Expression Theory
    Hormone + Cytoplasmic receptor → Hormone-receptor complex enters the cell nucleus

Modes of Regulation: Biological rhythmic secretion (biological clock), fluid regulation (direct feedback regulation, axis feedback regulation), neural regulation

  • Direct feedback regulation: Effect substance: Blood sugar ↑ → Insulin ↑ → Blood sugar ↓ → Insulin ↓

  • Axis feedback regulation: Hypothalamus-pituitary-target gland axis

  • Neural regulation: Sympathetic ↑ → Adrenal medulla hormone ↑; Sympathetic ↑ → Adrenal medulla hormone ↑

  • The hypothalamus is a crucial pathway linking the nervous and endocrine systems.

Important Endocrine Glands

Important Endocrine Glands: Hypothalamus and Pituitary Gland

Pituitary Gland: Neurohypophysis (does not release hormones but stimulates the hypothalamus to release hormones) + Adenohypophysis

Pituitary Portal System: Vascular network connecting the adenohypophysis and hypothalamus

Neurohypophysis (stores and releases 2 hormones produced by the hypothalamus), Adenohypophysis (produces and releases 7 hormones)

Antidiuretic Hormone: Synthesized and secreted by the hypothalamus, stored and released by the neurohypophysis

Important Endocrine Glands: Adrenal Glands

Important Endocrine Glands: Thyroid (Synthesis and release of thyroid hormones) and Parathyroid Glands

Synthesis of Thyroid Hormones: Thyroid hormones = Triiodothyronine T3 + Thyroxine T4.
Synthesis Steps: Iodination, iodine conversion (requires thyroid peroxidase TPO), coupling

Storage: Exist in extracellular follicular cavities, bind to thyroglobulin TG until secretion

Secretion: Regulated by Thyroid Stimulating Hormone TSH. Pinocytosis → hydrolysis → into blood

Parathyroid Glands secrete Parathyroid Hormone and Calcitonin

Important Endocrine Glands: Various Cells of the Pancreas

Pancreas: Dispersed endocrine cell clusters within the pancreas, divided into 4 types

Alpha cells: Glucagon; Beta cells: Insulin; Delta cells: Somatostatin; PP cells

Insulin

Function: Promotes synthetic metabolism (lowers blood sugar - promotes utilization/glycogen synthesis, lowers fats - promotes storage, raises proteins - promotes synthesis/reduces breakdown)

Regulation: Substrate regulation - blood sugar (↑ blood sugar → insulin secretion; ↓ blood sugar → glucagon secretion), other hormones (gastrointestinal hormones, etc.), neural regulation (sympathetic - inhibits secretion - raises blood sugar, vagus - promotes secretion)

“Most”

Pituitary Gland: The most complex endocrine gland in the body

Key Points

  • Understanding the endocrine system

  • Endocrine, autocrine, paracrine

  • Understanding hormones

  • Synthesis and release of thyroid hormones (understand) - what is needed for release - thyroglobulin

  • Antidiuretic hormone - urinary system

  • Insulin - digestion

  • Pituitary gland - regulation of endocrine functions

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