ENDOCRINE GLANDS OVERVIEW

ENDOCRINE GLANDS OVERVIEW

  • General Principles of the Endocrine System

    • Unlike the nervous system, which uses electrochemical impulses for rapid, localized responses, the endocrine system uses hormones secreted into the bloodstream to produce widespread, long-lasting effects on distant target cells.

    • Target Cells: Hormones only affect cells that possess specific protein receptors for that specific hormone.

    • Gland Types:

    • Endocrine glands: Ductless glands that release secretions into the blood or lymph.

    • Exocrine glands: Secrete products through ducts onto body surfaces or into body cavities (e.g., sweat or salivary glands).

  • Major Endocrine Organs and Their Functions

    • Anterior Pituitary (Adenohypophysis): Regulated by the hypothalamus; secretes Growth Hormone (GH), Thyroid-Stimulating Hormone (TSH), Adrenocorticotropic Hormone (ACTH), Follicle-Stimulating Hormone (FSH), Luteinizing Hormone (LH), and Prolactin (PRL).

    • Thyroid Gland: Located in the neck; produces T<em>3T<em>3 (triiodothyronine) and T</em>4T</em>4 (thyroxine) which regulate metabolic rate, and Calcitonin which lowers blood calcium levels.

    • Parathyroid Glands: Small glands on the posterior thyroid; secrete Parathyroid Hormone (PTH), the primary regulator of blood calcium levels (increasing levels when low).

    • Adrenal Cortices: The outer layer of the adrenal gland; produces corticosteroids such as Cortisol (stress response) and Aldosterone (sodium/potassium balance).

    • Thymus: Active in early life; secretes thymosins involved in T-lymphocyte maturation and immune function.

    • Pancreas: A mixed gland; the Islets of Langerhans secrete Insulin (to lower blood glucose) and Glucagon (to raise blood glucose).

    • Gonads (Testes/Ovaries): Produce sex hormones (Testosterone, Estrogens, and Progesterone) for reproductive maturation and function.

  • Neuroendocrine Organs

    • Hypothalamus: The master regulator; produces "releasing" and "inhibiting" hormones that control the anterior pituitary, and synthesizes Oxytocin and Antidiuretic Hormone (ADH) stored in the posterior pituitary.

    • Adrenal Medulla: Part of the autonomic nervous system; secretes Epinephrine and Norepinephrine during the "fight-or-flight" response.

    • Pineal Gland: Located in the brain; secretes Melatonin, which regulates sleep-wake cycles (circadian rhythms).

  • Hormone Chemistry and Mechanisms

    • Amino Acid-Based/Protein Hormones: Usually water-soluble; cannot cross the plasma membrane. They utilize second messenger systems (like cAMP) to trigger intracellular responses.

    • Steroid Hormones: Derived from cholesterol (e.g., testosterone, cortisol); lipid-soluble. They diffuse across the plasma membrane and bind to intracellular receptors to directly influence gene transcription.

    • Thyroid Hormones: Although amino acid-derived, they act like steroids by entering the cell and binding to nuclear receptors.

  • Regulation of Hormone Secretion

    • Humoral Stimuli: Secretion in response to changing blood levels of ions and nutrients (e.g., low blood calcium stimulating PTH).

    • Neural Stimuli: Nerve fibers stimulate hormone release (e.g., sympathetic fibers triggering the adrenal medulla).

    • Hormonal Stimuli: Release of a hormone in response to another hormone (e.g., TSH stimulating the thyroid to release T4T_4).

    • Feedback Mechanisms: Most secretion is regulated by negative feedback loops, where the effect of the hormone eventually shuts off the initial stimulus to maintain homeostasis.