Anatomy and Physiology - Chapter 18: The Endocrine System

Chapter 18: The Endocrine System

Key Concepts of the Endocrine System

  • General Functions and Components:

    • Integrates bodily functions, regulates homeostasis, growth, reproduction, metabolism through hormones.

  • Comparison of Nervous and Endocrine Systems:

    • Both regulate body functions; however, the nervous system uses electrical impulses while the endocrine system utilizes hormones released into the bloodstream.

Hormones

  • Chemical Nature:

    • Types: Peptides, steroids, amines.

  • Modes of Action:

    • Hormones can act directly on target cells or require a second messenger system.

  • Feedback Mechanisms:

    • Negative feedback: inhibits secretion to maintain homeostasis.
    • Positive feedback: enhances secretion to amplify effects (e.g., oxytocin during childbirth).

Key Definitions

  • Hormones: Chemical messengers secreted by glands.
  • Endocrine Gland: A gland that secretes hormones directly into the bloodstream.
  • Endocrine Cell: A cell that releases hormones.
  • Target Cell: Cells that respond to hormones due to specific receptors.
  • Autocrine: Hormones that act on the same cells that secrete them.
  • Paracrine: Hormones that act on nearby cells.

Hormonal Interactions

  • Types of Interactions:
    • Antagonistic: One hormone opposes the action of another.
    • Synergistic: Combined effect is greater than individual effects.
    • Permissive: One hormone enhances the target cell's responsiveness to another hormone.
  • Drugs: Can act as agonists (activating the receptors) or antagonists (blocking the receptors).

Major Endocrine Glands

  1. Hypothalamus: Controls anterior and posterior pituitary gland.
  2. Pituitary Gland:
    • Anterior: secretes hormones that regulate other glands (e.g., GH, TSH).
    • Posterior: stores hormones produced by the hypothalamus (e.g., ADH, oxytocin).
  3. Thyroid & Parathyroid Glands:
    • Regulate metabolism and calcium levels.
  4. Adrenal Glands:
    • Cortex (cortisol production) and medulla (epinephrine, norepinephrine).
  5. Pancreas:
    • Islets produce insulin and glucagon.
  6. Gonads:
    • Ovaries produce estrogen; testes produce testosterone.

Interaction Between Hypothalamus and Pituitary Gland

  • Hypothalamus connects via the hypophyseal portal system (anterior) and hypothalamohypophyseal tract (posterior).
  • Anterior Pituitary Control: Regulated by releasing and inhibiting hormones.
  • Posterior Pituitary Control: Hormones produced in hypothalamus, released when stimulated.

Control of Hormone Secretion

  • Hypothalamic Hormones:
    • GHRH: Stimulates GH secretion.
    • GHIH: Inhibits GH secretion.
    • TRH: Stimulates TSH secretion.
    • CRH: Stimulates ACTH secretion.
    • GnRH: Stimulates FSH and LH secretion.

Growth Hormone (GH)

  • Functions:
    • Stimulates growth, cell reproduction, and regeneration.
  • Disorders:
    • Hypersecretion: Gigantism in children; Acromegaly in adults.
    • Hyposecretion: Dwarfism.

Thyroid Hormones (T3 and T4)

  • Functions:
    • Increase basal metabolic rate, body heat production, and regulate metabolism.
  • Disorders:
    • Hypothyroidism: Weight gain, cold intolerance.
    • Hyperthyroidism: Weight loss, heat intolerance.

Adrenal Glands

  • Hormones:
    • Cortisol: Involved in stress response, increases glucose levels.
    • Aldosterone: Regulates blood pressure by controlling sodium and water reabsorption.

Pancreatic Hormones

  • Insulin: Lowers blood glucose levels; promotes glucose uptake in cells.
  • Glucagon: Increases blood glucose levels; promotes glycogen breakdown.
  • Diabetes Mellitus: Characterized by high blood glucose levels, either due to insulin deficiency (Type 1) or insulin resistance (Type 2).

Effects of Aging on the Endocrine System

  • Decrease in hormone production over time, affecting metabolism, reproduction, and overall health.