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.