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General Physiology chapter 6 2nd MED 2024 2025

2nd Medical Educational Year

  • Academic Year: 2024 – 2025

  • Prepared by: Dr. Ghenwa Nasr

  • Lebanese University

  • Faculty of Medical Sciences, Hadath Campus

  • Course: General Physiology – PHYS111

Chapter 6: The Endocrine System

1. General Overview of the Endocrine System

  • Structure: Comprises ductless glands (endocrine glands), hormone-secreting cells in organs (brain, heart, kidneys, liver, stomach).

  • Function: Hormones = chemical messengers; travel in blood to target cells.

  • Target Cells: Influence cells are known as target cells for specific hormones.

  • Homeostasis: Endocrine system coordinates organ systems to maintain homeostasis.

  • Hormonal Interaction: One hormone's effects can be potentiated, inhibited, or counterbalanced by another.

2. General Characteristics of Hormones and Hormonal Control Systems

2.1. Hormones and Endocrine Glands

  • Gland Definition: Group of cells that excretes substances (hormones, sweat, saliva, etc.).

  • Endocrine vs Exocrine:

    • Endocrine: Ductless; releases hormones into blood via interstitial fluid.

    • Exocrine: Secretes into ducts (e.g., sweat to skin, digestive aids to intestines).

  • Anatomical Connection: Endocrine system components not directly connected but functionally integrated.

  • Multiple Hormones: A gland can secrete various hormones simultaneously.

2.2. Hormone Structures and Synthesis

  • Classes: Hormones are classified into amine, peptide, and steroid hormones.

2.2.1 Amine Hormones

  • Derivation: Derived from the amino acid tyrosine.

  • Examples: Thyroid hormones, catecholamines (epinephrine, norepinephrine, dopamine).

  • Adrenal Glands:

    • Composed of adrenal medulla (secretes catecholamines)

    • Adrenal cortex (secretes steroid hormones).

2.2.2. Peptide and Protein Hormones

  • Composition: Majority are polypeptides; size range varies.

  • Synthesis Process:

    • Synthesized as preprohormones in ribosomes.

    • Processed to prohormones, cleaved in the endoplasmic reticulum, and packaged in Golgi apparatus.

    • Released as active hormone through exocytosis.

2.2.3. Steroid Hormones

  • Synthesis: Derived from cholesterol.

  • Location: Mainly by adrenal cortex and gonads (testes/ovaries).

  • Mechanism: Binding of anterior pituitary hormone activates pathways for synthesis.

2.3. Hormone Transport in the Blood

  • Solubility:

    • Peptides/catecholamines are typically water-soluble, circulating dissolved in plasma.

    • Steroid/thyroid hormones are lipid-soluble, often protein-bound and poorly soluble in blood.

  • Free vs Bound: Only free hormones can act on target cells.

2.4. Hormone Metabolism and Excretion

  • Clearance: Rate of clearance depends on secretion rate and removal rate (excretion/metabolism).

  • Major Organs: Liver and kidneys play predominant roles in hormone metabolism.

  • Examples of Metabolism Effects:

    • Some hormones metabolized to become active (e.g., T4 to T3).

2.5. Mechanisms of Hormone Action

2.5.1. Hormone Receptors

  • Functionality depends on specific receptors in target cells.

  • Receptor location varies by hormone type: membrane-bound for peptides/catecholamines, intracellular for steroids/thyroid hormones.

2.5.2. Events Elicited by Hormone-receptor Binding

  • Signal transduction mechanisms initiated upon hormone binding involve various cellular responses.

  • Influence on gene transcription and protein synthesis, addressing rapid and slower cellular effects.

2.5.3. Receptor Binding

  • Down-regulation and up-regulation can occur based on hormonal exposure levels, affecting responsiveness.

  • Permissiveness: One hormone must be present for another to exert its effects (e.g., epinephrine requires thyroid hormones).

2.5.4. Pharmacological Effects of Hormones

  • Excessive hormone doses can cause unwanted effects (e.g., prednisone usage with side effects).

2.6. Inputs That Control Hormone Secretion

  • Hormone secretion primarily regulated by:

    • Plasma concentrations of ions/nutrients

    • Neurotransmitter stimulation

    • Other hormone inputs

2.6.1. Control by Plasma Concentrations of Mineral Ions or Organic Nutrients

  • Example: Insulin secretion increases with increasing plasma glucose levels, facilitating glucose uptake.

2.6.2. Control by Neurons

  • Sympathetic pathways influence hormone secretion (e.g., adrenal medulla response).

2.6.3. Control by Other Hormones

  • Hormones often stimulate others; termed tropic hormones.

2.7. Types of Endocrine Disorders

  • Categories:

    • Hyposecretion: too little hormone.

    • Hypersecretion: too much hormone.

    • Hyporesponsiveness: target cells are less responsive.

    • Hyperresponsiveness: target cells are overly responsive.

2.7.1. Hyposecretion

  • Causes include gland damage, enzyme deficiencies, dietary deficiencies (e.g., iodine).

  • Treatments typically involve hormone replacement.

2.7.2. Hypersecretion

  • Can be caused by tumors, leading to continual hormone production.

  • Treatment options include surgery, radiation, or pharmacological inhibition.

2.7.3. Hyporesponsiveness and Hyperresponsiveness

  • Example: Type 2 diabetes shows cellular resistance to insulin.

3. The Hypothalamus and Pituitary Gland

3.1. Control Systems Involving the Hypothalamus and Pituitary Gland

  • Pituitary Gland: Located below hypothalamus; composed of anterior and posterior lobes.

  • Connection: Infundibulum bridges hypothalamus and pituitary; hormones released into capillaries.

3.1.1. Posterior Pituitary Hormones

  • Hormones synthesized in hypothalamus, released in posterior pituitary (e.g., oxytocin, vasopressin).

3.1.2. Anterior Pituitary Gland

  • Hormonal control via hypothalamic responses, stimulating various hormonal outputs.

3.1.3. Hormones and the Hypothalamus

  • Hypophysiotropic hormones regulate anterior pituitary secretion; these hormones are crucial for overall endocrine function.

4. The Thyroid Gland

4.1. Synthesis of Thyroid Hormone

  • Produces T4 (thyroxine) and T3 (triiodothyronine).

  • Iodine Uptake: Via sodium-iodide transporters in epithelial cells.

  • Thyroid Hormone Release: T4 and T3 released through endocytosis and proteolysis of thyroglobulin.

4.2. Control of Thyroid Function

  • Control via TSH produced by anterior pituitary gland; negatively feedback by T3 and T4 levels.

4.3. Actions of Thyroid Hormone

  • Metabolic Actions: Boost glucose and lipid metabolism, supporting increased basal metabolic rate.

  • Permissive Actions: Up-regulates catecholamine actions (e.g., epinephrine).

  • Growth and Development: Essential during fetal development, impacts neurodevelopment.

4.4. Hypothyroidism and Hyperthyroidism

  • Hypothyroidism: Characterized by low hormone levels; symptoms include fatigue, weight gain.

  • Hyperthyroidism: Often autoimmune; leads to weight loss, anxiety, increased heart rate.

5. The Endocrine Response to Stress

5.1. Functions of Cortisol in Stress

  • Response Mechanism: Stress increases cortisol via hypothalamic-pituitary axes.

  • Effects: Gluconeogenesis and lipolysis increase energy availability; non-essential functions like reproduction are inhibited.

5.2. Other Hormones Released During Stress

  • Co-releasing hormones include beta-endorphin and vasopressin with cortisol; sympathetic nervous system activation results in epinephrine secretion.