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.