Chapter 19

Respiratory System Overview

1. Respiratory Functions

  • External Respiration: Also known as ventilation, it is the process of exchanging air between the lungs and the environment.

2. Respiratory Tract Structure

  • Divisions: The respiratory system is divided into the upper and lower respiratory tracts.
    • Upper Respiratory Tract: Extends from the nose to the larynx (voice box).
    • Lower Respiratory Tract: Everything below the larynx.

3. Upper Respiratory Tract Anatomy

  • Nasal Cavity: Includes the nostrils and is lined with tiny external hairs (nose hairs) that filter and protect.
  • Mucous Membranes:
    • Composed of specialized cells known as goblet cells that produce mucus, a mix of the protein mucin and water.
    • The mucus traps pathogens and particles; cilia move trapped substances toward the throat for expulsion or swallowing.
  • Conchae (Turbinates): Bony structures that slow down airflow, allowing it to warm and humidify.
  • Sinuses: Four pairs reduce skull weight and resonate sound; these are prone to infections (sinusitis) affecting voice quality.

4. Pharynx

  • Anatomy: Divided into three parts:
    • Nasopharynx: Contains the Eustachian tube opening (auditory tube).
    • Oropharynx: Middle section.
    • Laryngopharynx: Connects to the trachea (windpipe) and esophagus.

5. Larynx

  • Components: Includes three cartilages:
    • Thyroid Cartilage: Largest, known for the laryngeal prominence (Adam's apple), responds to testosterone leading to its growth in males.
    • Cricoid Cartilage: Marks the transition between the upper and lower respiratory tracts.
    • Epiglottis: Made of elastic cartilage, covers the trachea during swallowing to prevent food from entering.
  • Vocal Cords:
    • True Vocal Cords: Vibrate to produce sound when air passes through.
    • False Vocal Cords: Provide support but do not produce sound.

6. Lower Respiratory Tract Anatomy

  • Trachea: Composed of 15-20 C-shaped cartilaginous rings, extends down to the carina where it bifurcates into left and right primary bronchi.
  • Sensory Receptors: Last present at the carina.
  • Bronchial Tree: The airway system which becomes narrower with smaller airways.
    • Primary Bronchi: Right bronchi is larger and shorter than the left because of heart positioning.
    • Secondary and Tertiary Bronchi:
    • 2 secondary bronchi on the left, 3 on the right due to lung size differences.
  • Bronchioles: Terminal bronchioles lead to the respiratory bronchioles where gas exchange begins.

7. Lung Anatomy

  • Lobes: Two lobes in the left lung (due to heart space) and three in the right lung.
  • Hilum: Entry and exit point for structures like bronchi and blood vessels.
  • Pleura: Visceral pleura touches the lung, parietal pleura lines the chest cavity, enabling pressure changes during breathing.

8. Breathing Mechanics

  • Inspiration: Inhalation where the diaphragm contracts and the thoracic cavity volume increases, decreasing intra-thoracic pressure.
  • Expiration: Primarily passive, involving relaxation of lung muscles and elastic recoil unless actively pushing out air.
  • Boyle's Law: Inverse relationship between volume and pressure.

9. Respiratory Measurement and Volumes

  • Total Lung Capacity (TLC): Approximately 6 liters.
  • Tidal Volume (TV): 500 mL; volume of air inhaled or exhaled under normal conditions.
  • Inspiratory Reserve Volume (IRV): ~3 liters; extra air inhaled after a normal breath.
  • Expiratory Reserve Volume (ERV): Volume of air exhaled after a normal breath (typically 1-1.5 liters).
  • Residual Volume (RV): Volume of air left in the lungs after maximum exhalation to prevent lung collapse, which cannot be measured directly.
  • Vital Capacity (VC): Max air exhaled after max inhalation, excluding RV.
  • Functional Residual Capacity (FRC): Volume remaining after normal expiration.

10. Respiratory Physiology

  • Minute Ventilation: Calculated as tidal volume multiplied by respiratory rate (normal rate: 16-20 breaths/min).
  • Dead Spaces: Structural regions (anatomical and physiologic) where no effective gas exchange occurs.
  • Chemoreceptors: Central chemoreceptors in the medulla monitor carbon dioxide levels; peripheral chemoreceptors in arteries monitor oxygen levels.
  • Partial Pressure Relationship:
    • Nitrogen (77%) dominates the atmosphere, followed by Oxygen (21%) and carbon dioxide (0.03%).

11. Gas Transport and Exchange

  • Oxygen Transport: 98% bound to hemoglobin, 2% dissolved in plasma.
  • Carbon Dioxide Transport: 7% dissolved in plasma, 25% bound to hemoglobin, 70% converted to bicarbonate.
  • Henderson-Hasselbalch Equation: Represents equilibrium between carbon dioxide and bicarbonate affecting blood pH.
  • Respiratory Rates: Adjust based on physical activity and CO2 levels; higher CO2 or lower O2 triggers increased breathing depth/rate to balance.

12. Pathophysiological Considerations

  • Surfactant Role: Prevents collapse of alveoli by reducing surface tension.
  • Alveolar Macrophages: Engulf pathogens and debris to maintain respiratory health.
  • Bicarbonate Buffering: Buffers blood acidity helping maintain pH levels during metabolic activities.
  • Carbamino Hemoglobin: Hemoglobin bound to carbon dioxide for transportation.