Respiratory Anatomy and Physiology

Learning Outcomes

• Be able to:
  • Define airway resistance.
  • Realise the importance of radius to airway resistance.
  • Know the distribution of airway resistance through the bronchial tree.
  • Understand how breathing is brought about.
  • Understand the relationship between the lungs and the chest wall in producing intrapleural pressure.
  • Define compliance & hysteresis.
  • Explain the significance of the properties of the liquid lining the lungs.
  • Understand the importance of bronchomotor tone and how it is controlled.
  • Understand why extra energy is required in dynamic situations.

Introduction to Respiration

• Energy necessary to sustain life comes from oxidative metabolism.
• Oxygen must be delivered to mitochondria for energy production.
• The balance of life is acquiring oxygen and expelling carbon dioxide.

Major Respiratory Structures

• Nasal Cavity: Air entry point, filters, warms, and moistens the air.
• Oral Cavity: Alternate airway, less efficient due to less filtration.
• Pharynx: Connects nasal and oral cavities to the larynx.
• Trachea: Major airway leading to the bronchi.
• Larynx: Houses vocal cords and acts as a passage for air.
• Bronchial Tree:
  • Right Primary Bronchus
  • Left Primary Bronchus
  • Secondary (lobar) and Tertiary (segmental) Bronchi
  • Bronchioles and Terminal Bronchioles
• Alveoli: Site of gas exchange, characterized by:
  • Approximately 300 million alveoli in the lungs.
  • Extensive surface area (~100 m²) facilitating gas exchange.
  • Thin walls and elastic properties.

Pleurae and Intrapleural Pressure

• Pleurae: Double-layered serosa surrounding the lungs (visceral and parietal pleura).
  • Parietal pleura covers thoracic wall and diaphragm.
  • Visceral pleura covers lung surfaces, dipping into fissures.
• Intrapleural pressure is negative (-3 mm Hg at rest). It is influenced by:
  • Elastic recoil of the chest wall pulling outward.
  • Elastic recoil of lungs pulling inward.

Laws of Gas Behavior

• Charles’ Law: At constant pressure, the volume of a gas increases with temperature.
  V \propto T
• Boyle’s Law: Pressure and volume of a confined gas are inversely proportional at a constant temperature.
  PV = k ( or ) V \propto \frac{1}{P}

Pulmonary Ventilation Mechanics

Inspiration

1. Inspiratory Muscles Contract: Diaphragm descends; rib cage rises.
2. Thoracic Volume Increases: Lungs expand, increasing intrapulmonary volume.
3. Pressure Drop: Intrapulmonary pressure drops below atmospheric pressure.
4. Air Inflows: Air enters the lungs until pressures equalize.

Expiration

1. Muscles Relax: Diaphragm rises; rib cage descends due to elastic recoil.
2. Thoracic Volume Decreases: Lung volume decreases.
3. Pressure Rise: Intrapulmonary pressure increases above atmospheric pressure.
4. Air Exits: Gases flow out until pressures equalize.

Factors Affecting Pulmonary Ventilation

• Airway Resistance: Major source of resistance due to friction.
  • Relation: \Delta P = R \times F
• Nature of Airflow:
  • Laminar vs Turbulent Flow: Laminar is preferred for less resistance.
• Compliance: The ability of lungs to expand; higher compliance indicates better expansion for a given pressure.
• Surface Tension: Surfactant plays a key role in reducing surface tension in alveoli.

Hysteresis

• Difference in lung volume between inflation and deflation due to surface tension changes.

Gas Exchange and Transport in Lungs

• Partial Pressure: The driving force for gas exchange; defined by Dalton’s Law.
• Henry’s Law: Describes gas solubility in liquids.
• Oxygen and Carbon Dioxide Transport:
  • Oxygen is carried by hemoglobin; carbon dioxide is more soluble in blood.
• Gas exchange occurs primarily by diffusion, driven by partial pressure gradients.

Importance of Alveoli

• Structure:
  • Large surface area enables efficient gas exchange.
  • Thin walls decrease diffusion distance.
  • Moist environment facilitates gas transfer.

Dynamics of Gas Exchange

• PO2 and PCO2 gradients drive diffusion:
  • Oxygen diffuses from high (alveoli) to low (blood) pressure areas.
  • Carbon dioxide diffuses in the opposite direction.
• Influencing factors:
  • Thickness of the respiratory membrane.
  • Surface area.
  • Ventilation-perfusion coupling ensures matching ventilation with blood flow.

Oxygen Transport Mechanics

• Hemoglobin combined with oxygen:
  • Each hemoglobin can bind 4 O2 molecules.
• Factors influencing binding: Haldane Effect (increased oxygen loading at high PO2) and Bohr Effect (increased unloading at low PO2).
• Myoglobin provides oxygen storage in muscle with a higher affinity than hemoglobin.

Summary

The respiratory system is vital for gas exchange, driven by principles of physics regarding pressure and volume, and relies on the structure of alveoli facilitating diffusion. Mastering these concepts aids in understanding respiratory physiology and the implications of pulmonary health.