Lung compliance and airway resistance

Lung Function Tests

  • FEV1/FVC: A simple, useful test for lung function.

    • FVC: Forced Vital Capacity

    • FEV1: Forced Expiration Volume in 1 second

    • Normal Person: In a healthy individual, the FEV1/FVC ratio is approximately 80%.

  • Types of Lung Diseases:

    1. Obstructive Lung Disease:

    • Both FEV1 and FVC decrease.

    • The ratio of FEV1/FVC decreases.

    1. Restrictive Lung Disease:

    • Both FEV1 and FVC decrease.

    • The ratio of FEV1/FVC remains stable.

Factors Affecting Lung Compliance and Airway Resistance

  • Lung Compliance:

    • Defined as the volume change produced by a unit change in transmural pressure.

    • Important for understanding lung stiffness; indicated by the ease of lung expansion.

    • Rigid structures have low compliance; soft structures have high compliance.

  • Resistance to Airflow:

    • Involves both the size and length of the airways.

Measuring Lung Compliance

  • Compliance Formula:
    CL = \frac{∆VL}{∆P_{TM}}

  • Graphical Representation:

    • Lung compliance is reflected in the slope of the pressure-volume curve.

    • Compliance decreases as lung volume increases.

  • Variation of Compliance:

    • Compliance is not constant; it changes with varying conditions.

Determinants of Lung Compliance

  • Factors:

    1. Surface Tension:

    • Most significant reason affecting compliance.

    • Exists at the water-air interface within the alveoli.

    • Cohesive forces between water molecules are stronger than the force between liquid and air, creating pressure.

    1. Stretchability of connective tissues:

    • Changes in connective tissue can influence compliance positively or negatively.

Surface Tension in the Lungs

  • Functionality:

    • To expand the lungs, surface tension pressure has to be overcome.

    • Internal negative pressure aids in inflation.

  • Dynamic Lung Pressure-Volume Curve:

    • Saline inflation offers a simplified model without surface tension compared to air inflation, which deals with the water-air interface.

Pulmonary Surfactant

  • Composition:

    • 60% Diphosphatidyl Choline (DPPC)

    • 20% other phospholipids

    • 10% other lipids

    • 10% protein.

  • Functions:

    • Reduces surface tension, increasing lung compliance and facilitating easier lung expansion

    • Secreted by type II alveolar cells.

    • Secretion is increased by deep breaths.

  • Clinical Relevance:

    • Respiratory Distress Syndrome (RDS):

    • Leading cause of death in premature infants due to a lack of surfactant, leading to exhaustion, lung collapse, and death.

LaPlace Law

  • Equation: P = \frac{2T}{r}

    • Smaller alveoli experience greater pressure and result in increased tendency to empty into larger alveoli.

    • Surfactant stabilizes smaller alveoli by decreasing surface tension, preventing collapse.

Hysteresis in Dynamic Lung Pressure-Volume Curve

  • Causes of Hysteresis:

    1. Surface tension.

    2. Frictional resistance to air movement.

    3. Surfactant recruitment during inflation.

    4. Lung diseases like pulmonary fibrosis impact compliance and breathing efficiency.

Factors Influencing Airway Resistance

  • Critical Factors:

    1. Smooth muscle contraction in airway walls.

    2. Thickening of airway walls due to inflammation (

    • e.g., asthma).

    1. Excessive secretion of mucus.

    2. Destruction of lung tissue (e.g., emphysema).

  • Poiseuille's Law:

    • Resistance from airflow in small tubes is related to the tube caliber.

  • Airflow Patterns:

    • Laminar Flow: Predominant in small airways; low resistance.

    • Turbulent Flow: Occurs with higher Reynolds number, especially in larger airways (e.g., trachea).

Specific Conditions and Impact on Lung Function

  • Obstructive Lung Diseases:

    • Increased airway resistance seen in conditions like asthma, emphysema, and chronic bronchitis.

    • Asthma leads to inflammation and excessive mucus production, causing airway narrowing.

    • Emphysema involves destruction of alveolar walls, increasing resistance.

  • Restrictive Lung Diseases:

    • Characterized by low lung compliance and decreased vital capacity.

    • Examples include pulmonary fibrosis and pulmonary edema, both of which reduce lung compliance and diffusion capacity.

Summary of Lung Volume Control

  • Core variables affecting lung functionality:

    • Lung compliance, Volume, Lung volume, Transpulmonary pressure, Surface tension, Elastic tissue in lung wall, Elastic recoil, Surfactant action, Intrapleural pressure, and motor neuron activity regarding the diaphragm and intercostals muscles.

Graphical Representations

  • Visual diagrams illustrating conditions in lung diseases, differing lung compliance states, and histological changes in lung structures should be utilized for better understanding.