Respiratory System: Pressures and Volumes

Compliance in the Respiratory System

  • Lung Compliance: Refers to the ease with which lungs can be inflated at a given pressure change.

    • Comparison: Like blowing up a stiff vs. a stretched balloon.

Compliance vs. Resistance

  • Compliance:

    • Indicates how easily the chest wall expands.

    • Decreased Compliance: More difficult to expand.

    • Increased Compliance: Easily expanded.

  • Resistance:

    • Refers to the amount of resistance to air movement from atmosphere to alveoli.

Factors Affecting Resistance

  • Decrease in Elasticity: Affects lung expansion.

  • Bronchiole Diameter Changes: Can lead to increased resistance.

  • Atelectasis: Refers to alveolar collapse.

Surface Tension and Compliance

  • Surface Tension:

    • Important for lung compliance; relates to forces of surface molecules in alveoli.

    • Effect: Air moves from high pressure alveoli (smaller radius) to lower pressure (larger radius).

    • Surfactant:

      • Hydrophilic heads attached to fluid and hydrophobic tails toward air.

      • Forms a monolayer reducing surface tension more in smaller alveoli due to higher surfactant concentration.

Airflow Dynamics

  • Volume of Air Movement:

    • Related to the pressure difference between lungs and atmosphere, inversely to resistance.

    • Can be Laminar (smooth) or Turbulent (chaotic).

    • Silent Zone: Occurs in very small airways with low flow velocity, contributing little resistance.

Lung Volumes Components

  • Tidal Volume (Vt): Amount of air exchanged with each breath (normal is about 500 ml).

  • Inspiratory Reserve Volume (IRV): Maximum air inhaled after normal expiration (about 3,100 ml).

  • Expiratory Reserve Volume (ERV): Maximum exhaled after normal expiration (about 1,200 ml).

  • Residual Volume (RV): Air remaining in lungs after maximal expiration (about 1,200 ml).

  • Functional Residual Capacity (FRC): Volume after normal expiration; balance of inward lung recoil and outward chest recoil (about 2,400 ml).

Lung Capacities

  • Vital Capacity (VC): Amount exhaled after maximal inspiration = Vt + IRV + ERV (around 4,600 ml).

  • Inspiratory Capacity (IC): Maximum air inhaled from normal expiration = Vt + IRV (about 3,500 ml).

  • Total Lung Capacity (TLC): Sum of all lung volumes (around 5,800 ml); about 20-25% less in females than males.

Work of Breathing

  • Minute Volume: Total air exchanged in 1 minute = Vt x Respiratory Rate (RR), conventionally in liters/minute.

Ventilation and Gas Exchange

  • Ventilation: Movement of gases in/out of lungs via pressure gradient from chest volume changes.

    • Inspiration: Air drawn in; Expiration: Air forced out.

    • Dependent on airway resistance and lung compliance.

Distribution of Ventilation

  • Varies with body position; gravity affects intrapleural pressure causing uneven expansion between apex and base.

Dead Space

  • Definition: Air that does not participate in gas exchange.

    • Anatomic Dead Space: In conducting airways; Alveolar Dead Space: In respiratory portion of lungs.

Pulmonary Perfusion

  • Function: To provide blood flow to alveoli for gas exchange.

Shunting and Gas Exchange

  • Shunting: Refers to perfusion without ventilation or vice versa.

Respiration Stages

  1. Pulmonary Ventilation: Gas movement from atmosphere to alveoli.

  2. Alveolar Gas Exchange: Gas exchange from alveoli to pulmonary capillaries (external respiration).

  3. Gas Transport: Transport to body cells.

  4. Systemic Gas Exchange: Gas exchange from capillaries to systemic cells (internal respiration).

Mechanics of Breathing

  • Inspiration: Air flows into lungs; Expiration: Decrease in intrapulmonary volume.