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27 Ventilation and Gas Exchange Review

  • Alveoli Structure and Function

    • Alveoli are covered with pulmonary blood vessels and elastic fibers.

    • Site of gas exchange.

    • Airway smooth muscle regulates air entering the alveoli through processes such as bronchoconstriction (narrowing) and bronchodilation (widening).

  • Cell Types in Alveoli

    • Type I Cells:

    • Facilitate gas exchange.

    • Type II Cells:

    • Secrete surfactant that improves alveoli expansion and maintains alveolar surface tension.

    • Also involved in the mucociliary escalator that clears pathogens from the lungs.

  • Gas Exchange Mechanism

    • Gases move following partial pressure gradients (high to low).

    • Dalton’s Law: Total pressure of gas mixtures equals the sum of partial pressures of individual gases.

    • Understanding gases involved (e.g., O2, CO2) is essential for appreciating gas exchange dynamics.

  • Boyle’s Law

    • Pressure (P) and volume (V) have an inverse relationship.

    • If volume decreases, pressure increases and vice versa.

    • Importance in the mechanics of breathing, as changes in thoracic cavity volume affect lung pressure and airflow.

  • Pleural and Alveolar Pressures

    • Two membranes surround the lungs (visceral and parietal pleura).

    • Intrapleural pressure (Pip) is always lower than atmospheric pressure (Patm) and alveolar pressure (Palv).

    • This pressure differential keeps lungs inflated.

  • Breathing Mechanics

    • Inspiration:

    • Diaphragm and intercostal muscles contract, increasing thoracic cavity volume and decreasing Palv (air moves into lungs).

    • Expiration:

    • Diaphragm relaxes, decreasing thoracic cavity volume and increasing Palv (air moves out of lungs).

  • Pressure Interactions During Breathing

    • Air moves from areas of higher pressure to lower pressure.

    • For inhalation: Patm > Palv (air moves into the lungs).

    • For exhalation: Palv > Patm (air moves out of the lungs).

  • Pneumothorax

    • Collapse of lung due to disruption of pressure gradients (e.g., a traumatic injury) where Patm equalizes with Pip/palv, collapsing the lung.

    • Treatment: Chest tube insertion to drain air and restore pressure differential.

  • Regulation of Airway Resistance

    • Diameter of airways impacts airflow resistance:

    • Bronchodilation: Caused by epinephrine binding to beta-2 adrenergic receptors (reduces resistance).

    • Bronchoconstriction: Caused by acetylcholine (Ach) binding to muscarinic receptors and histamines (increases resistance).

  • Physiological Responses

    • Histamine release leads to bronchoconstriction and potentially anaphylaxis (widespread vasodilation and reduced MAP).

    • Leukotrienes contribute to inflammation and bronchoconstriction (targeted by medications like Montelukast for asthma treatment).

  • Clinical Considerations

    • Not all presence of histamines leads to anaphylaxis or leukotrienes to asthma; context-dependent roles in respiratory response.

  • Summary of Key Points

    • Breathing mechanics involve the interplay of pressure changes, pulmonary pressures, and smooth muscle regulation of airways.

    • Understanding these physiological processes is crucial for managing conditions like asthma and recognizing complications such as pneumothorax.