M04.07.03 Lung Mechanics

Understanding lung mechanics is crucial for comprehending respiratory physiology and the forces that govern breathing. This guide covers the muscles involved in respiration, the processes of inspiration and expiration, and the forces acting on the lung system.

Muscles of Respiration

Inspiration

The major muscle of inspiration is the diaphragm. Its contraction enlarges the vertical dimensions of the chest. Additionally, the external intercostal muscles assist by lifting the ribs, increasing the anterior-posterior dimensions of the chest.

Expiration

Under resting conditions, expiration is typically a passive process resulting from the relaxation of the muscles of inspiration and the elastic recoil of the lungs. For forced expiration, the abdominal wall muscles (including the external oblique, rectus abdominis, internal oblique, and transverse abdominal muscles) and internal intercostal muscles contract, compressing the chest wall and pushing the diaphragm upwards.

Forces Acting on the Lung System

Pressure Units

In respiratory physiology, units of pressure are usually given in cm H2O. The conversion between units is as follows:

• 1 cm H2O = 0.74 mm Hg
• 1 mm Hg = 1.36 cm H2O

Lung Recoil and Intrapleural Pressure

Understanding lung mechanics involves recognizing the main forces acting on the respiratory system:

• Lung Recoil: Represents the inward force created by the elastic properties of the alveoli.
  • As the lung expands, recoil increases; as it decreases in size, recoil decreases.
  • Recoil always acts to collapse the lungs.
• Chest Wall Recoil: Represents the outward force of the chest wall.
  • Functional Residual Capacity (FRC) is the point where the outward recoil of the chest wall counterbalances the inward recoil of the lung.
• Intrapleural Pressure (IPP): Represents the pressure within the fluid between the visceral pleura (attached to the lung) and the parietal pleura (attached to the chest wall).
  • The outward recoil of the chest and inward recoil of the lung creates a negative (subatmospheric) IPP.
  • IPP serves as the external pressure for all structures within the chest wall.
• Transmural Pressure Gradient (PTM): This represents the pressure gradient across any tube or sphere.
  • Calculated as inside pressure minus outside pressure.
  • A positive PTM (inside greater than outside) indicates a net force pushing outward, while a negative PTM (outside greater than inside) indicates a net force pushing inward.
  • At FRC, the negative IPP results in a positive PTM, preventing alveolar collapse (atelectasis).
  • For the entire lung, PTM is referred to as transpulmonary pressure (TPP).

Before Inspiration

Before inspiration, the glottis is open, and all respiratory muscles are relaxed, indicating FRC. This represents the equilibrium point of the respiratory system. At FRC:

• Intrapleural pressure is negative due to the inward elastic recoil of the lungs countered by the outward recoil of the chest wall.
• Alveolar pressure is zero, with atmospheric pressure conventionally set to zero.

During Inspiration

Inspiration occurs through the contraction of the diaphragm and external intercostal muscles, expanding the chest wall. This leads to:

• More negative intrapleural pressure (IPP).
• Increased PTM (TPP), resulting in lung expansion.
• According to Boyle’s law, increased alveolar volume, which decreases alveolar pressure, causes air to rush into the lungs.

End of Inspiration

Inspiration continues until alveolar pressure equilibrates with atmospheric pressure, resulting in a larger lung volume. Under resting conditions, approximately 500 mL of air enters the lung system, normalizing alveolar pressure.

Expiration

Expiration under resting conditions occurs via the relaxation of inspiration muscles:

• Relaxation causes intrapleural pressure to return to -5 cm H2O.
• The decrease in IPP reduces PTM, resulting in decreased alveolar volume and increased alveolar pressure, causing air to flow out of the lungs.
• Airflow continues until alveolar pressure returns to zero, at this point, airflow stops and the lung system returns to FRC.

Pressure Dynamics

• Intrapleural Pressure during a Normal Cycle: Remains subatmospheric.
• Intraalveolar Pressure: Slightly negative during inspiration and slightly positive during expiration, always returning to zero at the end of each cycle.

Points to Remember

• The diaphragm is the primary muscle of inspiration.
• Expiration is generally a passive process under resting conditions.
• The intrapleural pressure is negative, which is essential for lung expansion.
• Transpulmonary pressure prevents alveolar collapse.
• Normal tidal volume during quiet breathing is about 500 mL.

Tables

Bibliography Reference Links

1. West, J. B. (2012). Respiratory Physiology: The Essentials. Lippincott Williams & Wilkins.
2. Guyton, A. C., & Hall, J. E. (2016). Textbook of Medical Physiology. Elsevier.
3. Borish, L. C., & Vassallo, R. (2020). Physiology of the Respiratory System. In: Medical Physiology. Elsevier.