Definition of lung compliance

Definition of lung compliance

Definition of lung compliance

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Definition of Lung Compliance - The Lung's Stretchiness

  • Lung Compliance (CL) is the measure of the lung's ability to stretch and expand in response to pressure changes. It quantifies the lung's "distensibility."

  • It's the inverse of elastance (stiffness).

    • High Compliance: Lungs are easy to inflate (e.g., emphysema). Think of a well-used party balloon.
    • Low Compliance: Lungs are stiff and difficult to inflate (e.g., fibrosis, ARDS). Think of a thick, new balloon.
  • Formula:

    • $C_L = \frac{\Delta V_L}{\Delta P_{TP}}$
    • $\Delta V_L$: Change in lung volume.
    • $\Delta P_{TP}$: Change in transpulmonary pressure (Alveolar Pressure - Intrapleural Pressure).

Pressure-volume loops and lung compliance, normal, and low (fibrosis) lung compliance)

  • Key Determinants:
    • Elastic Properties: Intrinsic stretchiness from elastin and collagen fibers in the lung parenchyma.
    • Surface Tension: Force at the air-liquid interface in alveoli, which is reduced by surfactant.

⭐ In a healthy adult, the compliance of the lungs and chest wall combined is approximately 100 mL/cm H₂O, which is about half the compliance of the lungs alone (200 mL/cm H₂O).

Factors Affecting Compliance - What Helps & Hinders

Lung compliance ($C = \Delta V / \Delta P$) is dictated by the lung's intrinsic elastic recoil and the surface tension at the alveolar air-fluid interface. Factors altering these properties will change compliance.

Factors Increasing Compliance (↑C)Factors Decreasing Compliance (↓C) - "Stiff Lung"
* Emphysema/COPD: Destruction of elastic fibers results in a "floppy," easily distended lung.* Pulmonary Fibrosis: Excess collagen deposition makes the lung stiff and difficult to inflate.
* Normal Aging: Gradual loss of elastic recoil increases compliance.* Pulmonary Edema/Pneumonia: Fluid or consolidation in alveolar spaces resists expansion.
* ARDS: Diffuse alveolar damage, edema, and loss of surfactant.
* Neonatal RDS: Surfactant deficiency causes high surface tension.

Clinical Significance - Floppy vs. Stiff Lungs

  • High Compliance (Floppy Lungs): Lungs are easier to distend but have poor elastic recoil.

    • Causes: Emphysema (e.g., α1-antitrypsin deficiency), normal aging.
    • Mechanism: Loss of elastic fibers makes lungs easy to inflate but difficult to exhale from.
    • Clinical Result: Leads to air trapping, ↑ Functional Residual Capacity (FRC), and a "barrel chest." Expiration becomes an active process, increasing the work of breathing.
  • Low Compliance (Stiff Lungs): Lungs are harder to distend, requiring greater pressure.

    • Causes: Pulmonary fibrosis, ARDS, pulmonary edema, Neonatal RDS.
    • Mechanism: Increased fibrous tissue or alveolar fluid/inflammation stiffens the lung.
    • Clinical Result: Requires ↑ inspiratory work to inflate the lungs.

⭐ In Neonatal Respiratory Distress Syndrome (NRDS), a deficiency of surfactant increases alveolar surface tension. This causes widespread atelectasis (alveolar collapse), severely ↓ lung compliance.

  • Lung compliance is the change in lung volume per unit change in transpulmonary pressure ($C_L = \Delta V / \Delta P_{tp}$).
  • It is the primary measure of the distensibility or "stretchiness" of the lungs.
  • High compliance (e.g., emphysema, aging) means lungs are easy to inflate but have ↓ elastic recoil.
  • Low compliance (e.g., fibrosis, ARDS, pneumonia) indicates "stiff" lungs that are difficult to inflate.
  • Compliance is the inverse of elastance ($C = 1/E$).
  • Surfactant increases lung compliance.

Practice Questions: Definition of lung compliance

Test your understanding with these related questions

An 85-year-old man with hypertension and type 2 diabetes mellitus is brought to the emergency department because of a 2-day history of shortness of breath. He has smoked one pack of cigarettes daily for 30 years. His temperature is 36.9°C (98.4°F), pulse is 100/min, respirations are 30/min, and blood pressure is 138/75 mm Hg. Pulmonary function testing shows decreased tidal volume and normal lung compliance. Which of the following is the most likely underlying etiology of this patient's tachypnea?

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Flashcards: Definition of lung compliance

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The compliance of the lungs and chest wall is _____ proportional to the wall stiffness

TAP TO REVEAL ANSWER

The compliance of the lungs and chest wall is _____ proportional to the wall stiffness

inversely

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