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Mechanics I

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Ernest J. Sukowski, Ph.D., Dept. of Physiology and Biophysics,

Rm. 3-228, ph. ext. 3342

Respiratory Lectures 9-10 Textbook, “Respiratory Physiology, The Essentials”, 7th Ed., by John B. West.


Respiratory Lecture 9: Mechanics of Breathing I


Topics to be covered:


  1. Muscles of respiration
    1. Inspiration
    2. Expiration
  2. Elastic properties of the lung
    1. Pressure-volume curves
    2. Lung compliance
      1. Tissue elastic recoil
      2. Surface tension
  3. Regional differences in ventilation
    1. Ventilation; definition
    2. Differences in ventilation vertically down the upright lung
    3. Small airway closure
  4. Elastic properties of the chest wall


Main objectives:

  • To understand how air flow into the lungs is accomplished, and to identify the muscles involved in this process.
  • To understand the determinants of compliance of the lung and chest wall, and how compliance is measured.
  • To understand the role of surfactant in increasing compliance of the lung.
  • To explain why, for a given change in pressure, the change in volume is smaller for alveoli at the top of the lung than at the bottom of the lung.
  • To understand the elastic properties of the lung.
  • To understand why intrathoracic pleural pressure is negative.
  • To understand how changes in compliance of the lung alter intrathoracic pleural pressure and functional residual capacity (FRC).


Assigned reading: West, Chapter 7, pp 93-120


Problems: Questions 1-14 on pp 118-120. Problem set 9

Mechanics of Breathing I

A. Muscles of Respiration are all Skeletal Muscles

B. Elastic Properties of the Lung



  • Elastic = Ability to spring back and resist deformation.
  • Compliance = Ability to yield and be nonresistant (distensibility).
  • Recoil = Ability to rebound or spring back.


Demonstration/Magic Show



1. Pressure-volume curves of the lung describe lung compliance at different lung volumes and demonstrate hysteresis (Figs. 7.3 & 7.8 in text)

  • Specific Compliance = compliance/unit volume of lung
  • The lung is less compliant at high and low volumes.
  • Hysteresis: a different pathway of compliance changes between inspiration and expiration (Fig. 7.5 in West and Fig. 3 of notes).

2. Factors involved in lung compliance

  • Surface Tension - at the air-liquid interphase inside the lung; tends to collapse the lung to a smaller volume.

Laplace’s Law: Pressureinside = 2 x Surface Tension/Radius
  • Tissue Elastic Recoil - due to the geometric arrangement of elastin and collagen fibers. Consider the effects on compliance of lung fibrosis or elastin changes seen with aging.


In the closed thorax, the negative intrapleural pressure at FRC is due to the elastic recoil of the chest wall outward and the lung recoil inward. Intrapleural pressure becomes positive with forced expiration, and even more negative than at FRC with forced inspiration. Consider what would happen to the chest wall, lung, and intrapleural pressure in the event of a pneumothorax.


Surface Tension

Demonstration/Surface Tension


  • Surfactant, a detergent-like natural substance, reduces surface tension and keeps the lungs dry (Fig. 1).
  • Loss of surfactant results in:
    • Stiff lungs (elevated surface tension)
    • Areas of atelectasis
    • Alveoli filled with fluid
  • Consider the effect of surfactant on hysteresis.

The lack of surfactant increases the surface tension of the alveolus, drawing the alveolar walls inward (recoil). This causes a greater negative interstitial space, overcoming the colloid osmotic pressure (COP) of blood, resulting in more fluid filtering out of the capillaries into the interstitial space and into alveoli.

Surfactant reduces the surface tension, allowing the alveolus to enlarge. This produces a less negative interstitial space, less than the colloid osmotic pressure of blood, thereby keeping fluid from leaving the capillaries and keeping the alveoli dry.

  • In premature births, lack of surfactant causes infant respiratory distress syndrome. A similar condition can exist in adults and is called adult respiratory distress syndrome (ARDS).

Figure 2. This figure compares the compliance changes of the normal to various lung respiratory pathologies. Note that the curves for emphysema and asthma (during bronchospasm) are shifted upward and to the left while those for rheumatic valve disease and interstitial fibrosis are flattened. Elastic recoil tends to increase in patients with rheumatic valve disease who have a raised pulomonary capillary pressure and interstitial edema.

C. Regional Differences in Ventilation


1. Definition of Ventilation (V): the movement (flow) of air from outside, through air passages, to the terminal respiratory units (alveoli).

The amount of ventilation is determined by:

  • The distensibility of the lungs (compliance)
  • Factors governing air movement.

Muscular effort is required to enlarge the thorax and lungs, thereby generating a pressure difference to drive the air flow. Airway resistance (AWR) impedes air flow.


2. Differences in ventilation vertically down the upright lung (Fig. 3)

a. The bottom of the lung receives the greatest ventilation when one inspires from FRC for the following reasons:

  • There are different intrapleural pressures vertically, with the top alveoli experiencing a more negative pressure holding them open before inspiration. Top alveoli have less reserve to enlarge.
  • The weight of the lung compresses the lower alveoli (gravity effect). With inspiration and lowering of the diaphragm, the elastic components of the lung reduces this effect
  • The sum of all factors places the lower lung is on a more favorable segment of the compliance curve at FRC.


Figure 3. Intrapleural pressures and compliance vertically down the lung.

The narrow arrows inside the curve depict the %volume change at different areas of the lung. Note the longer arrow representing the base of the lung. The wider arrows show hysteresis.

3. Small airway closure

  • Small airways (respiratory bronchioles) in the bottom of the lung close as one exhales and approaches RV (closing volume is less than 10% of lung vital capacity in the healthy young adult). Note position of lower lung on the compliance curve (Fig 3). This results in air trapping in the lower lung and a greater percentage of the expired air coming from the upper portions of the lung at the end of a forced expiration to RV. With age, as the lung becomes more compliant, this closure of small airways occurs at higher lung volumes (air trapping may even be present at FRC).
  • Inspiration starting from RV results in the upper lung receiving the greatest ventilation initially.

D. Elastic Properties of the Chest Wall (Fig. 7.11 in West)

  • Consider how Fig. 7.11 might appear in patients with emphysema, diffuse interstitial lung fibrosis, and increased rigidity of the chest wall.

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