Mechanics I


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:

 

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

 

Definitions:

 

Demonstration/Magic Show

Demonstration/Slinky

 

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

2. Factors involved in lung compliance

Laplace’s Law: Pressureinside = 2 x Surface Tension/Radius

 

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

 

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.

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:

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:

 

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

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