Lung Compliance and Intrapleural Pressure Quiz

Intrapleural Pressure

• Intrapleural pressure is always less than intra-alveolar pressure.
• This causes the lung to be stretched, even during expiration.
• If intrapleural pressure equilibrates with atmospheric pressure, a pneumothorax occurs.

Lung Compliance

• Lung compliance refers to a change in lung volume for a given pressure change.
• High compliance means the lung is easy to expand.
• Factors decreasing lung compliance include a deficiency of surfactant, high lung volumes, and pulmonary congestion.
• Factors increasing lung compliance include lower lung volumes and advancing age.
• The deflation limb differs from the inflation limb because surface tension is higher during inflation than deflation.

Lung Volumes and Compliance

• Lung compliance is increased at low lung volumes and decreased at high lung volumes
• This relationship can be visualized on a pressure-volume curve: the slope of the curve represents compliance.

Lung Compliance and Lung Disease

• Pulmonary disorders are divided into obstructive and restrictive diseases.
• Restrictive diseases involve increased fibrous tissue, reducing lung compliance.
• An example of a restrictive disease is pulmonary fibrosis, sometimes caused by asbestos exposure.

Mechanics of Breathing

• The elastic forces of the thoracic wall favor thorax expansion, while the elastic forces of the lungs favor lung collapse.
• The difference in pressure between the alveoli and the pleural cavity, called the transmural pressure gradient, causes the lungs to expand when the thorax expands.
• The breathing cycle at rest involves inspiration and expiration.

Lung Volumes

• Lung volumes are measured by a spirometer.
• Tidal volume (VT) is the volume of air inhaled or exhaled during normal breathing (~ 500 ml)
• Inspiratory reserve volume (IRV) is the extra volume of air that can be inhaled after a normal inspiration (~ 3000 ml)
• Expiratory reserve volume (ERV) is the extra volume of air that can be exhaled after a normal exhalation (~1200 ml)
• Residual volume (RV) is the air remaining in the lungs after a maximal exhalation (~1200 ml)
• RV cannot be measured by spirometry because it cannot be exhaled.

Lung Capacities

• Lung capacities are combinations of two or more lung volumes.
• Total lung capacity (TLC) is the maximum volume of air the lungs can hold.
• Vital capacity (VC) is the maximum volume of air that can be exhaled after a maximal inhalation.
• Functional residual capacity (FRC) is the volume of air remaining in the lungs after a normal passive expiration; it is the resting volume of the lung.

Alveolar Stability

• The collapsing pressure on an alveolus is directly related to surface tension and inversely related to alveolar radius.
• A smaller alveolus has higher collapsing pressure than a large alveolus.
• A small alveolus has a better surface area to volume ratio for gas exchange.

Alveolar Surfactant

• Alveolar surfactant is a complex mixture of phospholipids, lipids, and proteins produced by type II alveolar epithelial cells.
• It reduces surface tension in alveoli, reducing the work of breathing and increasing compliance.
• It can be deficient in premature infants.

Interdependence of Alveoli

• Adjacent alveoli support each other.
• If one alveolus starts to collapse, it pulls on surrounding alveoli, which recoil and pull the collapsing alveolus open.

Airway Resistance

• Airway resistance is determined by the Poiseuille equation, which states that resistance is inversely proportional to the fourth power of the radius.
• The primary determinant of airway resistance is the radius.
• In healthy individuals, the radius is large enough to keep resistance low.

Factors Affecting Airway Resistance

• The autonomic nervous system (ANS) influences airway radius.
• The parasympathetic nervous system constricts bronchial smooth muscle, while the sympathetic nervous system relaxes it.
• Lung volume also influences airway resistance: airway resistance is lower at higher lung volumes.

Airway Resistance in Pulmonary Disease

• Obstructive lung diseases, such as asthma, chronic bronchitis, and emphysema, increase airway resistance due to:
  • Thickening of airway walls
  • Excessive secretions in the airway lumen