104. Mechanics of Breathing – Quiz

📌 Professor’s Notes: Quick Revision

High-yield revision notes for rapid review before university examinations.

• Breathing is a mechanical process in which air moves into and out of the lungs due to pressure changes produced by the respiratory muscles. Inspiration is an active process, whereas quiet expiration is normally passive because of elastic recoil of the lungs and chest wall.
• The diaphragm is the principal muscle of inspiration. Its contraction flattens the dome, increases the vertical diameter of the thorax, makes intrapleural pressure more negative, lowers alveolar pressure below atmospheric pressure, and draws air into the lungs. The external intercostal muscles elevate the ribs by bucket-handle (increases transverse diameter) and pump-handle (increases anteroposterior diameter) movements.
• Accessory muscles of inspiration (sternocleidomastoid, scalenes, neck and upper back muscles) are recruited only during deep or labored breathing. Their use at rest indicates respiratory distress.
• Quiet expiration is passive and results from elastic recoil. During forced expiration, the abdominal muscles and internal intercostal muscles actively decrease thoracic volume and increase intrapleural pressure. Patients with bronchial asthma often require active expiratory muscle contraction even during normal breathing.
• Intrapleural pressure is normally negative because the lungs tend to recoil inward while the chest wall tends to expand outward. It is about −5 cm H₂O at end expiration and becomes more negative during inspiration. Its functions are to expand the lungs during inspiration and maintain normal lung and chest wall shape. Loss of negative pressure causes pneumothorax, leading to lung collapse.
• Transpulmonary pressure is the difference between alveolar and intrapleural pressures. It is always positive during normal breathing and keeps the lungs inflated. Alveolar pressure becomes slightly negative during inspiration and slightly positive during expiration, producing airflow.
• Types of breathing:
  • Abdominothoracic breathing is predominant in males because diaphragmatic movement is greater.
  • Thoracoabdominal breathing is common in females and children because rib cage movement is more prominent.
• Lung volumes:
  • Tidal volume (TV): 500 mL.
  • Inspiratory reserve volume (IRV): Extra air inspired after tidal inspiration.
  • Expiratory reserve volume (ERV): Extra air expired after tidal expiration.
  • Residual volume (RV): Air remaining after maximal expiration.
• Lung capacities:
  • Vital capacity (VC): Maximum expired air after maximal inspiration.
  • Forced vital capacity (FVC): VC expired forcefully; used to obtain forced expiratory volume in one second.
  • Inspiratory capacity (IC) = TV + IRV.
  • Functional residual capacity (FRC) = ERV + RV; maintains continuous gas exchange and stable alveolar gas composition.
  • Total lung capacity (TLC): Air present after maximal inspiration. Residual volume and FRC are measured by helium dilution or body plethysmography.
• Lung compliance is the change in lung volume per unit change in pressure (ΔV/ΔP). High compliance indicates easy inflation but poor recoil (emphysema), whereas low compliance indicates stiff lungs (pulmonary fibrosis) requiring greater work for breathing. Dynamic compliance increases during exercise because more surfactant reaches the alveolar surface.