How do muscle contractions create the pressure changes in the thorax that enable ventilation of the lungs?

Sourav PanNovember 8, 2024

Answered

Muscle contractions play a crucial role in creating the pressure changes in the thorax that enable ventilation of the lungs. This process involves coordinated actions of various respiratory muscles, primarily the diaphragm and intercostal muscles, which facilitate the movement of air into and out of the lungs. Here’s how this mechanism works:

Mechanism of Ventilation

1. Inhalation (Inspiration)

• Diaphragm Contraction:
  • The diaphragm is the primary muscle involved in inhalation. When it contracts, it moves downward (flattens), increasing the vertical dimension of the thoracic cavity. This expansion creates more space for the lungs to fill with air.
• Intercostal Muscles:
  • The external intercostal muscles, located between the ribs, also contract during inhalation. This contraction elevates the ribs and expands the rib cage outward and upward, further increasing the volume of the thoracic cavity.
• Pressure Changes:
  • According to Boyle’s Law, as the volume of the thoracic cavity increases, the pressure within it decreases. The intrapleural pressure (pressure within the pleural cavity) becomes more negative, and the alveolar pressure drops below atmospheric pressure. This pressure gradient causes air to rush into the lungs from the atmosphere.

2. Exhalation (Expiration)

• Diaphragm Relaxation:
  • During exhalation, the diaphragm relaxes and moves back to its dome-shaped position, decreasing the volume of the thoracic cavity.
• Intercostal Muscles:
  • The external intercostal muscles also relax, allowing the rib cage to move downward and inward. In forced exhalation, internal intercostal muscles may contract to further decrease thoracic volume by pulling ribs down more forcefully.
• Pressure Changes:
  • As lung volume decreases during exhalation, intrapleural pressure becomes less negative and alveolar pressure rises above atmospheric pressure. This results in air being expelled from the lungs into the atmosphere until pressures equalize.

Summary of Pressure Changes

• Inspiration:
  • Muscle contraction → Increased thoracic volume → Decreased intrapleural and alveolar pressures → Air flows into lungs.
• Expiration:
  • Muscle relaxation (or contraction in forced expiration) → Decreased thoracic volume → Increased intrapleural and alveolar pressures → Air flows out of lungs.