Mechanism of Breathing and Pulmonary Mechanics

Two Stages of Breathing

  • Breathing is a biological process involving two distinct stages:
    • Inspiration: The phase during which atmospheric air is actively drawn into the lungs.
    • Expiration: The phase by which alveolar air is released out of the lungs into the environment.

Pressure Gradients and Atmospheric Interaction

  • The movement of air into and out of the lungs is fundamentally driven by the creation of a pressure gradient between the lungs and the atmosphere.
  • Inspiration Criteria: This occurs when the intra-pulmonary pressure (the pressure within the lungs) is lower than the atmospheric pressure.
    • This state is characterized as "negative pressure" in the lungs relative to the atmosphere.
  • Expiration Criteria: This occurs when the intra-pulmonary pressure is higher than the atmospheric pressure.

Role of the Diaphragm and Intercostal Muscles

  • Specific anatomical structures are responsible for generating the necessary pressure gradients:
    • The Diaphragm: A primary muscle located at the base of the thoracic cavity.
    • Intercostal Muscles: A specialized set of muscles located between the ribs, categorized into:
      • External Intercostals
      • Internal Intercostals

Physiological Process of Inspiration

  • Inspiration is initiated through the following sequence of events:
    • Contraction of the Diaphragm: This contraction increases the volume of the thoracic chamber specifically in the antero-posterior axis.
    • Contraction of External Intercostal Muscles: This action lifts the ribs and the sternum.
    • Thoracic Volume Expansion: The lifting of the ribs and sternum causes an increase in the volume of the thoracic chamber in the dorso-ventral axis.
    • Pulmonary Volume Increase: The overall increase in thoracic volume leads to a corresponding increase in pulmonary volume.
    • Pressure Reduction: As pulmonary volume increases, the intra-pulmonary pressure decreases to a level below the atmospheric pressure.
    • Air Inflow: This pressure differential forces air from the outside to move into the lungs.

Physiological Process of Expiration

  • Expiration follows a reversal of the inspiratory mechanics:
    • Relaxation: The diaphragm and the external intercostal muscles relax.
    • Structural Return: The diaphragm and sternum return to their original, normal positions.
    • Volume Reduction: This movement reduces the thoracic volume, which subsequently reduces the pulmonary volume.
    • Pressure Increase: The reduction in pulmonary volume leads to an increase in intra-pulmonary pressure to a level slightly above atmospheric pressure.
    • Air Expulsion: The higher internal pressure causes the expulsion of air from the lungs.

Muscular Control and Strength

  • Humans possess the ability to increase the strength of both inspiration and expiration beyond baseline levels.
  • This is achieved through the recruitment of additional muscles in the abdomen.

Clinical Assessment and Breathing Statistics

  • Breathing Rate: On average, a healthy human breathes approximately 12−16 times/minute12-16 \, \text{times/minute}.
  • Spirometer: This is a specialized medical device used to estimate the volume of air involved in breathing movements.
    • It is a critical tool for the clinical assessment of pulmonary functions and diagnosing respiratory efficiency.