Mechanism of Breathing — Core Principles
Core Principles
Breathing is a mechanical process involving inspiration (inhalation) and expiration (exhalation), driven by pressure changes within the lungs. During inspiration, the diaphragm contracts and flattens, while external intercostal muscles contract, pulling ribs up and out.
This increases the thoracic cavity volume, which in turn decreases intrapulmonary pressure below atmospheric pressure, causing air to rush in. Inspiration is an active process. During quiet expiration, these muscles relax.
The diaphragm moves up, and ribs move down and in due to elastic recoil, decreasing thoracic volume. This increases intrapulmonary pressure above atmospheric pressure, forcing air out. Quiet expiration is passive.
Forced expiration involves active contraction of internal intercostal and abdominal muscles to further reduce thoracic volume. The pleural fluid ensures the lungs expand and contract with the thoracic cavity, making Boyle's Law the fundamental principle governing airflow.
Often confused with
Side-by-side differences the NEET paper likes to test.
| Aspect | Mechanism of Breathing | Expiration |
|---|---|---|
| Nature of Process (Quiet Breathing) | Active | Passive |
| Muscles Involved (Quiet Breathing) | Diaphragm, External Intercostals (contract) | Diaphragm, External Intercostals (relax) |
| Muscles Involved (Forced Breathing) | Diaphragm, External Intercostals, Sternocleidomastoid, Scalenes, Pectoralis minor (contract) | Internal Intercostals, Abdominal muscles (contract) |
| Thoracic Cavity Volume | Increases | Decreases |
| Intrapulmonary Pressure | Decreases (below atmospheric) | Increases (above atmospheric) |
| Airflow Direction | Into lungs | Out of lungs |
| Energy Requirement | Requires energy (ATP) for muscle contraction | Does not require energy (ATP) for muscle relaxation and elastic recoil (quiet) |
Inspiration is an active process driven by muscle contraction, primarily the diaphragm and external intercostals, which expands the thoracic cavity and lowers intrapulmonary pressure to draw air in. Conversely, quiet expiration is a passive process, relying on the relaxation of these muscles and the elastic recoil of the lungs and thoracic wall, which reduces thoracic volume and raises intrapulmonary pressure to expel air.
Forced expiration, however, becomes active, engaging internal intercostals and abdominal muscles for a more forceful expulsion.
Why it is tested: For NEET, understanding the distinct characteristics of inspiration and expiration, especially the active/passive nature and specific muscle involvement in both quiet and forced breathing, is crucial. Questions frequently test these differences, often requiring identification of muscles or the sequence of pressure changes.