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.

Important Differences

vs Expiration

Aspect	This Topic	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.