Inspiration (Pulmonary Ventilation): Mechanism and Physiology

Definition

Inspiration is the active process of drawing air into the lungs. It is the first phase of the respiratory cycle.

Mechanism

1. Muscle contraction: Diaphragm contracts (flattens); external intercostals lift the rib cage.
2. Thoracic expansion: Chest volume increases.
3. Pressure gradient: Intrapulmonary pressure drops below atmospheric pressure.
4. Airflow: Air flows in until pressures equalize.

Key Features

• Active process (requires muscular effort).
• Contrast with expiration, which is usually passive at rest.