Breathing Mechanics: Inspiration and Expiration

Inspiration (breathing in)

During inspiration, the diaphragm contracts and flattens downward, and the external intercostal muscles contract, pulling the ribcage upward and outward. These movements increase the volume of the thoracic cavity.

As volume increases, the pressure inside the lungs decreases below atmospheric pressure. Air flows in through the nose or mouth, down the trachea, through the bronchi and bronchioles, and into the alveoli: moving from higher pressure (atmosphere) to lower pressure (inside the lungs).

Expiration (breathing out)

During normal (quiet) expiration, the diaphragm relaxes and curves upward, and the external intercostal muscles relax, allowing the ribcage to move downward and inward under its own weight and the elastic recoil of the lungs. The thoracic volume decreases.

The decreased volume increases pressure inside the lungs above atmospheric pressure. Air is pushed out of the lungs through the airways. Normal expiration is largely a passive process: it relies on relaxation and elastic recoil rather than active muscle contraction. During forced expiration (such as during vigorous exercise), the internal intercostal muscles and abdominal muscles contract actively to push more air out.

Summary table

Follow this chain in your answer: muscles → volume → pressure → air movement. Answers that jump from muscle action directly to air movement miss the pressure-change step.