(b) Explain the mechanism of (i) Quiet and (ii) Forced Expiration.

Explanation

(b) Quiet expiration, In quiet expiration, (i) Diaphragm moves upwards into the chest cavity, (ii) External intercostal muscles relax, passively (iii) Thoracic ribs move downwards and inward. As a result, the thoracic cavity is decreased in volume and intrathoracic pressure rises to-2.5 mm Hg. This is reflected upon intrapulmonary pressure which rises to + 3 or + 4 mm Hg. For this alveolar air with `CO_(2)` comes out through the air passages into the atmosphere.
Forced expiration : In forced expiration, in addition to the normal process of quiet expiration, certain accessory muscles like abdominal muscles (Transversus abdominis, External and Internal oblique, etc.) and back muscles (Sternocleidomastoid Scalene) contract actively.
As a result, the intrapulmonary pressure may rise upto + 10 mm Hg or more. For this, alveolar air with `CO_(2)` , comes out rapidly through the air passages into the atmosphere.