During quiet breathing, how is expiration achieved?

During quiet breathing, expiration occurs primarily due to the elasticity of the lungs. When the diaphragm and intercostal muscles relax after inhalation, the lung tissue, which possesses elastic properties, tends to recoil. This elastic recoil helps push air out of the lungs and into the atmosphere without requiring additional muscle contraction or active energy expenditure.

The lungs are designed to stretch when filled with air and then return to their resting state once the inhaled air has been expelled. This passive process is essential for efficient and effortless breathing, particularly during rest. Consequently, during quiet breathing, the act of exhaling relies significantly on the natural elasticity of the lung tissue, making this the correct answer.