Exhalation, or expiration, is the process of expelling air from the lungs. During quiet breathing, it's a passive process driven by the elastic recoil of the chest wall and lungs. When inspiratory muscles relax, the diaphragm moves upward, and the ribs are depressed, reducing lung volume. Forceful exhalation involves the active contraction of muscles like the abdominal and internal intercostals, further decreasing lung volume.