In emphysema the patient becomes breathless because

Explanation

Smoke attracts phagocytes that release elastase, which digests the elastic tissue, so alveoli merge into large air spaces with much less total surface and the lungs lose the recoil needed to breathe out. Gas exchange falls and expiration becomes an effort, so even mild exertion leaves the patient gasping. The damage is permanent, since alveolar walls cannot regenerate.