A large group of diseases with different causes but with the same or similar clinical and pathological changes. These are due to chronic, nonmalignant, noninfectious diseases of the lower respiratory tract characterized by inflammation and disruption of the walls of the alveoli. This manifests clinically as a limitation in the ability of the lungs to transfer oxygen from the alveoli to the pulmonary capillary bed. Patients with these disorders are dyspneic first in connection with exercise and, later, as the disease progresses, even at rest.