Injuries to the respiratory center or to spinal nerve tracts that transmit motor impulses may paralyze breathing muscles. Paralysis may also be due to a disease, such as poliomyelitis, that affects parts of the central nervous system and injures motor neurons. The consequences of such paralysis depend on which muscles are affected. Sometimes, by increasing their responses, other muscles are able to compensate for functional losses of a paralyzed muscle. Otherwise, mechanical ventilation is necessary.

Bronchial asthma is usually an allergic reaction to foreign antigens in the respiratory tract, such as plant pollen that enters with inhaled air. The walls of the small bronchioles become edematous, the cells lining the respiratory tubes secrete abnormally large amounts of thick mucus, and the smooth muscles in these tubes contract, constricting the bronchioles and reducing the diameters of the air passages. Breathing becomes increasingly difficult, and it produces a characteristic wheezing sound as air moves through narrowed passages.

A person with asthma usually finds it harder to force air out of the lungs than to bring it in. This is because inspiration utilizes powerful breathing muscles, and, as they contract, the lungs expand, opening the air passages. Expiration, on the other hand, is a passive process due to elastic recoil of stretched tissues. Also, it compresses the tissues and constricts the bronchioles, further impairing air movement through the narrowed air passages.

Emphysema is a progressive, degenerative disease that destroys many alveolar walls. As a result, clusters of small air sacs merge to form larger chambers, decreasing the total surface area of the alveolar walls. At the same time, the alveolar walls lose their elasticity, and the capillary networks associated with the alveoli become less abundant.

Because of the loss of tissue elasticity, a person with emphysema finds it increasingly difficult to force air out of the lungs. Abnormal muscular efforts are required to produce the elastic recoil of inflated tissues needed for expiration. Exposure to respiratory irritants, such as those in tobacco smoke and polluted air, is believed to cause emphysema.