Chronic obstructive pulmonary disease (COPD) is a condition in which the bronchioles (breathing tubes) leading to the lung develop thickened membranes that secrete large amounts of sputum and pus or in which the tine alveoli (air sacs) within the lungs become inelastic and sometimes break open and trap air. COPD causes breathing to become more difficult because air is taken in and cannot be expelled completely.