Chronic obstructive pulmonary disease. Artwork of a bronchus and alveoli of the lungs in chronic obstructive pulmonary (or lung) disease (COPD). COPD is a combination of emphysema and chronic bronchitis. Lung bronchi (upper right) divide to form alveoli (air sacs). These are the terminal portions of the lung's air passages, where oxygen and carbon dioxide transfer between the air and the blood (capillaries are red and purple). The cutaway sections reveal that the alveoli are filled with sputum (phlegm, yellow). COPD causes shortness of breath and can lead to oedema and heart failure. It is usually caused by smoking and damage is irreparable.