The tubes of the lung are arranged like the branches of a tree, with the diameter of the tubes becoming narrower further along the branches. It is at the narrower ends of these 'branches' that oxygen from inhaled air is transferred into the bloodstream. In chronic bronchitis, the lungs produce large quantities of excess phlegm and the lining of the tubes becomes thickened. The tubes become narrowed as a result, which leads to difficulties in moving air in and out of the lungs. In emphysema, the smaller 'branches' become damaged and prevent air from entering and leaving the areas where the oxygen is transferred.
Damage also occurs to the blood vessels in the lungs that 'collect' the oxygen.
The most common cause of both chronic bronchitis and emphysema is cigarette smoking. A small proportion of patients who have emphysema have an inherited deficiency of a particular chemical that helps to protect the lungs. They develop emphysema before 40 years of age, especially if they smoke. However, it must be emphasised that smoking accounts for most cases of COPD and if people did not smoke it would become a very rare disease. Chronic bronchitis and emphysema develop over many years and most people who have the conditions do not notice any problems until middle age.