Emphysema can best be characterized as the progressive destruction of the grape-like air sacs (alveoli) that perform the lung's basic function of exchanging oxygen in the air for carbon dioxide in the cardiovascular system.

The small air sacs are unable to completely deflate (over inflation) and unable to fill with fresh air for adequate ventilation. Emphysema is not reversible, but the disease is manageable through medications, exercise and good nutrition.

In emphysema caused by smoking, which constitutes the majority of cases, the very small airways (bronchioles) that join the alveoli are damaged and the walls lose elasticity.

Pockets of dead air form in the damaged lung areas restricting the ability to exhale, reducing normal lung function. Inhalation is not usually impaired in the early stages, but in the late stages of the disease, oxygen and carbon dioxide levels are abnormal and breathing becomes labored.
Emphysema patients have typically lost between 50% and 70% of their lung function by the time symptoms begin to appear.

Experts believe the process leading to emphysema is mostly due to an imbalance in chemicals that protect the lungs from infection and damage. Any condition that causes an imbalance in these substances may trigger emphysema.

Cigarette smoke contains irritants that inflame the air passages, setting off these biochemical events that damage cells in the lung, thus increasing the risk both for emphysema and lung cancer.

Because smoking is overwhelmingly the cause of emphysema and chronic bronchitis, they often develop together and frequently require similar treatments.

In a rare, inherited form of emphysema known as alpha-1-antitrypsin deficiency, both the walls of the bronchioles and alveoli to which they connect, usually in the lower lungs, are diseased.