Human lungs are light, soft and elastic when healthy and always contain some air in living individuals. Asymmetrical, the left lung is slightly smaller than the right due to the location of the heart.

The lungs do not function independently, but are part of the larger respiratory system. The inner side of each lung is attached to veins, pulmonary arteries, nerves, bronchi, and lymphatic vessels. The bronchi branch repeatedly within the lungs, becoming smaller tubules known as bronchioles. The bronchioles lead to the alveoli, diminutive air sacs that are the location of gas exchange between the respiratory spaces and the capillaries.

Relatively vulnerable, the lungs are frequently a site of infection and diseases, such as emphysema, cancer, tuberculosis, and pneumonia. Asthma, however, is perhaps the most commonly incurred lung condition, currently affecting approximately five percent of the populations in developed countries. Rates are continuing to rise and speculation about the cause is abundant. Second-hand smoke, air pollution, and crowded cities are the most frequently blamed environmental factors, but some scientists argue that vaccinations may also be suspect because of their interference with the immune system.