The lung is a remarkable organ.  It is the only organ to receive virtually the entire cardiac output.  It’s main function is to get oxygen from the air into your blood and remove carbon dioxide (a byproduct of metabolism) from your blood. This gas exchange occurs in air sacs called alveoli.  A pair of adult lungs contains 300 million alveoli with about 1000 capillaries per alveolus, creating a gas exchange surface area of up to 100 m².  Diseases of the lung parenchyma or blood vessels result in damage to this gas exchange surface.

The air that we breathe in gets to these air sacs by going through the bronchial tubes.  The largest airway is the trachea (windpipe), which branches into the left and right main bronchi that lead to the respective lungs.  The bronchial tubes (bronchioles) continue to divide, much like the branches of a tree, for up to 23 generations.  Diseases of the bronchial tubes can occur too.  The commonest airway diseases are asthma and COPD (Chronic Obstructive Pulmonary Disease), although the latter is often a combination of both airways disease and parenchymal disease.