The respiratory tract is the site of an exceptionally large range of disorders for three main reasons: 1) it is exposed to the environment and therefore may be affected by inhaled organisms, dusts, or gases; 2) it possesses a large network of capillaries through which the entire output of theheart has to pass, which means that diseases that affect the small blood vessels are likely to affect the lung; and 3) it may be the site of “sensitivity” or allergic phenomena that may profoundly affect function