The lungs are two masses of spongy tissue on either side of the heart within the rib cage. They are surrounded by two pleural membranes. Visceral pleura is attached directly to the outer surface of the lungs. Parietal pleura lines the chest cavity. Between them is a layer of lubricating fluid, just as there is between the pericardial membranes of the heart.

The lungs themselves are subdivided into the lobes that receive the secondary bronchi. The lobes in turn are subdivided into lobules enclosed in elastic connective tissue and receiving a lymphatic vessel, an arteriole, a venule, and a terminal bronchiole. The terminal bronchiole branches into two or more respiratory bronchioles that subdivide into the alveolar ducts, which feed into the air sacs of the lungs, the alveolar sacs.

Gas exchanges between the blood and lungs take place in alveolar sacs, which resemble a bunch of tiny grapes or a cluster of bubbles. Each grape or bubble is an alveolus, and it is through the walls of each alveolus that the gas transfer takes place. The alveolus is lined by epithelium and surrounded by capillaries, so that the air in the alveoli is separated from the blood by a thin wall just two cells thick. This wall is only 0.5 micrometers thick. The lungs contain some 300 million alveoli, with a combined surface area of 70 square meters .

Pneumonia can cause death primarily because the fluids produced by infected respiratory tissues physically block the gas exchanges within the alveoli and thus reduce the supply of oxygen to the brain and rest of the body. Pneumonia can have numerous causes, ranging from bacterial and viral infections to irritations provoked by inhaling dangerous gases that damage and inflame the linings of the lungs. In response to such irritations, the alveoli fill with fluid and dead white blood cells. Bacterial pneumonia now rarely kills, for we can use antibiotics to end the infection in its early stages. Also, a vaccine has recently been developed that protects high-risk patients against the most common bacterial form of pneumonia. Viral pneumonia can still be fatal. Certain cells in the alveolar epithelium secrete surfactant, a substance that help keep the alveoli from collapsing when they
lose air and shrink during exhaling. Premature babies that are born with an inadequate supply of surfactant cannot keep their lungs inflated. The results are infant respiratory distress syndrome; physicians can now supply as an aerosol spray surfactant produce by genetically engineered bacteria, and death is less likely then it used to be.

The alveolar wall also contain dust cell that phagocytize and remove dust particles and debris, which otherwise would collect deep within the lungs. They remove the dead white cells after a bout with pneumonia. They also remove the tar deposited in the lungs by tobacco smoke, allowing smokers who stop smoking to improve significantly the condition of their tar blackened lungs.

Lung cancer is not generally a cancer of the alveolar cells. Rather, it develops when chemicals such as those in tobacco smoke damage bronchial cells enough to cause them to become malignant. The resulting bronchogenic carcinoma can escape from the bronchial walls to spread in the actual lung tissue. The growth of cell blocks alveoli and bronchial tubes and prevents oxygen from reaching the blood. The cell also metastasize, or break loose from the tumor and ride the blood or lymph to other parts of the body, including the brain. After metastasis, even complete replacement of the lungs with a transplant often cannot cure lung cancer is rarely detected before metastasis has occurred. It is among the most frequently fatal of cancers.