lung

The lungs are the pair of soft, spongy organs that fill most of the chest and do the body's breathing. Each one feels light and airy, like a sponge or fresh bread, because it is honeycombed with hundreds of millions of tiny air pockets where air meets blood.

There are two lungs, one on each side of the heart, divided into lobes — three on the right, two on the left, leaving room for the heart. Air arrives through the branching bronchi and ends in the alveoli; blood arrives through the pulmonary arteries and wraps each alveolus in fine capillaries. Spread flat, the gas-exchange surface of both lungs would cover a sizable room.

Besides exchanging oxygen and carbon dioxide, the lungs help filter small blood clots, fine-tune blood acidity, and warm and humidify air. They are protected and moved by the rib cage, the diaphragm and the slippery pleural membranes, and they are the central concern of nearly everything in pulmonology.