The lungs are two flexible, elastic organs that draw in and collect surrounding air. They are protected by the rib cage as they are made of very thin tissue that is relatively easy to puncture. Air travels from the mouth and nose down to the trachea, which splits into two channels called the bronchi (plural for bronchus). After this initial split, the passages continue to diverge and get smaller until they end in tiny clusters of grape-like air sacs or alveoli (plural for alveolus). These air sacs contain the region where gas exchange takes place. Capillaries enter alveoli to uptake the air in the lungs and carry it back to the heart and, eventually, the body tissues. At the same time that the blood takes up oxygen, it releases carbon dioxide picked up from the body.
We breathe because our tissues breathe! The lungs basically work as a pair of bellows – as the chest expands they draw in air (of which oxygen is a component). Upon exhaling the chest uses its elastic properties to passively “snap back” to its original size. Oxygen and carbon dioxide can travel between the lungs and bloodstream because the capillaries that surround the air sacs are permeable to gases. In essence, this means that oxygen and carbon dioxide molecules are small enough to passively diffuse through the tissues that compose both the blood vessels and alveoli. This same principal applies to gas exchange between the blood in the capillaries and tissues in the body. In heart failure conditions, however, if the blood is leaving the lung against a high pressure downstream in the left atrium, fluid can actually diffuse into the lungs. This condition, known as pulmonary edema, can inhibit gas exchange and make it very difficult and uncomfortable to breathe.