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The lungs are spongy, saclike respiratory organs in the chest cavity which, together with the heart, work to remove carbon dioxide from the blood and provide it with oxygen.

adult human lung
An adult human lung is shown schematically, starting with trachea (top left) and continuing with the many ramifications, upward and downward, of the left main bronchus. The total area of the lungs 300 million cup-shaped alveoli is some 70 square meters.

Anatomy of the lungs

lungs and bronchi
The trachea (windpipe) branches into two smaller airways: the left and right bronchi, which lead to the two lungs. The left lung is longer, narrower, and has a smaller volume than the right lung it shares space in the left side of the chest with the heart. The right lung is divided into three lobes and each lobe is supplied by one of the secondary bronchi. It has an indentation, called the cardiac notch, on its medial surface for the apex of the heart. The left lung has two lobes.

The bronchi themselves divide many times before branching into smaller airways called bronchioles. These are the narrowest airways – as small as one half of a millimeter across. The larger airways resemble an upside-down tree, which is why this part of the respiratory system is often called the bronchial tree. The airways are held open by flexible, fibrous connective tissue called cartilage. Circular airway muscles can dilate or constrict the airways, thus changing the size of the airway.

how the lungs work
At the end of each bronchiole are thousands of small air sacs called alveoli. Together, the millions of alveoli of the lungs form a surface of more than 100 square meters. Within the alveolar walls is a dense network of tiny blood vessels called capillaries. The extremely thin barrier between air and capillaries allows oxygen to move from the alveoli into the blood and allows carbon dioxide to move from the blood in the capillaries into the alveoli.

Each lung is enclosed by a double-layered serous membrane, called the pleura. The visceral pleura is firmly attached to the surface of the lung. At the hilum, the visceral pleura is continuous with the parietal pleura that lines the wall of the thorax. The small space between the visceral and parietal pleurae is the pleural cavity. It contains a thin film of serous fluid that is produced by the pleura. The fluid acts as a lubricant to reduce friction as the two layers slide against each other, and it helps to hold the two layers together as the lungs inflate and deflate.

The lungs are soft and spongy because they are mostly air spaces surrounded by the alveolar cells and elastic connective tissue. They are separated from each other by the mediastinum, which contains the heart. The only point of attachment for each lung is at the hilum, or root, on the medial side. This is where the bronchi, blood vessels, lymphatics, and nerves enter the lungs.

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Sources: National Cancer Institute and Merck