Lungs are soft, spongy, cone-shaped organs in the thoracic (chest) cavity. The lungs consist largely of air tubes and spaces. The balance of the lung tissue, its stroma, is a framework of connective tissue containing many elastic fibers. As a result, the lungs are light, soft, spongy, elastic organs that each weigh only about 0.6 kg (1.25 pounds). The elasticity of healthy lungs helps to reduce the effort of breathing.
The left and right lungs are situated in the left and right pleural cavities inside the thoracic cavity. They are separated from each other by the heart and other structures of the mediastinum, which divides the thoracic cavity into two anatomically distinct chambers. As a result, if trauma causes one lung to collapse, the other may remain expanded. Below the lungs, a thin, dome-shaped muscle called the diaphragm separates the chest from the abdomen. When you breathe, the diaphragm moves up and down, forcing air in and out of the lungs. The thoracic cage encloses the rest of the lungs.
Each lung occupies most of the space on its side of the thoracic cavity. A bronchus and some large blood vessels suspend each lung in the cavity. These tubular structures enter the lung on its medial surface.
Parietal refers to a membrane attached to the wall of a cavity; visceral refers to a membrane that is deeper—toward the interior—and covers an internal organ, such as a lung. Within the thoracic (chest) cavity, the compartments that contain the lungs, on either side of the mediastinum, are lined with a membrane called the parietal pleura. A similar membrane, called the visceral pleura, covers each lung.
The parietal and visceral pleural membranes are separated only by a thin film of watery fluid (serous fluid), which they secrete. Although no actual space normally exists between these membranes, the potential space between them is called the pleural cavity.
A thin lining layer called the pleura surrounds the lungs. The pleura protects your lungs and helps them slide back and forth against the chest wall as they expand and contract during breathing. A layer of serous membrane, the visceral pleura, firmly attaches to each lung surface and folds back to become the parietal pleura. The parietal pleura, in turn, borders part of the mediastinum and lines the inner wall of the thoracic cavity and the superior surface of the diaphragm.
Each lung is a blunt cone with the tip, or apex, pointing superiorly. The apex on each side extends into the base of the neck, superior to the first rib. The broad concave inferior portion, or base, of each lung rests on the superior surface of the diaphragm.
On the medial (mediastinal) surface of each lung is an indentation, the hilum, through which blood vessels, bronchi, lymphatic vessels, and nerves enter and exit the lung. Collectively, these structures attach the lung to the mediastinum and are called the root of the lung. The largest components of this root are the pulmonary artery and veins and the main (primary) bronchus. Because the heart is tilted slightly to the left of the median plane of the thorax, the left and right lungs differ slightly in shape and size.
Within each root and located in the hilum are:
- a pulmonary artery,
- two pulmonary veins,
- a main bronchus,
- bronchial vessels,
- nerves, and
- lymphatics.
Generally, the pulmonary artery is superior at the hilum, the pulmonary veins are inferior, and the bronchi are somewhat posterior in position. On the right side, the lobar bronchus to the superior lobe branches from the main bronchus in the root, unlike on the left where it branches within the lung itself, and is superior to the pulmonary artery.
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