The function of the lower airway is to deliver oxygen to the alveoli. Its external boundaries are the fourth cervical vertebra and the xiphoid process, which is the narrow, cartilaginous, lower tip of the sternum. Internally, the lower airway spans the glottis to the pulmonary capillary membrane. The trachea, or windpipe, is the conduit for air entry into the lungs. This tubular structure is approximately 10 to 12 cm in length and consists of C-shaped cartilaginous rings. The trachea begins directly below the cricoid cartilage and descends anteriorly down the midline of the neck into the thoracic cavity. Once in the thoracic cavity, the trachea divides at the level of the carina into the two mainstem bronchi (right and left). The hollow bronchi are supported by cartilage and distribute air into the right and left lungs. The lungs consist of the entire mass of tissue that includes the smaller bronchi, bronchioles, and alveoli. The lungs are surrounded by a serous membrane called the pleura. All lung tissue is covered with a thin, slippery outer membrane called the visceral pleura. The parietal pleura lines the inside of the thoracic cavity. A small amount of fluid is found between these two layers and serves as a lubricant to prevent friction during breathing.
On entering the lungs, each bronchus divides into increasingly smaller bronchi, which in turn subdivide into bronchioles. The bronchioles are thin, hollow tubes made of smooth muscle. The tone of these smooth muscles allows the bronchioles to dilate or constrict in response to various stimuli. The smaller bronchioles branch into alveolar ducts that end at the alveolar sacs. The alveoli, located at the end of the airway, are millions of thin-walled, balloonlike sacs that serve as the functional site for the exchange of oxygen and carbon dioxide. Surrounding each of these sacs is an intricate bed of blood vessels, known as pulmonary capillaries. Oxygen diffuses through the lining of the alveoli into the pulmonary capillaries where, depending on adequate blood volume and pressure, it is carried back to the heart for distribution to the rest of the body. At the same time, carbon dioxide (waste) diffuses from the pulmonary capillaries into the alveoli, where it is exhaled and removed from the body. The chest cage (thoracic cavity) contains the lungs, one on each side.
Anatomy of the Lower Airway
The boundaries of the thorax are the rib cage anteriorly, superiorly, and posteriorly and the diaphragm inferiorly. Each individual rib plays a part in the overall protection of the thorax. In between each rib are intercostal muscles that can assist with breathing; however, they generally are not used unless the patient is in respiratory distress. Within the chest cage, you will find the lungs, which hang freely within the chest cavity. Between the lungs is a space called the mediastinum, which is surrounded by tough connective tissue. This space contains the heart, the great vessels, the esophagus, the trachea, the major bronchi, and many nerves. The mediastinum effectively separates the right lung space from the left lung space. In addition to the respiratory and circulatory structures found in the chest cage, an important structure of the nervous system is also found in the thorax—the phrenic nerve. The phrenic nerve innervates the diaphragm muscle, allowing it to contract. Contraction of the diaphragm occurs in a downward direction and is necessary for adequate breathing to occur.

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Dr. D.Zeqiraj medical doctor at QKUK- Pristina, department of Infectious disease.