Upper Respiratory Tract

The structures of the upper respiratory tract are the nasal cavity, pharynx, and larynx. As air is brought into the body, the nasal cavity and pharynx filter and remove particles in the air and begin to humidify and warm it to body temperature. The mucosal lining of these structures has cells that secrete mucus and cells that are ciliated. Cilia and mucus trap particles; a sneeze removes large particles.

With illness and elevated body temperature, the mucous membrane tends to dry out, so the body secretes more mucus. This mucus dries out, and a cycle begins. The action of the cilia is inhibited by drying of mucus. The patient tends to breathe by mouth, which decreases the humidification of mucus and increases its viscosity.

The larynx, which extends from C3 to C6, controls airflow; and when it contracts rapidly, the epiglottis prevents food, liquids, or foreign objects from entering the airway.
Lower Respiratory Tract

The lower respiratory tract is composed of conducting airways of the tracheobronchial tree and the terminal respiratory units. There are approximately 23 generations (branchings) of the structures within the tracheobronchial tree, which extends from the trachea to the terminal respiratory units of the lungs.
The initial branchings of the tracheobronchial tree are depicted in. The first 16 airway branchings of the lower respiratory tract primarily conduct air, whereas the last 6 are respiratory airways that end (in the mature lung) in approximately 300 million alveoli The diameter of the airways becomes increasingly smaller with each successive generation of the tracheobronchial tree.

Trachea. The trachea is an oval, flexible tube supported by semicircular rings of cartilage. It extends from C6 in an oblique, downward direction to the sternal angle level of rib 2 and T6, at which point it bifurcates. The posterior wall is smooth muscle, and it contains an equal number of ciliated epithelial cells and mucus-containing goblet cells.

Mainstem bronchi. The trachea branches into two mainstream bronchi: the right, which is directed almost vertically, and the left, which is directed more obliquely.

Lobar bronchi. The two mainstem bronchi then divide into five lobar bronchi: three on the right and two on the left. Mainstem and lobar bronchi have a great amount of cartilage, which helps maintain airway patency.

Segmental bronchi. Each of the lobar bronchi divide into two or more segmental bronchi: 10 on the right and 8 on the left. Segmental bronchi have scattered cartilage, smooth muscle, elastic fibers, and a capillary network. The mainstem, lobar, and segmental bronchi have a mucous membrane essentially the same as the trachea.

Bronchioles. Segmental bronchi divide into subsegmental bronchi and bronchioles, which have less and less cartilage and ciliated epithelial cells. These bronchioles divide into the terminal bronchioles, which are distal to the last cartilage of the tracheobronchial tree. Terminal bronchioles contain no ciliated cells. Terminal bronchioles divide into respiratory bronchioles and provide a transitional zone between the bronchioles and alveoli.

Alveoli. The respiratory bronchioles divide into alveolar ducts and alveolar sacs. One duct may supply several sacs. The ducts contain smooth muscle, which narrows the lumen of the duct with contraction. The alveoli are located in the periphery of the alveolar ducts and sacs and are in contact with capillaries (alveolar-arterial membrane). Gas exchange occurs here.

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