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The respiratory system provides for exchange of O2 and CO2 to and from the blood. Respiratory organs include the lungs and a branching system of bronchial tubes that link the sites of gas exchange with the external environment. Air is moved through the lungs by a ventilating mechanism, consisting of the thoracic cage, intercostal muscles, diaphragm, and elastic components of the lung tissue. The system can be divided anatomically into the upper and lower respiratory tracts (Figure 17–1). Functionally the system has two components:

  • The conducting portion, which consists of the nasal cavities, pharynx, larynx, trachea, bronchi (Gr. bronchos, windpipe), bronchioles, and terminal bronchioles.

  • The respiratory portion, where the system’s main function of gas exchange occurs, consisting of respiratory bronchioles, alveolar ducts, and alveoli.


Anatomy of the respiratory system.

Anatomically, the respiratory tract has upper and lower parts. Histologically and functionally, the respiratory system has a conducting portion, which consists of all the components that condition air and bring it into the lungs, and a respiratory portion, where gas exchange actually occurs, consisting of respiratory bronchioles, alveolar ducts, and alveoli in the lungs. Portions of two sets of paranasal sinuses are also shown here.

Alveoli, the cellular sites of the exchange of O2 and CO2 between inspired air and blood, are small, air-filled, saclike structures, which make up most of the lung structure.

The conducting portion cleans and humidifies inspired air and provides conduits for air movement to and from alveoli. To ensure an uninterrupted supply of air, a combination of cartilage, collagen and elastic fibers, and smooth muscle provides the conducting portion with rigid structural support and the necessary flexibility and extensibility.


The left and right nasal cavities each have two components: the external, dilated vestibule and the internal nasal cavity. Skin of the nose enters the nares (nostrils) partway into the vestibule and includes sweat glands, sebaceous glands, and coarse, moist vibrissae (hairs), which filter out particulate material from inspired air. Within the vestibule, the epithelium loses its keratinized nature and undergoes a transition to typical pseudostratified columnar epithelium which also lines the nasal cavities.

The nasal cavities lie within the skull as two cavernous chambers separated by the osseous nasal septum. Extending from each lateral wall are three bony shelf-like projections (Figure 17–1) called conchae, or turbinate bones. The mucosa covering these and other parts of the nasal cavity walls has a lamina propria with important roles in conditioning inhaled air. A complex vasculature with loops of capillaries near the epithelial surface carries blood in a general direction counter to the flow of inspired air and releases heat to warm that air while ...

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