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INTRODUCTION

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 connecting the sites of gas exchange with the external environment. Air moves through the lungs by a ventilating mechanism that involves the thoracic cage, intercostal muscles, diaphragm, and elastic components of the lung tissue. As shown in Figure 17–1 anatomically the system consists of the upper and lower respiratory tracts. Functionally the system has two descriptive 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 (Figure 17–1).

FIGURE 17–1

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 for 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 the 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 both rigid structural support and the necessary flexibility and extensibility.

NASAL CAVITIES

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 lines the nasal cavities.

The nasal cavities lie within the skull as two cavernous chambers separated by the bony nasal septum. Three curving projections called conchae (L. concha, seashell) or turbinate bones, extend from each side of the septum, forming partially separate passages that create turbulence in the air passing along them. The highly vascular mucosa covering the conchae and other parts of the nasal cavities conditions inhaled air. Loops of capillaries very near the epithelial surface carry blood in a general direction against the flow of inspired air, releasing ...

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