The respiratory system functions in exchanging carbon dioxide for oxygen, which is then distributed to all of the tissues of the body. To accomplish

this function, air must be brought to that portion of the respiratory system where exchange of gases can occur. The respiratory system, therefore, has two portions:

•conducting portion

•respiratory portion.

Some of the larger conduits of the conducting portion are extrapulmonary, whereas its smaller components are intrapulmonary. The respiratory portions, however, are completely intrapulmonary. The luminal diameters of the various conduits can be modified by the presence of smooth muscle cells along their length (Table 12-1).

CONDUCTING PORTION OF THE RESPIRATORY SYSTEM

The extrapulmonary region of the conducting portion consists of the nasal cavities, pharynx, larynx, trachea, and bronchi. The intrapulmonary region entails the intrapulmonary bronchi, bronchioles, and terminal bronchioles (see Graphic 12-1).

Extrapulmonary Region

The mucosa of the extrapulmonary region of the conducting portion modifies the inspired air by humidifying, cleansing, and adjusting its temperature. This mucosa is composed of

•pseudostratified ciliated columnar epithelium (respiratory epithelium) with numerous goblet cells and an

•underlying connective tissue sheath that is well endowed with seromucous glands.

Modulation of the temperature of the inspired air is accomplished mostly in the nasal cavity by the rich vascularity of the connective tissue just deep to its respiratory epithelium.

Nasal Cavity and Olfaction

In certain areas, the mucosa of the nasal cavity is modified to function in olfaction and is referred to as the olfactory mucosa. The glands in the lamina propria of this region, known as Bowman’s glands, produce a thin mucous secretion that dissolves odoriferous substances, and the olfactory cells of the pseudostratified columnar olfactory epithelium perceive these sensory stimuli. Olfactory cells are

•bipolar neurons whose receptor ends are modified, nonmotile cilia that arise from a swelling, the olfactory vesicle, and extend into the overlying mucus. The axon

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of each olfactory cell arises from the basal end of the cell and passes through the cribriform plate at the roof of the nasal cavity to enter the floor of the cranial cavity to synapse with mitral cells of the olfactory bulb. Each olfactory cell lives approximately for 4 months.

•Odorant binding proteins (integral membrane proteins that are odorant receptors) lying within the plasma membrane of the cilia are sensitive to molecules of specific odor groups, where each of these molecules is known as an odorant.

When an odorant binds to its corresponding odorant receptor, one of two possibilities occurs.

The receptor itself may be a gated ion channel, and, upon binding the odorant, the ion channel opens or

the bound receptor activates adenylate cyclase, causing the formation of cAMP, which, in turn, facilitates the opening of ion channels.

The opening of the ion channel results in ion flow into the cell with subsequent depolarization of the plasmalemma, and the olfactory cell becomes excited.

The action potentials generated by the depolarizations of the olfactory cells are transmitted, via synaptic contacts, to the mitral cells of the olfactory bulbs.

The axons of the mitral cells form the olfactory tract, which transmits signals to the amygdala of the brainstem.

The odorant must satisfy at least three requirements: it must be volatile, water soluble, and lipid soluble, so that it can:

enter the nasal cavity (volatility),

penetrate the mucus (water solubility), and

have access to the phospholipid membrane (lipid solubility).

•In addition to the olfactory cells, two other cell types compose the olfactory epithelium, namely, sustentacular cells (supporting cells) and basal cells.

Sustentacular cells do not possess any sensory function, but they manufacture a yellowish-brown pigment that is responsible for the coloration of the olfactory mucosa; additionally, they insulate and support the olfactory cells.

Basal cells are small, dark cells that lie on the basement membrane and probably are regenerative in function forming sustentacular, olfactory, as well as more basal cells.

Axons of the olfactory cells are collected into small nerve bundles that pass through the cribriform plate of the ethmoid bone as the first cranial nerve, the olfactory nerve. Thus, it should be noted that the cell bodies of the olfactory nerve (cranial nerve I) are located in a rather vulnerable place, in the surface epithelium lining the nasal cavity.

TABLE 12-1 • Summary Table of Respiratory System

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