Nose and Paranasal Sinuses

Organization of the Respiratory System

The upper respiratory tract includes the nose, nasal cavity, paranasal sinuses, and pharynx. The lowerrespiratory tract includes the larynx, trachea, and lungs. The lungs contain the bronchi, bronchioles, and alveoli. FIGURE 21-1 shows the structures of the respiratory system.

Nose and Paranasal Sinuses

The only externally visible part of the respiratory system is the nose, which is the primary passageway for incoming air. It has a variety of respiratory func-tions, conditioning incoming air, filtering and clean-ing the air, functioning as a resonating chamber for speaking, and containing the smell or olfactory receptors. The conditioning of incoming air consists of moistening and warming.

The paired external nares, commonly known as nostrils, open into the nasal cavity. The nasalvestibule is a space inside the flexible nasal tissues.Its epithelium has coarse hairs extending across the external nares. Large particles in the air, including dust, sand, and insects, become trapped in these hairs, preventing them from entering the nasal cavity.

Nasal Cavity

The nasal cavity is a hollow space located behind the nose divided into right and left portions by the nasalseptum. This structure is made up of the vomer boneand the perpendicular plate of the ethmoid bone posteriorly­ and septal cartilage anteriorly­. Posteriorly, the nasal cavity is continuous with the nasal portion of the pharynx via the posterior nasal apertures, which are funnel-like structures also known as choanae.

The roof of the nasal cavity comprises the skull’s ethmoid and sphenoid bones. Its floor is made up of the palate, a structure that separates it from the oral cavity below. Anteriorly, the palate is supported by processes of the maxillary bones and the palatine bones. This area is called the hard palate. There is also a soft palate, which is the unsupported poste-rior section.

Most of the nasal cavity is lined with respiratorymucosa, which is pseudostratified ciliated epitheliumcontaining many scattered, mucus-secreting gobletcells. This epithelium includes a network of bloodvessels and rests on a lamina propria, which has a rich supply of seromucous nasal glands. Every day, these glands secrete approximately 1 liter of mucus, which contains the antibacterial enzyme known as lysozyme. The filtration mechanisms that prevent contamination of the respiratory system are referred to as the respira-tory defense system.

As air passes through from the nostrils and over the mucosal surfaces, heat from the blood in these underlying capillary plexuses warms the air to more closely match the body’s temperature. Incoming air is also moistened from H₂O evaporation out of the mucous lining. The lysozyme-containing mucus, pushed by the cilia of the epithelial lining, entraps dust and other small particles and carries them toward the pharynx. The lysozyme attacks and chemically destroys bacteria. The respiratory mucosa cells also secrete defensins, which function as natural antibiot-ics. The defensins kill invading microorganisms. The respiratory mucosa has ciliated cells that create a slight current, moving the contaminated mucus posteriorly toward the throat. Once swallowed, the mucus and its contained microorganisms are destroyed by the ­gastric juice of the stomach. The nasal cavity acts as a resonating chamber in speech.

There is a rich supply of sensory nerve endings in the nasal mucosa. When they contact irritating dust, pollen, and other particles, the sneeze reflex is triggered. When we sneeze, air is forced outward vio-lently, which effectively expels irritants.

The bones and bone processes that project in a curved shape from the nasal cavity’s lateral walls, dividing it into passages, are called the nasal conchae (FIGURE 21-2). These include the superior, middle, and inferior nasal conchae. Together, they increase themucosal surface area that is exposed to air as well as the air turbulence in the nasal cavity.

Paranasal Sinuses

The paranasal sinuses are a ring of air-filled spaces inside the skull bones that open into the nasal cavity. Located inside the maxillary, frontal, ethmoid, and sphenoid bones of the skull, they are lined with mucous membranes that are continuous with the lining of the nasal cavity. These sinuses reduce the skull’s weight and resonate, affecting the quality of the voice. They also help to warm and moisten the incoming air. Mucus produced by the paranasal sinuses eventually flows to the nasal cavity. When you blow your nose, a suction-ing effect is created that helps to drain the sinuses.