4. Circulatory and Respiratory system

Upper respiratory tract

The upper respiratory tract refers to the parts of the respiratory system that lie outside the thorax, more specifically above the cricoid cartilage and vocal cords. It includes the nasal cavityparanasal sinusespharynx and the superior portion of the larynx. Most of the upper respiratory tract is lined with the pseudostratified ciliated columnar epithelium, also known as the respiratory epithelium. The exceptions are some parts of the pharynx and larynx.

Nasal cavity

Nasal cavity (Cavitas nasi); Image: Begoña Rodriguez

Nasal cavityCavitas nasi1/6Synonyms: Nasal cavity proper, Cavum nasi

The upper respiratory tract begins with the nasal cavity. The nasal cavity opens anteriorly on the face through the two nares, and posteriorly into the nasopharynx through the two choanae. The floor of the nasal cavity is formed by the hard palate, while the roof consists of the cribriform plate of the ethmoid bone posteriorly, and the frontal and nasal bones anteriorly. The nares and anterior portion of the nasal cavity contain sebaceous glands and hair follicles that serve to prevent any larger harmful particles from passing into the nasal cavity. 

The lateral walls of the nasal cavity contain three bony projections called nasal conchae (superior, middle and inferior), which increase the surface area of the nasal cavity. The nasal conchae also disrupt the laminar flow of air, making it slow and turbulent, thereby helping to humidify and warm up the air to body temperature. 

The roof of the nasal cavity contains the olfactory epithelium which consists of specialized sensory receptors. These receptors pick up airborne odorant molecules and transform them into action potentials that travel via the olfactory nerve to the cerebral cortex, allowing the brain to register them and provide a sense of smell.

Another pathway for the entry of air is the oral cavity. Although it is not classified as a part of the upper respiratory tract, the oral cavity provides an alternative route in the case of obstruction of the nasal cavity. The oral cavity opens anteriorly on the face through the oral fissure, while posteriorly, it opens into the oropharynx through a passage called the oropharyngeal isthmus. 

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