Nasal Cavity

Air passages consist of the conducting portion and the respiratory portion. The conducting portion includes the air passages that lead to the sites of respiration within the lungs where gas exchange takes place. The two components are the passages external to the lungs and the passages within the lungs. The respiratory portion includes the respiratory bronchioles (both air conduction and has exchanges occur here), alveolar ducts, alveolar sacs, and alveoli.

The respiratory region of the nasal cavity has a mucosa layer made of ciliated pseudostratified columnar cells. It consists of five cells types:

• Ciliated cells for ciliary action
• Goblet cells to secrete mucus
• Brush cells with microvilli for sensory function
• Small Granule cells or Kulchitsky cells, which are endocrine cells of the APUD system
• Basal cells serve as stem cells

The lamina propria consists of a rich vascular network and mucous glands with serous demilunes. The vascular network is made of complex capillary loops arranged in rows near the surface. Blood flows perpendicular to the air flow, which warms up the air. Blood vessels become engorged and leaky during allergy, infection, or common cold. The mucous glands produce secretions to supplement goblet cell secretions, which trap particulate matter in the air. Trapped particles are swept into the pharynx by ciliary action.

The olfactory region of the nasal cavity is located on the roof of the nasal cavity and slightly extends over the medial and lateral walls. It is lined with olfactory mucosa containing olfactory epithelium and olfactory glands. The epithelium is composed of four cell types (but no goblet cells), including:

• Olfactory receptor cells have bipolar neurons and a single dendritic process that projects above the epithelial surface as a knob-like structure known as the olfactory vesicle. It has long thin non-motile cilia that extend from the olfactory vesicle or dendritic process. The axonal process leaves from the base of the cell.
• Supporting or sustentacular cells are the most numerous and have numerous microvilli. They secrete odorant binding protein (OBP) and make adherent junctions with olfactory cells. They provide physical and metabolic support to the olfactory cells. Their function is comparable to that of neuroglial cells.
• Brush cells are found in smaller numbers and contain large, blunt microvilli. The sabal surface is in contact with general sensory afferents of the trigeminal nerve. Their function is general sensation of the olfactory mucosa.
• Basal cells are small, rounded cells located near the basal lamina. They are the stem cells for olfactory cells and supporting cells.

The lamina propria layer contains olfactory glands (or Bowman’s glands), and unmyelinated nerves and blood vessels. The Bowman’s glands contain lipofuscin granules that predominate in the glands. Due to this the mucosa is a yellow-brown colour. Serous secretions of the gland trap and dissolve odoriferous substances.