The sinuses are paired, air-filled cavities within the bones of the face and skull, connected to the nasal passages. They lighten the skull, condition inhaled air, and contribute to vocal resonance.
Mapping the Four Major Sinuses
The head contains four pairs of paranasal sinuses, named for the bone in which they reside. The largest are the Maxillary sinuses, located in the cheekbones beneath the eyes. These pyramid-shaped cavities are often the first affected during a common cold.
The Frontal sinuses are positioned in the forehead bone, just above the eyebrows. They are typically triangular and often separated into two distinct pockets. Inflammation within this pair usually causes pressure and pain in the forehead area.
Deep within the skull, between the eyes and behind the nose bridge, are the Ethmoid sinuses. These are a honeycomb-like collection of small air cells, located near the eye sockets and upper nasal cavity. Further back, within the sphenoid bone at the skull’s base, are the Sphenoid sinuses.
The Sphenoid sinuses are located behind the eyes and upper nasal area, near the optic nerves and the pituitary gland. All four pairs connect to the nasal cavity through narrow drainage pathways. This allows for air exchange and fluid clearance, explaining why inflammation can cause widespread facial pressure.
The Internal Structure and Lining
The interior of a healthy sinus cavity is uniformly lined with specialized respiratory mucosa. This lining is thin, slightly moist, and a pale, healthy pink color. The mucosa is an active, functional layer responsible for the sinus’s health.
This tissue is composed of ciliated pseudostratified columnar epithelium, which includes mucus-producing goblet cells. These cells secrete a thin, watery layer of mucus that traps inhaled contaminants like dust, bacteria, and allergens. The constant production of this fluid helps maintain a clean environment within the cavity.
Microscopic, hair-like projections called cilia protrude from the lining cells. These cilia beat in a coordinated, wave-like motion, creating a cleaning system known as the mucociliary escalator. This action steadily sweeps the layer of mucus and trapped debris toward the small drainage openings.
These openings, called ostia, are the small bottlenecks through which the sinuses connect to the nasal cavity. A healthy ostium is open and clear, allowing the mucus to drain freely into the nose. The effective function of the cilia and the patency of the ostia are what define a healthy, clear sinus interior.
Appearance Under Common Conditions
When a person is healthy, the sinus cavity is predominantly air-filled, and the mucosal lining is a non-inflamed, pale pink color. The mucus layer is thin and clear, maintaining its moisture without pooling or obstructing the drainage points. This open, air-filled state is the necessary condition for proper sinus function and drainage.
In contrast, a common condition like sinusitis, or sinus inflammation, dramatically alters the internal appearance. The first and most noticeable change is the visible thickening and swelling of the mucosal lining, a state known as edema. Due to increased blood flow and inflammation, the thin, pale pink tissue becomes visibly red and engorged.
This swelling causes the internal space of the sinus cavity to shrink, making it appear congested and closed off rather than open and air-filled. The thickened lining begins to produce an excessive amount of mucus that is often thicker and more opaque than normal. This fluid may appear yellow or green, indicating a high concentration of inflammatory cells fighting an infection.
The swollen mucosa effectively constricts the small ostia, which are already narrow by design, leading to a visible blockage of the sinus opening. This obstruction traps the accumulating thick mucus and air inside the cavity, leading to the painful pressure associated with a sinus infection.