The paranasal sinuses are a group of four paired, air-filled cavities situated within the bones of the face and skull. They are centered around and directly connected to the nasal passages, making them part of the body’s respiratory system. These cavities often remain unnoticed until an infection, allergy, or other issue causes them to become congested or inflamed.
The Four Pairs of Paranasal Sinuses
The intricate structure of the sinus system is composed of four distinct pairs, each named for the facial bone in which it is housed. The largest are the maxillary sinuses, located in the cheekbones just below the eyes. Above them, positioned in the forehead area, are the frontal sinuses, which sit just superior to the eyebrows.
The third pair, the ethmoid sinuses, is a cluster of small air cells positioned between the eyes and at the bridge of the nose. This group is often described as three sub-groups: anterior, middle, and posterior. Finally, the sphenoid sinuses are the deepest pair, located in the sphenoid bone at the very back of the nasal cavity, behind the eyes.
Essential Roles of the Sinuses
The sinuses serve several physiological purposes that benefit the body. They decrease the overall weight of the skull, as these air-filled pockets make the bony structure lighter than if it were solid. They also act as natural shock absorbers, providing a buffer that helps protect delicate structures like the brain and eyes in the event of facial trauma.
The sinuses also play a role in how we sound when we speak, as they act as resonating chambers for the voice. Sound waves bounce off the cavity walls, which helps to amplify the voice and gives it a unique timbre. More importantly for respiratory health, the sinuses are lined with a specialized tissue called mucosa.
The mucosa lining produces a thin, flowing layer of mucus that is continuously secreted. This mucus has an immune function, helping to trap airborne particles, dust, and pathogens that are inhaled. The sinuses also serve to humidify and warm the air before it travels deeper into the airways and lungs. The continuous flow of mucus sweeps these trapped particles toward small openings called ostia, allowing the material to drain into the nasal cavity where it is ultimately swallowed.
Why Sinuses Cause Discomfort
The feeling of pressure and pain associated with a cold or allergies results from a disruption in the normal drainage process. The delicate lining of the sinuses is highly sensitive to irritants, and exposure to viruses, bacteria, or allergens causes the mucosa to become inflamed. This swelling is a protective response, but it can be problematic because the small channels that allow mucus to drain, the ostia, are easily restricted.
When the ostia swell shut, the mucus continues to be produced but has no way to exit the sinus cavity. This trapped fluid rapidly begins to build up within the rigid, bony walls of the sinuses. The resulting increase in internal pressure translates directly into the familiar feeling of facial fullness, throbbing pain, and congestion across the cheeks, forehead, and behind the eyes.