The sinuses are air-filled spaces within the bones around the nose and eyes that connect with the nasal cavity.
Original educational illustration created by the author; anatomy is simplified for clarity. Source: Samargandy S, 2026.
Where are the sinuses?
There are four main sinus regions:
- Maxillary sinuses — within the cheeks
- Ethmoid sinuses — between the eyes
- Frontal sinuses — above the eyes
- Sphenoid sinuses — deeper behind the nose
The size and shape of the sinuses vary naturally from person to person.
How do the sinuses connect to the nose?
The sinuses are lined by a thin mucous membrane similar to the lining of the nose. Mucus normally drains through small pathways from the sinuses into the nasal cavity.
These drainage pathways can be narrow and vary between people.
What happens when the sinuses become inflamed?
Swelling of the lining can interfere with normal drainage and ventilation. This may happen with sinusitis, nasal polyps, allergies, infection, or other inflammatory conditions.
Ongoing inflammation can cause congestion, nasal discharge, facial pressure, and reduced sense of smell.