Sinus ostia
Encyclopedia
A sinus ostium is the opening that connects a sinus
Paranasal sinus
Paranasal sinuses are a group of four paired air-filled spaces that surround the nasal cavity , above and between the eyes , and behind the ethmoids...

 to the nasal cavity
Nasal cavity
The nasal cavity is a large air filled space above and behind the nose in the middle of the face.- Function :The nasal cavity conditions the air to be received by the other areas of the respiratory tract...

 itself. It is a tight area that tends to have a higher percentage of cilia than the surrounding mucosa.

If the sinus ostium is blocked this will cause an accumulation of fluid in the sinus.
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