Sinuses are naturally occurring airfilled spaces in the skull. Each sinus is covered with a mucosal covering. The sinuses are all connected to the nose and mucus produced in the sinuses naturally drain out through the nose.
In sinusitis, the mucous membranes become inflamed and swollen. This obstructs the flow of mucus out of the sinuses and leads to build-up of mucus in the sinuses leading to sinusitis.