Maxillary 3... During infections and sinus blockage, mucous or pus fills the sinus. Fluid is pulled from the surface of the fluid level and then transported to the exit. The surface area of the sinus cavity that is below the fluid level cannot provide useful mucocilliary flow. It can be a fine balance between the rate that fluid is produced and the speed of the cilliary pumping mechanism. 1999 Peter Casano, M.D.