A thin layer of respiratory mucosa lines the parsnasal sinuses. mucus produced in the sinuses normally drains out of small aperatures and adds to the mucus in the nasal cavity.The open sinuses also help lighten the skull and resonate the voice sounds.sinusitis most often occurs when infections,allergies, or tisue irritants cause the sinus mucosa to become inflamed.the edematous membranes block the ostia drainageways that lead to the nasal cavity and mucus accumulates in the open sinus chamber.