Paranasal Sinuses and Sinusitis.
Several open, air-filled chambers called paranasal sinuses are present in the
bones surrounding the nasal cavity. In the cheeks are two large maxillary sinuses,
just above the orbits are two frontal sinuses, between the eyes are many small
ethmoid air cells (=sinuses), and at the base of the skull are two sphenoid sinuses.
A thin layer of respiratory (nasal) mucosa lines the paranasal sinuses. Mucus
produced in the sinuses normally drains out of small aperatures (= ostia) and
adds to that already present in the nasal cavity. The open sinuses also help
lighten the skull and resonate the voice sounds produced by the larynx.
Sinusitis most often occurs when infections, allergies, or tissue irritants cause
the sinus mucosa to become inflamed. The edematous (=swollen) membranes block
the ostia drainageways that lead to the nasal cavity and mucus accumulates in
the open sinus chamber. Air trapped in the sinus is absorbed into the blood stream,
creating a negative pressure or vacuum. As the vacuum builds, so does the sense
of pain. The vacuum may draw fluids into the sinus from the blood stream. Bacteria
and other microbes often grow in these fluids leading to more edema and inflammation.
