Anatomy of the Paranasal Sinuses

The hollow air spaces in the body are known as sinuses. There are about 60 sinuses spread throughout your body. When you think of “sinusitis” or “sinus infection”, you are talking about the paranasal sinuses. Each of the paranasal sinuses has an opening (ostium) into the nasal cavity. To function normally and stay healthy, each sinus cavity must be able to drain mucus and exchange air through these openings.

The paranasal sinuses are composed of:

  • Ethmoid sinuses
  • Frontal sinuses
  • Maxillary sinuses
  • Sphenoid sinuses

The Ethmoid Sinuses are located behind the bridge of the nose in the ethmoid bone. These sinuses consist of 6-12 thin-walled cavities. These are divided into anterior, middle and posterior groups. The posterior group drains into the nasal cavity towards the rear. Sometimes one or more of the posterior group opens into the sphenoid sinus. The middle group and anterior group drain into the middle of the nasal cavity.

The Frontal Sinuses are located behind your eyebrows in the frontal bone. They can differ in size from left to right and in about 5% of people they are not present at all. The frontal sinuses are absent at birth, but are well-developed by age 8, and reach their full size around puberty. The frontal sinuses drain into the middle part of the nasal cavity.

The Maxillary Sinuses are the largest of the paranasal sinuses. They are located behind each cheekbone and are roughly triangular in shape. The maxillary sinuses drain into the middle par t of the nasal cavity. The opening into the nasal cavity is found high up on the sinus wall and these sinuses do not drain well with the head upright.

The Sphenoid Sinuses consist of one or two sinuses located deep behind the bridge of the nose in the sphenoid bone. These sinuses drain into the back part of the nasal cavity. The openings of the sphenoid sinuses are also located high on sinus wall and do not drain well when the head is upright.

The sinus lining is composed of epithelium cells (with and without cilia), goblet cells, and basal cells. There are also wandering immune cells present (lymphocytes and mast cells). The sinus lining forms a physical barrier that keeps bacteria, pollutants and allergens from entering our bloodstream and tissues.

The sinus lining also produces mucus from the goblet cells. This mucus traps the pollutants, bacteria and allergens. The mucus also has special antibodies and enzymes that;

  • prevent viruses and bacteria from sticking to the sinus lining
  • help our white blood cells to recognize viruses and bacteria as invaders and to kill them

The ciliated epithelial cells work together to sweep out the mucus that has foreign materials and microorganisms. This process is known as mucociliary clearance. These ciliated cells are very sensitive to humidity, pollutants and toxins. If they don’t function well, we can expect sinus infections to occur.

The purpose of the sinuses is unknown, but here are some of the possible functions:

  • Makes the front of the skull lighter
  • Makes the voice more resonant
  • Provide a crumple zone for facial blows
  • Protects the eyes and teeth from rapid temperature changes
  • Heating and moistening incoming air