A sinus is essentially a space within the body. Three such sinuses are the paranasal sinuses, the lymph sinues, and the anal sinuses. Each of these different sinuses has a distinct function that helps the body run smoothly. The paranasal sinuses help to filter pathogens, dust, and other foreign bodies out of the body, while the lymphoid sinuses allow lymph to pass through lymph nodes. Lastly, the anal sinuses help feces to pass through the anus more easily.

Perhaps the most well known sinuses are in paranasal sinuses. These are the structures that get infected when a person has sinusitis. Paranasal sinuses are spaces within the bones of the skull and are lined with mucus membranes. They exist to help the nose keep foreign objects out of the body and to help moisten the air that is inhaled. Moisture that accumulates in the paranasal sinuses usually drains out of the sinus cavity and into the nose.

There are four paranasal sinuses: the ethmoid, the frontal, the maxillary, and the sphenoid. The ethomoid sinus is located in the bones between the eyes, and the maxillary sinuses are located within the cheekbones. These are the only paranasal sinuses a person is born with. The sphenoid sinuses are located behind the eyes, and the frontal within the bones in the forehead. These sinuses develop by the time a person reaches adulthood.

In addition to the paranasal sinuses, are lymph sinuses. A lymph sinus is a space within the lymph node that lymph passes through. Lymph is a clear fluid that leaks out of capillaries and is taken up by lymph vessels. The lymph then flows into lymph nodes through lymph sinuses where it is cleaned.

Three different sinuses exist within the lymph nodes: the subcapsular, cortical, and medullay. Within these lymph sinuses are macrophages that seek to eliminate pathogens and foreign bodies from the lymph. Thus cleaned, the lymph can continue on until it returns to the bloodstream.

The anal sinus is another sinus cavity within the body. Like the paranasal sinuses, the anal sinus contains mucus membranes that secrete mucus. The mucus secreted there helps to lubricate the anal canal. The anal sinuses are located just inside of the anus. When feces pass by and press on the sinuses, they release mucus that allows feces to more easily and comfortably pass.