A sinus is essentially a space within the body. Three different ones are the paranasal sinuses, the lymph sinuses, and the anal sinuses, each of which has a distinct function that helps the body run smoothly. The paranasal ones help to filter pathogens, dust, and other foreign bodies out of the body, while the lymphoid ones allow lymph to pass through lymph nodes. The anal sinuses help feces to pass through the anus more easily.
Perhaps the most well known are in paranasal sinuses, the structures that get infected when a person has sinusitis. They are spaces within the bones of the skull that are lined with mucus membranes. These spaces 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 them usually drains out of the cavity and into the nose.
There are four paranasal sinuses: the ethmoid, the frontal, the maxillary, and the sphenoid. The ethomoid one is located in the bones between the eyes, and the maxillary ones are located within the cheekbones. These are the only paranasal ones that a person is born with. The sphenoid sinuses are located behind the eyes, and the frontal within the bones in the forehead. These develop by the time a person reaches adulthood.
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 these cavities, where it is cleaned.
Three different ones can be found within the lymph nodes: the subcapsular, cortical, and medullay. Within them are macrophages that seek to eliminate pathogens and foreign bodies from the lymph. Once cleaned, the lymph can continue on until it returns to the bloodstream.
Like the paranasal ones, the anal sinuses contain membranes that secrete mucus, which helps to lubricate the anal canal. They are located just inside of the anus. When feces pass by and press on them, they release mucus that allows feces to more easily and comfortably pass.