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What is a Sinus?

By M.R. Anglin
Updated Mar 03, 2024
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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.

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Discussion Comments

By DylanB — On Feb 27, 2013

@healthy4life – I've had those sinus headaches before, and they are very painful. Mine usually happen when I have a really stuffy head because of a cold, though.

I don't suffer from allergies, so I don't deal with chronic sinusitis like some people I know. When I get a cold, though, I get a taste of what they go through on a regular basis. All of my head sinuses become clogged, and the lymph sinuses in my neck swell up and become sore.

When you have a cold, nothing you take really seems to provide relief. Over-the-counter stuff isn't powerful enough to open up my sinuses, and the most it can do is help the pain from the sinus pressure. Since colds are caused by viruses instead of bacteria, there is no cure.

By feasting — On Feb 26, 2013

Do you have a blocked lymph sinus when your lymph nodes swell? I know that they swell because they are working extra hard to fight off an infection, but is part of the reason because the sinuses can't handle the increased flow of lymph?

By healthy4life — On Feb 25, 2013

I've had sinus pressure behind my eyes and in my cheeks before, and it was very unpleasant. It felt like the area was about to explode, and I had some sharp pains in my sinuses.

I had to take painkillers to deal with it. I took a combination decongestant/painkiller to try to deal with the source of the pressure, but it only helped during the four hours while the medication was in my system.

I finally caved and went to a doctor. She told me I had a sinus infection and gave me antibiotics.

Bacteria were actually to blame for the trapped mucus in my sinuses. I doubt I could have ever gotten over the infection without antibiotics.

By kylee07drg — On Feb 25, 2013

We usually think of sinus drainage as a bad thing, but when it comes to the anal sinuses, it's actually helpful. I did not know until reading this article that these sinuses were where the mucus in the bowel movements came from, but I have seen some in the toilet before and wondered what was going on with that.

By Jellybeaner — On Feb 14, 2013

Who knew that there is an array of sinuses all over our body?

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