What exactly is the mechanism or physiology that explains why gargling with warm saltwater relieves the pain of a sore throat?

R.D.

This is an educated guess. Sore throats are sore due to inflammation. Usually, the inflammation is due to a viral or bacterial infection. (The common cold and strep throat are examples of each.) However, sore throat may also be related to allergies or physical injury to the throat. In each case, an inflammatory response has been triggered. Pain, swelling and an itchy or irritated sensation are the common symptoms of this inflammatory response.
The medical term for swelling is "edema." As part of the inflammatory response, there is an increase in the water content of the involved tissues. With respect to sore throat, a sense of fullness and difficulty swallowing are both related to edema of the tissue lining the throat (called mucosa).

Water tends to follow salt. Anyone who has ever salted a snail knows what happens to the snail. Here's a less disgusting analogy: Have you ever salted slices of raw eggplant? The salt draws moisture from the eggplant. After a few minutes, you can see beads of fluid on the surface of the eggplant.

Everyone has a different recipe for a saltwater gargle, but all recipes tend to be fairly salty. If the gargle has a higher salt concentration than your cells' salt concentration, it will tend to draw out some of the edema fluid from the mucosa of the throat. This will make SOME of your "sore throat" symptoms better. The relief is very real, but also tends to be short-lived, since the gargle has done nothing to remove the cause of the sore throat.

In some cases, gargling with saltwater also has a cleansing action. If you are suffering from postnasal drainage due to a bacterial sinus infection or allergies, the mucus that drains from your nasal cavity is full of chemicals that tend to promote an inflammatory response. As this stuff drains down the back of your throat, it leaves a trail of inflamed tissues in its wake. Frequent gargling with saltwater will help minimize this problem.

As with everything else in medicine, you can certainly overdo this one! Make the gargle salty enough, and you will dry out the mucosa of the throat. This could lead to even greater irritation than what you started with. My advice: The gargle should taste a little salty, but not overwhelmingly so. Begin with one teaspoon of salt per pint of water. Once the salt has dissolved, taste it. If it tastes like spit, add a bit more salt until it begins to taste salty.