Why does salt in a wound hurt so much?
In: Biology
The reason that salt stings a cut is that as the salt dissolves, it causes the fluid surrounding damaged tissues to become extremely hypertonic (which means that the concentration of salt and other electrolytes is higher than it is in normal body fluids).
Pain-sensing neurons have receptors on them that respond to a variety of stimuli. For example, there is a specific receptor that responds vigorously to capsaicin, which is the substance that gives jalapeño and habanero peppers their kick. So, quite literally, when you are adding hot pepper to food, you are (carefully, I hope!) inducing the sensation of pain to complement the other flavors of the meal.
More recently identified is a receptor that responds to changes in electrolyte concentration—such as the change that a large amount of salt induces. Thus, putting salt on a wound stimulates pain-sensing neurons in much the same way hot pepper does.
When tissue is damaged, many pain receptors become sensitized—that is, they need a much lower level of stimulation to respond than they normally would. (For example, after you have burned your mouth, try eating some spicy food that you could normally handle—ouch!) Salt may then compound the pain by further damaging injured tissues and making them more sensitive in general.
There is probably more going on than just this, but to me what comes to mind is its dehydrating effects. Salt relative to its size takes a lot of water to dissolve and in the way dry things do it attracts water. Salt even has utility along these lines, in the past being used to dry out and preserve meat.
So salt is going to rapidly dehydrate whatever flesh it has immediate contact with, this flesh being in a wound is unprotected by skin which would insulate against this. The pain is this dehydration happening until the cells taking the brunt of it die off.
Latest Answers