The acetic acid in the vinegar will gently cauterize the inside of the nose and stop the flow of blood. You can also fight off your nosebleeds by having more zinc in your diet, a task that can be accomplished by consuming more whole wheat bread. Vitamin K helps your body with blood clotting, so you should also try to consume more dark-green leafy vegetables, and having more Vitamin C will reduce your chances of getting a nosebleed in the first place. If you already have a nosebleed, you can also apply pressure to your upper lip. This is where a major blood vessel that travels to your nose is located, so if you hold a piece of cotton against it, you can slow down the bleeding.
Check out HowStuffWorks for more tips on how to fight nosebleeds.
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