What does spitting. up blood when u brush your teeth?

Depends: If there is just a little blood, it could be coming from inflammation of your gums. A little of this once in a while is not uncommon. But if this is a regular problem, you should see your dentist as you may have chronic gum disease that needs treatment. If there is a lot of blood on a regular basis, get your mouth checked by a doctor to make sure there is no oral tumor present.

Gium disease: If you are bleeding after brushing or flossing most likely is due to gingivitis or periodontal disease. Plaque accumulates in areas you can't brush or floss well resulting in irritation to your gums just like a splinter embedded into your skin will do. Gums become"puffy" bleeding easily. Generally with a professional dental cleaning the problem will be corrected with improved home daily hygiene.