text: Aspirin helps these problems by stopping cells from making prostaglandins. Remember the enzyme, COX-2?  It is a protein made by your body's cells whose job is to take chemicals floating around in your tissues and turn them into prostaglandins.  \nCOX-2 can be found in lots of normal tissues, but much more of it is made in tissue that has been damaged in some way. Aspirin, it turns out, sticks to COX-2 and won't let it do its job; it's like a lock you put on your bicycle.  The bicycle won't move with the lock on, and COX-2 can't work with aspirin stuck in it.  So by taking aspirin, you don't stop the problem that's causing the pain, like the tight muscles in your scalp, or the cramping in your abdomen, or the hammer-damaged finger.  But it does "lower the volume" on the pain signals getting through your nerves to your brain. \n\nOne common question about aspirin and other medicines is, "How does it know how to get to where the pain is?"  The answer is that it doesn't! When you take aspirin, it dissolves in your stomach or the next part of the digestive tract, the small intestine, and your body absorbs it there. Then it goes into the bloodstream and it goes through your entire body. Although it is everywhere, it only works where there are prostaglandins being made, which includes the area where it hurts.  \n\nYou may ask, "How come I have to keep taking aspirin if it works so well?"  As with almost all chemicals, your body has ways of getting rid of aspirin.  In this case, your liver, stomach, and other organs change aspirin to... surprise! Salicylic acid!  This chemical then slowly gets changed a bit more by the liver, which sticks other chemicals onto the salicylic acid so that your kidneys can filter it out of your blood and send it out in your urine. This whole process takes about four to six hours, so you need to take another pill at that time to keep the effect going. \n\nThe problem with the fact that aspirin goes through your entire bloodstream is that your body needs prostaglandins for some reasons.  One place they are useful is in the stomach; it turns out another enzyme called COX-1 makes a prostaglandin that seems to keep your stomach lining nice and thick.  Aspirin keeps COX-1 from working (it keeps most prostaglandins from being made equally well -- it's "nonselective"), and your stomach lining gets thin, allowing the digestive juice inside to irritate it.  This is probably the biggest reason why aspirin and its relatives upset stomachs (not only because it's an acid, as Hoffmann had thought).  \n\nCOX-2 also works in some normal tissues like the brain and kidney; at normal amounts, one dose of aspirin probably doesn't affect these areas much. And there are other places in the body where prostaglandins have a job in normal tissues, such as the blood..