A scanning electron microscope image from normal circulating human blood.

Photographers Bruce Wetzel/Harry Schaefer, courtesy National Cancer Institute

Aspirin and the Blood

One 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.

You 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.

The 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).

COX-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.

In the last few decades, it has been found that aspirin's action of stopping prostaglandin production has effects on things besides pain, inflammation, and the stomach.

For example, some types of prostaglandins cause tiny particles in your blood (known as platelets) to stick together to form a blood clot. By inhibiting prostaglandin production, aspirin slows down clot production. Although this can be bad, such as with a bloody nose -- in which case you want a clot to form -- blood clots can be damaging as well, such as in causing heart attacks by clogging the blood vessels that bring oxygen and energy to the beating heart. For this reason, many adults now take aspirin to prevent heart attacks, and it also helps people who have already had a heart attack stay alive. Surely Hoffmann (and the Bayer company) could never have predicted this effect. And as noticed at least as far back as Hippocrates in ancient Greece, aspirin and its relatives also lower fevers; this seems to be an effect on a part of the brain known as the hypothalamus, which controls temperature (as well as other body functions).

A lot of research is being done now to find out if aspirin can be used for other problems; it has already shown some promise in helping with problems as diverse as cataracts in the eyes, some cancers, gum disease, and high blood pressure during pregnancy!

Next, we'll check out aspirin's side effects.