How Do Drugs Work?

Did you ever wonder how aspirin knows to go to your head when you have a headache and to your elbow when you have "Tennis Elbow"? Or how one or two small aspirins containing only 325-650 mg of active drug can relieve a headache or ease the inflammation of a strained muscle or tendon in a 195 lb. athlete?
The answer to the first question is that drugs are distributed throughout the body by the blood and other fluids of distribution (see distribution below). Once they arrive at the proper site of action, they act by binding to receptors, usually located on the outer membrane of cells, or on enzymes located within the cell.