It’s not unusual for your hands to swell when you run because running causes increased blood flow to your heart, lungs, and muscles being worked out. This increased circulation to certain areas actually decreases to your hands, which will lower their temperature. In order to try to return to a normal temperature and help circulation, the blood vessels in your hands will naturally open wider. When this happens, it can be seen as swelling. If your fingers in particular appear to be swelling the most, this can happen from hanging your hands below your heart while running. Basically, your body is too busy increasing circulation to your largest muscles when you run that the smaller ones, like your hands and fingers, become less efficient in moving excess blood and fluids, which is why your hands swell.