Why do veins become more prominent after exercising?

I recently read an article in Scientific American on this topic, and I think the answer they give (about which I had no idea!) is really interesting. You can find it here. Basically, it appears that the increased pressure on the arterial side of our blood vessels increases the pressure in the muscle capillaries (the capillary's hydrostatic pressure). As a result of this, more fluid is filtered out into the interstitial spaces of the muscle, which causes it to become a little harder and distended. The surface capillaries are thus pushed against the skin surface and flatten out.