The physics of blood vessels, or of a liquid flowing through any tube is that of laminar flow. It has to do with relative friction of flowing fluid near the walls, and results in higher velocity near the center. Here, the fluid is water, flowing from a pipe in a pattern lit by alternating lights around this fountain. The clearer center of this cascade of water is moving with higher velocity, in the parabolic shape typical.

It also makes for a pretty cool image, I thought...