So within the st segment, all of the cells within the ventricles have been depolarized, and repolarization of those cells basically begin. Now, the semi lunar valves, both of them, actually open up and our blood begins to eject into these blood vessels. This takes place in this flat region, the st segment, where the voltage is basically flat. Now let's move on to our T wave. And then let's take a look at the U wave. Now, what exactly happens within our T wave?