13 years ago
When the ventricles are contracting, they are contracting to force the blood in the arteries; either to the pulomonry or the aorta. Blood always moves from a high pressure to a low pressure. The semi lunar valves will open when the pressure in the ventricles is higher then that in the arteries, forcing the blood into the ventricles.
Hope this helps. You really need to learn this good. I just learned it yesterday, and it really helps. x