Just to add on, the reason why the oxygenated blood from the left ventricles being pumped into the right ventricles instead of the other way around is the pressure provided by pumping action of the left ventricle is much higher than the right. After all the left ventricle needs to ensure they apply sufficient pressure so that blood can travel to a various part of the body. Just, take a look at the left ventricular muscle, much more significant than the right ventricle, isn't it?

Nice article, really simple and easy to understand.