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Preload and Afterload


I'm in my second level of nursing school and we are studying perfusion. I know I am going way too deep into the mechanics here, but I am the type of learner that i have to see the ENTIRE puzzle in order to understand the concept. Unfortunately, I'm missing a few of the puzzle pieces and need some plain and simple explanations.

While I understand the technical definitions of these words "preload" and "afterload", I am having trouble with the concepts of "which comes first?" and "how does one affect the other".

I know that afterload is the pressure that the heart has to beat against in order to eject blood from the left ventricle. The way I see it, if the afterload is high due to obstruction or vasoconstriction, then the preload will be lower b/c cardiac output is decreased due to said obstruction or narrowing and less blood is returning upon venous return.

If afterload is normal then preload will be normal as CO adequate to get the blood through the body and back to the heart.

If afterload is low b/c of excess vasodilation or decreased pressure and there's not a large amount of pressure to pump against, then the preload could then be too high b/c more blood is filling the heart upon venous return.

Now my actual question: how do each of these affect each other? Will low preload cause high or low afterload and will high preload cause high or low afterload?

Will high afterload cause low or high preload and will low afterload cause low or high preload?

As previously mentioned, I'm sure I'm making this much harder than necessary but I just need a little explanation in "scenario" form to make the puzzle pieces fall into place.