How do kidneys regulate systemic blood pressure? (indirect vs direct) 

You are right, I should have specified that that rule applies when all other variables remain constant (meaning that the blood vessels diameter remains the same) I sort of implied that in my pipe analogy, saying that if you have the same volume of water in the same size pipe the pressure will be higher, but I should have made it more explicit.

I also could have related it to the effects of hardened arteries... I very well may release this video again. I put it together very quickly because I have students who have a test tomorrow!