Why does the kidney release renin when the renal artery is blocked by artherosclerotic plaque?
There are a few huge regulators of blood pressure in your body. Your heart, blood vessels, and kidney are some of the primary things mediating whether your blood pressure increases or decreases. Your heart can vary its force or frequency of contraction. Your arteries can constrict or dilate to increase or decrease the size of the "container" holding blood. Finally, your kidneys pretty much are a significant determining factor in circulating volume and long and short term blood pressure regulation through its filtering and excretion of fluid and through a small set of cells called the juxtaglomerular apparatus (JGA).
These little guys are the primary reason why an atherosclerotic plaque would cause renin release. When you have a plaque proximal to the JGA, you decrease perfusion to it. This set of cells senses a blood pressure drop, and it works to correct it. The primary method of doing so? Activating the Renin-Angiotensin system! Cells inside the JGA secrete renin into the blood to activate a cascade of reactions that culminate in a chemical (angiotensin II) that clamps many arterioles (smaller container = higher pressure) and another chemical (aldosterone) that tells the kidney to recycle more water from urine (more fluid = higher pressure).
The JGA thinks that you're dying because the pressure it sees is low, so it does what it's designed to do: increase BP!
I hope that clears things up!