ACE inhibitor and Filtration Fraction

[cheruka]

Here is what I have from a qbank:
"Angiotensin II is the primary regulator of the efferent arteriole. Angiotensin II normally constricts the efferent (outgoing) arteriole thereby decreasing renal plasma flow which leads to an increase in the filtration fraction. However, when a patient takes an ACEi, the constricting effect on the efferent arteriole is blocked because ACEi’s decrease the production of angiotensin II. This decrease in angiotensin II leads to an increase in renal plasma flow and thus a decrease in the filtration fraction."
Filtration Fraction = GFR/RPF

If The ACEi does not let the efferent arteriole constrict, then the GFR will be low, making the Filtration Fraction low. Why would a ACEi affect RPF (renal plasma flow)?

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[Lanolin]

Ace inhibitors decrease intraglomerular pressure by decreasing resistance in the efferent arteriole. Essentially they are saying the same thing from a different angle. A decrease in resistance on the efferent side allows more blood through the kidney (RPF) but less pressure and time to filter it at the glomerulus (GFR). Ace I's will decrease angio II production thus reducing efferent artery constriction and subsequent intraglomerular pressure, both of which lead to decreased filtration even though more blood is flowing through the glomerulus for any given time point (RPF).

Think of it like a hose with a a hole in it (the filtration system loops). If you pinch the end it will increase pressure @ the hole and increase flow through it (renal filtration). If you let the water flow it'll take the route of least resistance.

 

[cheruka]

Thanks for the reply.
With ACEi, why is there increased RPF? Is it because it has vasodilatory effects everywhere, not just on the efferent arterioles?