Kidney Autoregulation

[Those2Chapmans]

hi, this is actually my first post. i have been using allnurses as a reference site since nursing school, but haven't ever posted. now that i'm starting school up again i thought i should give it a try.

i have two questions as i sit here studying our lovely kidneys. i've gotten sort of hung up on the concepts and could use some help.

1. i understand that the first step to urine formation is our glomerular filtration rate. this rate is dependent on blood flow to the kidneys, pressure in the bowman's space, as well as plasma oncotic pressure. okay, soooo... why is there an inverse relationship w/the pressure in bowman's space and the rate of filtration? i'm trying to think about when there is increased capsule pressure (increased blood flow in the afferent arterioles, correct?) why would the filtration rate decrease? wouldn't it make more sense that the filtration would increase with increased pressure and therefore increased blood flow? or is it that w/increased blood flow to the glomerular space the kidneys aren't able to filter as fast?

2. in understanding renal blood flow regulation mechanisms ... we have our autoregulation, our sns, and our raas systems. in autoregulation as arterial bp increases the afferent arterioles vasoconstrict. inversely, when our bp decreases the afferent arterioles vasodilate. if the bp is already low, for instance, why would the arterioles dilate? wouldn't that cause the bp to go even lower and not help the gfr?

i know this is a really specific question and gets into the nitty gritty, but for some reason i just can't wrap my head around this. i actually remember back to a class in adult health nursing where i got this wrong. i guess i never learned it correctly the first time.

i appreciate it.

thanks,