RAAS system.

[sabsaf123]

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Sorry if this is a dumb question, I googled but couldnt find an answer to my specific question.

So I understand that the RAAS system leads to increased Na+ and H2O reabsorption and constriction of the efferent arteriole. More water reabsorption leads to higher BP and by constricting the efferent arteriole the systemic BP will obviously go up BUT it will also increase the GFR. If you increase the GFR aren't you just going to pee out more fluid and thus decrease your BP?

It seems to me like the system is doing two opposing things. You are increasing BP cuz of efferent arteriole constriction but you are also decreasing BP cuz of increased GFR....?

 

[theonlytycrane]

Angiotensin II constricts the efferent arteriole -> increased GFR -> ***but with compensatory Na+ reabsorption in the PCT and distal nephron***. GFR goes up, but you're pulling back in more Na+ (and H2O that follows).

Just a note that constricting the efferent arteriole increases GFR (and the downstream effects of this increase BP). Efferent arteriole constriction doesn't increase systemic BP by itself (without these downstream effects).

 

[sabsaf123]

Angiotensin II constricts the efferent arteriole -> increased GFR -> ***but with compensatory Na+ reabsorption in the PCT and distal nephron***. GFR goes up, but you're pulling back in more Na+ (and H2O that follows).

Just a note that constricting the efferent arteriole increases GFR (and the downstream effects of this increase BP). Efferent arteriole constriction doesn't increase systemic BP by itself (without these downstream effects).

View attachment 245278

Thank You!

 

[Dro133]

The above explanation makes sense to me. I also found this explanation on Wikipedia which makes sense to me...as far as I gather, efferent arteriole constriction increases the filtration pressure, but actually decreases renal blood flow due to an increase in systemic resistance. And like the previous poster said, I'd imagine the increase in sodium reabsorption would offset any increase in sodium filtration.

"Normally the afferent arteriole is of larger diameter than the efferent. This means there is high resistance as the blood is forced from a wider vessel to a narrower one and this promotes filtration. If the arterial blood pressure remains constant then contracting either vessel reduces blood flow as it increases resistance. However contracting either has opposite effects on the filtration pressure. If you contract the afferent arteriole there will be less of a pressure difference between the afferent and efferent arteriole so there will be reduced filtration pressure. However if you constrict the efferent arteriole you are increasing the pressure difference between the two and filtration pressure increase.

Overall the constriction of the afferent arteriole decreases both blood flow and filtration pressure where as constricting the efferent arteriole decreases blood flow but increases filtration pressure. (Both of these statements are assuming a constant blood pressure). The fact that both can be altered allows independent regulation of both GFR and blood flow."