Filtration fraction aortic stenosis?

Hello everyone,

I don't understand the importance of this.

Wiki: In renal physiology, the filtration fraction is the ratio of the glomerular filtration rate (GFR) to the renal plasma flow (RPF).

 

Filtration Fraction , FF = GFR/RPF

 

The filtration fraction, therefore, represents the proportion of the fluid reaching the kidneys which passes into the renal tubules. It is normally about 20%.

 

The GFR on its own is the most common and important measure of renal function. However, in a condition such as renal artery stenosis, the blood flow to the kidneys is reduced. The filtration must therefore be increased in order to perform the normal tasks of the kidney in balancing fluid and electrolytes in the body. This would be reflected by a high filtration fraction, showing that the kidneys have to do more work with the fluid they are receiving. Diuretics such as loops and thiazides decrease the filtration fraction.

 

Catecholamines (Norepinephrine and Epinephrine) increase the filtration fraction by vasoconstriction of afferent and efferent arterioles, which is possibly activated by alpha 1 adrenergic receptors.

Now I don't exactly understand what is the importance of filtration fraction. If normal GFR is 125 ml, in aortic stenosis filtration fraction increases, what does this mean? Does it mean that kidney has to now work harder to maintain 125 ml GFR? But in aortic stenosis both GFR and renal flow decreases. I don't understand the significance of filtration fraction?

Also why does catecholamines constrict the efferent arteriole more than the efferent, I can understand how this would increase pressure up stream and increase GFR, is that the reason? Thanks