glomerular filtration rate

[biology_06er]

Can someone pls explain the following form my test book...its talking baout the myogenic pathway

the myogenic mechanism occurs when stretching triggers contraction of smooth muscle cells in the wall of the afferent arterioles. As blood pressure (BP) rises GFR also rises bc renal blood flow increases.

I don't get it....how does the renal blood flow increase if the pressure increases...shouldn't it decrease...if the arterioles are constricting-then higher pressure lower volume of flow??...

umm and also could someone please explain what happens if you don't drink enough water in terms or filteration, secretion and reabsorbtion--what i think is that less water in our body the blood volume decreases hence we have more solute than water so our urine is a bit concentrated (filled with na+ glucose etc)---i dnt think this is right but anyhelp for both questions is much appreciated

Thanks in advance
biology_06er