Physics of blood flow, glomerular filtration rate (+ TBR)

[Skarl]

Why does vasoconstriction of arterioles increase blood pressure yet vasoconstriction of the afferent arteriole decreases GFR/pressure. Furthermore, there is a TBR question which states:

"A reduction in the diameter of a precapillary vessel would:" and the answer is "B. result in a reduction of capillary hydrostatic pressure"

Constriction (reduction of radius) increases blood pressure, decreases GFR pressure, and decreases capillary hydrostatic pressure.

How can I reconcile these differences?

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[smukke]

Think of the arterioles running through the glomerulus as 3 segments of a hose.
The afferent part (impermeable hose), the part in the glomerulus (permeable part, like a leaky hose with holes), and the efferent (impermeable hose).

If you pinch off part of the afferent, it will cause a local vasoconstriction and increase the pressure there (and to the parts before it) but beyond that point there will be less water flow and therefore less pressure (decreased cap hydrostatic pressure) to push water through the leaks (decreased GFR pressure).

 

[Skarl]

Think of the arterioles running through the glomerulus as 3 segments of a hose.
The afferent part (impermeable hose), the part in the glomerulus (permeable part, like a leaky hose with holes), and the efferent (impermeable hose).

If you pinch off part of the afferent, it will cause a local vasoconstriction and increase the pressure there (and to the parts before it) but beyond that point there will be less water flow and therefore less pressure (decreased cap hydrostatic pressure) to push water through the leaks (decreased GFR pressure).

Clearest/most straightforward explanation I've gotten yet. Thanks a bunch!