Why does hypotension decrease perfusion to the kidney?

[nm825]

I'm a little confused here. If vessels are dilating, wouldn't this mean the kidney is able to get more flow, i.e. blood heading to the kidney has less viscosity?

Is it because if you are vasodilating, your TPR is decreasing all over, and because you have a fixed amount of blood in a closed system, the blood that would be flowing to the kidney now has room to go elsewhere?

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[Neurality]

I believe you're on the right track here. A reduction in TPR reduces MAP which means less blood per/time is reaching filtration. This is just in general though as local endothelial factors or damage in arteries supplying the kidney may also play a role. Also, it depends on what the source of the hypotension is (simply reduced TPR/MAP vs reduced EDV/EF) Just my 2 cents. 🙂

 

[SurfingDoctor]

There are several concepts, but the main concepts are 1) vasodilation (decreased resisteance) maintains flow (but not increase per se) when blood pressure drops, but only over a certain range. For the renal system (and others) this is termed autoregulation. But just like hooking up a hose to a faucet, if you turn the faucet down, a wider hose doesn't make more water come out. 2) hypotension is a pathologically low pressure and thus physiological processes, like autoregulation, no longer work. Resistance or vasodilation become maximized and flow becomes pressure dependent. 3) vasodilation in general causes both arterial and venous dilation, for the flow issues mentioned. However significant venous dilation causes venous pooling and lack of venous return to the heart. Lack of venous return leads to reduced cardiac output, and thus, the cycle continues.

By the way, these concepts are not unique to the kidneys, but most central organs, particularly the brain and heart.

 

[Hangry]

Depends what is causing your hypotension. If you're losing volume your vessels are clamping down, increasing (or attempting to increase) SVR. If you are in septic shock they are dilating and blood pressure is low because fixed volume in a larger space.

A bigger picture thing for you to think about is that the body will screw over the kidneys to spare more important organs like the heart and brain.

 

[Psai]

Vessels are dilating in response to the decrease in blood pressure but don't overcompensate. If you have a river and you have decreased rainfall that year, you're not going to get more water by widening the river

 

[OnePunchBiopsy]

It's like when you turn on a kitchen sink in a house with the water turned off: no water pressure.

(Hint: the sink is your kidney)