Why does renal or aortic stenosis cause hypotension and therefore a release of renin from the kidneys?

Question

I understand hypotension (low blood pressure) causes renin secretion in order to raise blood pressure, however WHY does stenosis cause HYPOtension and not HYPERtension? I thought vasoconstriction increases blood pressure. Thank you.

Answer 1

In the context of renal artery stenosis, I think your prof/textbook meant local, not systemic hypotension.

When the systemic blood pressure is normal, part of that pressure is blocked by the stenosis, resulting in low blood pressure in the renal artery between the stenosis and the kidney.  http://www.lakeridgehealth.on.ca/patient...

As far as that kidney is concerned, the whole body is hypotensive, and as a result, it releases renin, in an attempt to raise blood pressure (thus to improve renal perfusion).  By the time enough renin is released that the kidney is receiving enough pressure, the blood pressure in the rest of body is sky-high.  This is why (systemic) hypertension is a key feature of renal artery stenosis: http://en.wikipedia.org/wiki/Renal_arter...

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In aortic stenosis, the pressure differential is at the aortic valve.  So while the pressure inside the heart (left ventricle, to be exact) may be very high, because of the diseased aortic valve, the blood pressure inside the aorta (and the rest of the body, really) remains low.

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Finally, stenosis means narrowing at one point of an artery.  Kind of like a kink in the water hose.  Vasoconstriction means contraction of the arterial wall along a section (if not the whole thing).  The two concepts are different.

Hope this helps.