Re: why do the kidneys shut down during a hypertensive crisis?

Dear Dan:

A couple of things happen to cause acute renal failure during a hypertensive crisis.  
The most acute cause which can develop over the course of hours is congestive heart 
failure.  When blood pressure rises to a certain level, the heart begins to become 
unable to overcome this afterload as we call it.  The systolic ejection time (or the 
time during which blood is being propelled foreword by the heart) becomes shorter and 
shorter and thus cardiac output falls.  As this occurs, the kidneys see less and less 
blood flow and therefore less and less oxygen supply.  This will over time cause key 
filtering elements of the kidneys to cease functioning or even necrose (die).  This 
process is seen in very severe hypertension or in people with concomitant heart disease.  

Hypertension also causes activation of the blood clotting mechanism and can cause 
damage to blood cells themselves due to the pressure head driving them through small 
arterioles and capillaries.  This will cause thrombosis and embolism, where small blood 
clots form and travel to a point where they can pass no further.  All tissue supplied 
downstream from this point will die unless supplied by an alternate source.  When this 
occurs in the brain, we call it a stroke.  If it occurs in the kidney, the result is 
more subtle, but in aggregate many of these small infarctions may cause significant 
reduction of function.

Hypertension is harmful in itself to the kidneys, especially to the glomerular apparatus.  
It will cause hypertrophy and scarring, resulting in impaired function.  This process 
takes longer to develop.

Renal failure can cause hypertension as well.  This can complicate the management of a 
hypertensive patient with impaired renal function as both processes can aggravate the 
other.