Renal blood flow accounts for up to a quarter of the total cardiac output. 90% of this is directed to the cortex. Sympathetic stimulation causes a reduction in renal blood flow. There is a increase in blood flow with polycythaemia and a reduction with anaemia, thus maintaining a constant plasma flow, this is in contrast to the situation in most tissues. The blood flow to the kidneys remains constant via autoregulation over the range of blood pressure of 90 - 200 mm Hg.
The renal arteries bifurcate many times to give a series of end arteries. As a consequence the kidney is vulnerable to ischaemia if any branch of the renal artery is occluded.
Hypotension may damage the kidneys. Perfusion of the renal medulla is scanty in the healthy person and so minor interference quickly leads to medullary necrosis.
In contrast, hypertensive renal damage is usually more severe in the better-perfused cortex.
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