Narrowing of renal artery. Coloured X-ray of the arteries (red, upper left) of a patient's kidney, with stenosis (narrowing) of the renal artery. This is the right-hand kidney, seen at left of the spine (down centre) and below the ribs (top left). The red line running up from bottom centre is a catheter that has been inserted up through the aorta to the level of the kidneys. Radio-opaque material is being released from the end of the catheter (loop at upper right), and is passing into the kidney, allowing the arteries to be seen on the X-ray. The arterial narrowing is next to the loop, in front of the spine. Renal stenosis causes high blood pressure as the body tries to keep the blood pressure in the kidney high enough for the kidney to filter the blood.