Renal nutcracker syndrome is when the left renal vein, which drains blood from the left kidney, gets squeezed between two large arteries: the superior mesenteric artery and abdominal aorta. It’s kind of like the renal vein is the nut and the two arteries are the nutcracker.
Now, blood heading toward the lower body exits the left ventricle, swoops around the aortic arch, and then flows downward through the descending aorta, a large, muscular blood vessel about as thick as a thumb. The descending aorta runs along the back of the abdominal wall next to the spine, where it’s called the abdominal aorta, until it splits into the common iliac arteries. The abdominal aorta gives rise to many smaller arteries, including three unpaired arteries: the celiac artery, the superior mesenteric artery, and the inferior mesenteric artery. All three of these arteries branch off the anterior wall and supply blood to the digestive tract.
The renal vein carries blood returning from the kidney to the heart, passing between the aorta and the superior mesenteric artery. The angle formed between these two vessels is called the aortomesenteric angle. It’s usually around 45 degrees, with the renal vein cushioned by a bit of fat in the mesentery. If that aortomesenteric angle is reduced, the arteries begin to pinch the left renal vein like a nutcracker, preventing blood from returning back to the heart. This leads to a backup of blood in the left kidney, causing renal hypertension. Over time, the high pressures can cause small breaks in the renal blood vessels, and a bit of blood can even get into the urine. Also, because the left testicular vein drains into the left renal vein, blood can end up pooling in the left testicle as well.