Content Reviewers:Rishi Desai, MD, MPH
Contributors:Tanner Marshall, MS
Acute kidney injury, or AKI, is when the kidney isn’t functioning at 100% and that decrease in function develops relatively quickly, typically over a few days. Actually, AKI used to be known as acute renal failure, or ARF, but AKI is a broader term that also includes subtle decreases in kidney function.
AKI can essentially be split into three types, prerenal AKI meaning the cause of kidney injury’s coming before the kidneys, postrenal AKI—meaning after the kidneys, or intrarenal AKI—meaning within the kidneys.
Now the kidney’s job is to regulate what’s in the blood, so they might remove waste, or make sure electrolyte levels are steady, or regulate the overall amount of water, and even make hormones - the kidneys do a lot of stuff!
Blood gets into the kidney through the renal artery, into tiny clumps of arterioles called glomeruli where it’s initially filtered, with the filtrate, the stuff filtered out, moving into the renal tubule. Sometimes fluid or electrolytes can move back from the filtrate into the blood - called reabsorption, and sometimes more fluid or electrolytes can move from the blood to the fitrate - called secretion.
Along with fluid and electrolytes, though, waste-containing compounds are also filtered, like urea and creatinine, although some urea is actually reabsorbed back into the blood, whereas only a little bit of creatinine is reabsorbed. In fact, in the blood, the normal ratio of blood urea nitrogen, or BUN, to creatinine is between 5 and 20 to 1—meaning the blood carries 5 to 20 molecules of urea for every one molecule of creatinine, and this is a pretty good diagnostic for looking at kidney function!
Ultimately the filtrate is turned into urine and is excreted from the kidney through the ureter, into the bladder, and peed away. Meanwhile, the filtered blood drains into the renal vein.
Alright, so with postrenal AKI, there’s some obstruction to the outflow from the kidneys.
Reduced flow can be a result of something compressing the ureter like intra-abdominal tumors, or compressing the urethra further down, like from benign prostatic hyperplasia—a noncancerous growth of the prostate gland, both of which sort of pinch the ureter or urethra shut.
Also though it could be some blockage inside, like kidney stones getting stuck in the ureter or urethra, which actually plug it up. Now, if only one ureter’s obstructed, called unilateral obstruction, and the other kidney’s working fine, then renal function’s usually preserved. If, say, both ureters are obstructed, called bilateral obstruction, or the urethra gets blocked, then we’ve got a recipe for postrenal AKI.
Whatever the obstruction is, it ultimately causes this buildup of urine and pressure that backs up into the kidney; all the way to the millions of tiny renal tubules.