On the Nephrology ward, two people came in. The first one is 55 year old Matilda, who came in with oliguria, fever and a rash on her arm. Matilda also has a history of taking NSAIDs for her knee-pain. The second one is 34 year old George, which came in with oliguria, hypotension, tachycardia and cold extremities. George says that he’s also been having diarrhea for a few days now. BUN and creatinine levels were high for both of them.
Both Matilda and George have renal failure. Now, renal failure is when the kidneys aren’t functioning properly. Before talking specifics, let’s remember some basic renal physiology. 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, such as erythropoietin, which stimulates red blood cell production. Okay, so blood gets into the kidney through the renal artery, into tiny clumps of arterioles called glomeruli where filtration happens. After filtration, the stuff that’s filtered out, called the filtrate, moves into the renal tubules, where reabsorption and secretion of fluid and electrolytes happens. 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 way to assess kidney function! So, when we want to check renal function, we look at BUN and creatinine levels and if there’s something wrong, then levels of both BUN and creatinine will be high.
Now, back to renal failure. There are two types of renal failure: acute and chronic. Now, acute renal failure is now called acute kidney injury. This is when the kidney isn’t functioning at 100% and that decrease in function develops relatively quickly, typically over a few days. In this case, the individual typically presents with oliguria, even anuria and levels of BUN and creatinine will be high. Then there’s chronic kidney failure, which is now called chronic kidney disease. This is when the kidney function gradually decreases over a minimum of three months. This is usually caused by hypertension, diabetes mellitus, or congenital renal conditions.