As I understand it, a kidney is like a passive filter. Passive, as in, the kidney just sits there and the blood flows through like air through your HVAC filter. It is all through osmosis, a naturally occurring passive process, that allows the kidney to take out the bad stuff and keep the good stuff. Specifically, I understand the kidney takes out almost everything at first and slowly puts back the good stuff. So, how can a kidney fail or shut down???
I should have added: I did Google this, but I am not finding the answer about how a kidney fails. I get signs and causes (like illness), but not specifically how.
Filters can malfunction just as the HVAC filter, from a variety of causes. Inflammation is called nephritis.
Well, to begin with, passive filters can fail also. They can clog, or a bypass can open up around them.
The kidney isn’t made of filter paper, it’s made of living cells. If the cells fail, the kidney can’t contiue to function.
Well, there’s your problem. Kidneys are not just passive filters. They actively cleanse the blood plasma, concentrating wastes, re-absorbing various ions and particles, etc.
And as noted, even a passive filter, using only an osmotic gradient, will fail when specialized kidney cells making up the filter unit die off.
Avoid NSAID’s
If you have hypertension (high blood pressure), control it
(and, of course, we now blame simply everything on smoking - the new “Red Menace”)
As with any organ, if the individual cells die, the organ fails. What’s so different?
(and, no, you can’t simply replace a kidney with a sack of coffee filters - and expect the patient to live)
Kidneys are not passive filters. Some of the filtering is passive, other is active, meaning it requires some energy or exchange of ions.
There are various different ways a kidney can fail, as it has multiple components and different parts that can go wrong. In mammals, the kidney has two main parts, the cortex where the glomeruli (the initial filtration part) are, and the medulla which contains the various tubules and ducts that work to concentrate urine, making sure all impurities are dumped and everything useful is taken back.
Initially, when blood passes through the glomeruli, the plasma (fluid portion) is filtered out without much thought, so long as it is smaller than a specific space. This space is formed by a combination of the endothelial cells (capillary cells), the basement membrane (a sandwich of proteoglycans) and the glomerular cells in the other side (podocytes). If this first part is damaged (by various ways), then the filtration becomes even less specific, and things that should’t be filtrated out (like protein) are filtrated out.
Then the proximal and distal tubules are in charge of taking back some of the important solutes dumped, and also dumping additional solutes that may not have been filtered in enough amounts during the passing through the glomeruli. For example, remember a lot of solutes (ions) are dumped initially, including sodium. Sodium is very important, so a lot of that is taken back to the blood stream (usually in a passive manner). Another ion, potassium, may not filter out as much as needed, and so it is pumped out to the urine side.
Finally by the time the urine part gets to the distal and collecting ducts, the idea is to make the waste as concentrated as possible, so water may be removed from it. Obviously if things are damaged in the tubular part, the solutes removed/retaken are not in the correct ratios, and the urine is not as concentrated as it should be for the state of the animal (more hydrated animals need less concentrated urine, dehydrated animals would try to conserve as much water as possible).
IANAD, I am very interested in how to keep a Kidney healthy and in that, I follow my Doc’s instructions.
I donated a kidney to my brother about 7 years ago and plan on keeping my remaining Kidney in good working order as long as possible.
I entered into an agreement with the transplant center that I would get on medication to lower my B/P and the agreement was because I was on the upper threshold of “Normal”.
This was the first and only prescription I have.
Persistent Hypertension is a leading cause of kidney failure and is believed to be the cause of my brothers kidney disease.
Years before the transplant I learned that some medications are injurious to my kidneys. I worked in heavy maintenance and the area I worked in the mine was called the “Bull Gang”. We repaired huge machinery and the work was hard and physical. Some jobs were so grueling that before starting I would take a couple anti-inflammatory over the counter just so I could sleep at night after some of the work and of course usually had to continue for several days after.
These jobs would require running 1 1/4" drive impact wrenches over head and at arms length inside small compartments reaching into adjoining compartments (propel transmition c/o on large power shovels).
While the anti-inflammatory med would relieve the pains from the work I was wondering why I had pain in my back and this pain would be relieved when I stopped taking the anti—.
After the donation I was cautioned by the transplant team to not take anti-inflammatory meds if at all possible because they are hard on the kidneys! Hey I realized then that the back pain I had could certainly have been my kidneys protesting that med.
So here I sit, week 2 post op for a total knee replacement and I take no anti-'s!
And my wife worked for years as an RN in a Dialysis unit and when I asked her why so many people (like my brother) on dialysis, she said medications are hard on kidneys.
Just my $0.02 
Or you can be like me, where your kidneys have been taken over by mutant cysts, suffocating and rendering the kindeys useless. Image search polycystic kidney disease - but not before lunch.
Or it could stay out all night instead of studying for the urine test.
One of my Grandsons had a polycystic kidney that was seen on ultrasound before birth. He was born 10 weeks early and the Kidney had to be removed as soon as he was strong enough. He was so sick and irritable until the monstrous kidney was taken.
He is a very healthy and wonderful grandson today at 20 year old, but is disappointed because the Marines wont take him.
I have a relative with renal-artery stenosis, where blood flow is limited to an otherwise healthy kidney/organ. If blood can’t get to the kidney in sufficient volume, by blockage of the flow of blood to kidney(s), it cannot perform its full function. Kidney appeared fine on both Ultrasound and CT, fwiw. Just not able to receive enough of the product for filtration as designed, in a a manner of speaking. It took an invasive-style angiogram (injection of x-ray contrast next to said artery through catheter) to show how limited the flow of blood was to the kidney, fwiw.
Kidney receives enough oxygenated-blood to be ‘alive’ but not able to get enough ‘product’ to function efficiently, in a nutshell. Therefore kidney was not able to funtion much at all other than to stay ‘alive’. Not a rare syndrome, IME.