Why does the body create urine when it’s dehydrated? Shouldn’t it use the water to hydrate instead of creating a waste product?

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Why does the body create urine when it’s dehydrated? Shouldn’t it use the water to hydrate instead of creating a waste product?

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The body still has to get the waste products out of the system. The stuff filtered out by the kidneys needs to get out of the body somehow and it can’t just sit in the bladder forever.

When you get dehydrated _enough_ urination does stop, but until that point expelling toxic (to the body) waste is a very high priority.

I don’t think you want to try to push urea (waste) crystals through your body’s disposal system…

The body needs to produce urine as a way to discard waste products produced by the body. These wastes are not voluntary, they are side effects of the processes which the body does in order to remain alive. Leaving them inside the body will gradually kill it so removing them is necessary even when water isn’t easily obtained.

When dehydrated the body will make the urine more concentrated, putting more waste into less water which is visible in that the urine will be darker.

This is exactly what the body does. But if you are just partially dehydrated the body will still make urine in order to get rid of all the toxins you accumulate. The body can handle being partially dehydrated for a while and it is easily solved when you find water. However the toxins in the blood will damage the body more and create more permanent damage. So the best way to handle partial dehydration is to still make urine but make sure it is concentrated as much as it can to at least conserve some water.

Let me explain a bit how the kidney works. Blood flows into the kidney. It passes through a porous structure called the glomerulus. Some blood falls out of the artery into the glomerulus and some continues and leaves the kidney untouched. This is the first control of how much urine you create. When your body is dehydrated it reduces the amount of blood flowing to the glomerulus but there will always be some blood. After blood falls into the glomerulus a lot of stuff in it get pushed back into your arteries. There’s some stuff that doesn’t, that is, proto-urine. Your body can control how much to push back into your arteries but once it enters the kidney it uses salt to facilitate the transfer. This transfer of water with salt is semi automatic at some parts and other facilitated by energy. Anything left get eventually excreted as urine.

This process is designed to be always running. If you turn it off, it break the kidney. When you are super dehydrated you kidneys keep a small amount of liquid running to make sure everything will start to work again when you drink water

You’ve gotten good answers on the technical side. I’ll just add: we get a lot of questions here about “why doesn’t the body do this thing it would do if perfectly calibrated for this situation?” And the answer is often that the body is calibrated for the 99.99% scenarios, not the 0.01% scenarios. People get very thirsty reasonably often, but then they get to water and they’re fine. In those scenarios it would be quite bad if the body stopped removing waste for the hour before the person got to water.

The waste products being flushed out in the urine are toxic. They have to be removed to keep the person alive. If they are not removed, the person dies.