It doesn’t decide. It pumps water through the body pretty evenly (although blood vessels can expand and constrict to reduce blood flow to an area, which will also slightly reduce water flow). Each individual organ uses the water it needs, but the less water is available in total, the harder it is for any given organ to use it (this didn’t need to evolve or be controlled by anything, this is just a fact of how something called osmosis works). This means you can still lose water even when you have very little water left, but the proportion of water you lose to each thing is less. You’ll still sweat, but less sweat will be made. You’ll still urinate, but less urine will be made.
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