If urine is produced by the kidneys filtering water from blood – not directly connected to the digestive system – why do we feel the need to pee immediately after chugging water?

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Another bonus question about urine, while I’m pondering the subject: when we are very hydrated, urine looks extremely clear, almost like water. If you are essentially just taking water in and peeing water out when you are very hydrated, is it even worth drinking water at that point? Like, are there diminishing returns to the amount of hydration we can get? (I know there’s those extreme cases of dying from drinking too much water, but that’s not what I’m talking about here)

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You’re not just peeing out water, it’s also full of salts, ions, and minerals that your body wants to regulate the level of. Overhydration can cause issues maintaining the concentration of things like ions and minerals, but you have to drink a lot of water to get to that point.

Water can be quite rapidly absorbed into your bloodstream after drinking. As low as 5 minutes on an empty stomach. So you feel the need to pee because after chugging a lot of water, a lot of that water has been quickly absorb into your blood stream and filtered out by your kidneys into your bladder.

And yes, there are diminishing returns to drinking water for hydration, but you are not likely to ever reach that point without deliberately trying. While we do use water chemically, we also use it as a median to carry those wastes out. It’s like oil in a car. The car doesn’t use the oil directly and any oil you put in you’re going to get back out (for the most part) but that doesn’t mean your car isn’t putting it to use and that oil is wasted.

Take the tube inside a roll of paper towels, hold it horizontally, and pack it with ping pong balls. Now, shove another ping pong ball in one end, and a ball will pop out the other end.

That’s more or less it. Your body can hold so much fluid and you put more in, then some has to come out. If you put enough in in, the bladder will fill up quickly, and if it’s fast enough it can fill faster than stuff is filtered out (so it’s clear).

You do need to drink enough water to keep to flow going (so you can filter stuff out and pee it out). Drink too little, and you you dehydrate (volume of blood goes down, concentration of stuff in the blood goes up, hard on heart and kidneys). Drink too much, and you mess up the concentration of salts and minerals in your blood and your heart can fail.