I was thirsty; I drank some water. Then I had to pee. How did that water hydrate me when I peed it right out

First of all the amount you peed is lesser than the amount of water ingested.

Secondly what you are talking about is a cycle, and not a solitary segregated process, but rather a small but significant process in the jungle of "homeostasis."

Some amount of water which depends upon the bodies requirement was absorbed and the rest is waste which is excreted.

So THAT SMALL quantity of WATER ABSORBED HYDRATES YOU.

It can only be be possible that what you drink comes out immidiately as pee when the whole Gastrointestinal tract is absent.

The fluid which is peed out has its origin as water taken some hours before being filtered by the kidney.

First, you assume that you peed the same amount that you drank. If you peed less than you drank, then it's obvious.

So let's say that you 'filled the glass' as it were.

Your body needs the water to remove or 'wash away' the urea, and other things, like metals, in your body. So, basically, the urine that left was replaced by pure water and the process can continue. When the water doesn't get replaced, bad things happen...

bad things, man...

The water washed waste products out of your body. The pee was not the same clean water you drank. A living body is not so much an object as it is a process. It is like a flame. Matter comes and goes, but the process continues. I have heard that in a year, 99% of the atoms in your body are gone, and replaced by new atoms that you ate, drank or breathed in. You pee and poop all the time. You shed hair and skin and snot. Dead cells are always being washed out of your body and replaced with new cells. Your body is not a thing, it is a vortex of change; a temporary resting place for a few kilograms of mass with nothing better to do.

You didnt pee THAT water out, you peed water that was in your system, the new water cooled you off as it is going through your system.. One thing you can try next time you pee is to drink water at the same time, to see if it continues to come out as you drink.. TRY IT>>

Because 'pee' is just the part of water that your body dosent need.

I did not realize that there was such a big gap in the human evolution scale.

because it went from your stomack then in to your blood and to all your cells the pee just sits there doing nothing in your bladder