system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does drinking water when you need to pee, make that urge much stronger even though that water is far from reaching your bladder?
gpt
Tl;dr: if your bladder is already full of urine, your body was pretty happy with its water level. The body is very picky about its water level. If you add more water, it will quickly make more urine to allow for absorption of the water you just drank. An overfull bladder is potentially harmful, so normal urge to pee can be upgraded to "urgency" to protect the bladder when the body knows you're about to have an increase in urine production and your bladder is already full.

Long version:

Top answer covers where this reflex comes from from an evolutionary standpoint, but not the actual mechanism of *how* this happens.

The brain has a very specific group of nerves called the thirst center that use a few different means to tell how much water is in the body. The thirst center is *extremely* sensitive.

If you already have a full bladder, your body was likely already happy with the amount of water available in the body. The exceptions here would be if you are on certain types of medication or have some specific medical conditions.

If you add more water to the equation, the thirst center flips from "we are happy with this amount of water" to "we have too much water." Flipping this switch sends a few signals to certain blood vessles and the kidneys. The signal is an order for the kidneys to produce more urine by working harder and faster. We are wired this way because having too much water is extremely dangerous.

We don't know this for sure, but based on what we see in people who experience urinary incontinence, the sensation we typically experience telling us we need to pee is produced by certain nerves in the bladder as it fills. The more intense version you describe is called urgency. Urgency appears to involve sensors not just in the bladder, but also the kidneys. The kidneys that just got the order to work double time.

The kidneys know they are about to start making more urine. If the bladder is already full, adding more urine can damage the bladder itself or the muscles that allow us to retain urine and pee voluntarily. Thus the stronger "urgency" signal helps protect the bladder by encouraging immediate emptying before more urine is produced.

There is also speculation that the thirst center receives signals during the process of swallowing water. The theory is that the muscles that move stuff through your throat can tell the brain whether that stuff is liquid or solids. This part of the process appears to be a "shut off" signal to stop us from drinking too much water when thirsty. It may also contribute to "urgency" directly or by kick-starting the process by fast-tracking the message to the "thirst center" to begin the rest of the process.

Edit: clearer distinction between the Tl;dr and full text.