If you manage to get something spicy up there, it would definitely burn.
But, as to why it doesn’t generally… Urine is water and waste products that your kidneys filter out of your blood. The irritating chemicals in spicy foods never made it into your blood, so never make it into your urine. Instead, they stay in your gastro-intestinal tract until they’re pooped out.
Liquids don’t go right from your mouth to your bladder – the kidneys filter out water and certain waste products from your blood. And the chemical that makes food spicy, capsaicin, is actually not soluble in water – it’s hydrophobic, and dissolves in oils instead. Your kidneys try to handle only the water-soluble waste products, such as urea, which is our main way of getting rid of excess nitrogen compounds in the body.
So capsaicin doesn’t leave your body through pee, because only water-soluble waste products get sent that way. Some of it is broken down and digested by your body, and turned into other chemicals. But some of it (especially if you’re eating really spicy foods, which also irritate and speed up the digestive tract) will pass through undigested, which can burn on the way out.
Your urethra is connected to your bladder which is filled by the kidneys. It isn’t directly connected to the digestive system. Poop is the undigested bits and comes directly from what you eat.
The reason you have to pee more if you drink a lot of water is your kidneys will try to balance out the salinity levels in your blood which becomes diluted after your digestive system absorbs the water.
You poop out what’s left of what you eat. Your butthole and your mouth are part of one big tube with a lot of twists and turns in it where food has nutrients sucked out of it and what’s left is turned into poop.
Your pee doesn’t come from this system. It comes from your blood. Your kidneys filter your blood, and the water your kidneys pull out of your blood gets peed out of you.
So your butthole burns because the spicy chemicals that came in have to come out. They don’t get in your blood so when you pee later, you don’t have spicy pee.
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