Why is pee yellow?

This is a segment that I like to call Childhood Question.  It's where I take I question that I had as a kid, or that I heard a kid ask, or that I think a kid might ask...or just any question I feel like asking. 

Today's question: Why is pee yellow?

 Luckily, I took PDBio 305, or Human Physiology, and we discussed this very question.  It turns out that the answer (or part of the answer) has to do with why kids who have jaundice look yellow, or why bruises turn yellow.  The reason is bilirubin (pictured on the right).  Many people have heard of bilirubin , especially parents with children who had jaundice.  Bilirubin is the result of the break down of old red blood cells (pictured on the left).  When the red blood cells break down, the iron (Fe), in the middle of the hemoglobin, which makes the blood red, leaves and the whole thing "unfolds". To get rid of it, all of the bilirubin is delivered to the liver, which is then excreted in the bile, where it is then excreted in poop as stercobilin (which gives it its brown color), or reabsorbed back into the blood and then to the kidneys, and finally excreted in the urine as uroblin.*  Amazing!

So the reason kids can have jaundice is because they have liver problems, or they're destroying too many red blood cells, or there is something blocking the bile duct. It just makes all the bilirubin backed up in their body and makes their eyes and skin turn yellow.
Bruises make your skin turn yellow because the blunt force broke millions of red blood cells and the bilirubin pools there until you clear it all out.

Why are people worried about kids with jaundice and why isn't it a problem for adults?  That could be a question for another day.

*All of the things I know about the subject I owe to Dr. Reuben Rhees and his lecture note packet.  I have referred to that packet quite a few times since I finished that class.