Let me give you a little back story. As many of you know, I’m new to scienceblogs, and one of the first things we get to do is join a message board full of all the bloggers (sciblings) here. Well, suffice it to say, a contentious discussion took place between me and another scibling, which resulted in my being called a total n00b in front of the entire world.
So, what better way to make peace than by having me — an astrophysicist — answer a question about physiology?! Let’s take a look at what I’ve got, since I’m a frequent solicitor of questions:
Why is my pee yellow in the morning, but clear in the evening?
The human body is a favorite topic of mine, and has been ever since I answered the question of whether a human has more cells or a galaxy has more stars? Well, let’s take a look at what’s in your pee:
Sure, a whole lot of ions and chemicals, salts, proteins, enzymes, hormones, and other metabolites are found in urine. But what makes it yellow? The distinctive yellow color comes from urobilin, which is a chemical formed, eventually, from the breakdown of hemoglobin. You can even (kind of) see how it happens; start with a heme group:
and then break it down and remove the iron! Take a look at how alike these molecules are:
So when your urine contains a lot of urobilins, it’s quite yellow, and when it’s low on urobilins, it’s closer to clear. So that’s the easy answer, but we can dig a little deeper. First off, what are we doing destroying all of this hemoglobin? Don’t you need it to live?
Of course you do, but your circulatory system is constantly ordering replacement parts. Your body is constantly making new red blood cells, replacing the old ones every 4 months or so.
But there are about 30 trillion red blood cells in the human body, meaning you are both destroying (and making) new red blood cells at a rate of around 2.7 million cells per second. What’s more, is that every red blood cell has about 270 million hemoglobin molecules in it, with each one capable of carrying four oxygen molecules (and having four heme groups).
All told, assuming you sleep for 8 hours at night, your body has destroyed 78 billion red blood cells, now has to get rid of 8.4 x 10^19 heme molecules, and has ingested no water over that time. Now, most of these molecules wind up in your stool instead of your urine, but they’ve been able to determine that typical human urine contains around 8 mg of urobilin, which equates to about 10% of the broken-down heme molecules winding up in there.
But over the day, you’re drinking water, and so you’re diluting the amount of urobilin in your urine by a significant amount, allowing you to pee clear by the afternoon under normal circumstances. Someone like me — who drinks about 4 liters of water a day — gets their evening urine down to about one-sixth of the urobilin density found in their morning pee. Put simply, the determining factor for healthy people is hydration.
And so my friendly, horizon-expanding challenge back to you, anonymous scibling, is for you to solicit and answer a physical science question of your choosing! If you’d like a little help, I can point you to the most recent Carnival of Space (#97) for inspiration!
You’ve got a whole Universe out there to choose from…