Why does urine foam?

I guess the title says it all, really. I mean, I know it doesn’t foam all the time, but sometimes, you sure can put a head on the toilet water, ya know? So what’s in urine that causes that?

Google, “urine foam”, hit #1.


Does your pee-pee–scare you, Max?


I love the Internet. :smiley:

When I was struggling with the problem of foam in traffic paint I read a great big book on surfactants and I remember well the dogmatic statement: “Absolutely pure liquids do not foam.”

That has nothing to do with the question, but it’s nice to know something dogmatic like that. And it’s all I remember from the book.

Anyway, foam is usually (maybe always) caused by surfactants, which are substances that bind to and stabilize the air/water interface. (And, by definition, other kinds of interfaces.) Without a surfactant the interface isn’t stable and the little bubbles, which happen naturally with agitation, collapse. Urine, like beer and traffic paint, must contain some kind of surfactant that makes it foam.

It doesn’t take much for a substance to be amphiphilic enough to cause foam, and protein or bilirubin would definitely do it, but your urine isn’t supposed to have either one in it in any significant amount.

My guess as a paint veteran/first-year medical student is that it’s the urea that does it. Urea doesn’t seem like much of a surfactant, but there’s more of it in urine than anything else that seems likely, and like I said, it doesn’t take much to cause foam, especially with the kind of agitation you get when you pee in a toilet. And the structure of urea makes me think it could be at least a little amphiphilic.

I know for sure, though, that most proteins make great surfactants. I’ve never seen urine with protein in it, but it’s easy to believe that it would be foamy.

Right now it’s midnight, but if anyone doesn’t like my answer I know way too many kidney specialists I could ask tomorrow.

Actually, Bob, I commend your for a thoughtful and informative answer. Good show! Still, though, I would like to hear from urologists/kidney specialists (like your friends) about the more- and most-likely reasons for occasional urine foaminess, just so we can get a complete picture.

No, my pee-pee doesn’t scare me, DDG, ya goof. It’s just sometimes, you come up with the darndest questions, ya know? And I figured this would be a good GQ, since Cecil once covered “Why is shit brown?”.

Shit, get me good and mad and I foam too. Maybe you’ve been nagging your urine a little too much lately.

And it may not be what’s in the urine, but what’s already in the toilet water. So maybe you should fear the toilet water.

Or maybe you should stop peeing in your beer. :smiley:

This 2-post thread http://boards.straightdope.com/sdmb/showthread.php?threadid=67365 gives this link: http://biowww.clemson.edu/biolab/bubbles.html