Why Pee Is Yellow, Poo Is Brown and Bird Crap Is White

I feel like I struck question gold on this (no urine pun intended).  What's most awesome about the answer to this question is that it involves a whole slew of major organ systems in  your body.  On to the details!

When old blood cells retire the hemoglobin in them gets broken down into bilirubin.  This is no small thing.  You have 30 trillion red blood cells in your body and each of those cells has 270 million hemoglobin molecules in it.  Just to be able to have a stable red blood cell count and compensate for loss you have to make 2.7 million red blood cells a second!!   Bilirubin is also the yellow substance that causes bruises to appear yellow, as well as the yellow pallor of jaundice. 

I thought this was interesting.  It's all about moving oxygen and electrons!

Notice how it basically just loses the iron and gets splayed out.

This worn out hemoglobin gets disposed of in two places. One is our liver.  The liver is a thrifty organ and filters bilirubin out of the blood, concentrating it in the gall bladder as one of the main constituents in bile ('Bile' and the first half of 'bilirubin' come from the same Latin root meaning 'anger', 'wrath', or 'gall'.  'Rubin' means red, so literally it means 'red wrath'.).  Bile is squirted into our G.I. tract and does a great job emulsifying fats for us to process.  You may know someone that has had a gall stone and suffered from the symptom of intense pain after eating fatty foods.  Their body is trying to pump out bile to break down the fats, but is being painfully blocked.  If a severe enough condition exists, gall bladder removal surgery may be prescribed, in which case the patient will have to eat a low fat diet (or deal in other ways) since high fat content without a means of processing the fats can cause serious indigestion and bloating. Assuming proper function, the bilirubin in bile gets further broken down by bacteria in the gut into a brown substance called urobiligen, which is the main reason our feces is brown.

The liver, however, doesn't catch all of the bilirubin floating through the blood stream and the excess gets mopped up by the kidneys to be disposed of.  Before leaving the body it's further broken down into urobilin, which, you guessed it, is yellow.  So, no, it's not urea that makes our urine yellow.  Urea is colorless.

***Be really careful with this photo.  Those colors are for distinguishing them ONLY.  Not true colors.

Review:

Hemoglobin in blood breaks down into bilirubin which is filtered out by A) liver or B) kidney

A) Our liver concentrates it in the gallbladder as bile, which then pumps it into the intestines and is broken down by bacteria into the brown we know our feces as.

B) Our kidney further breaks bilirubin down into urobilin which is yellow and excreted in our urine.

From blood to Liver to Gallbladder to Intestines to Poo.

Blood to Kidneys to Wee.

Red (of blood) to Green (of bile) to Brown (of poo).


Red (of blood) to Yellow (of wee).

So, the next time you use the ole water closet, remember that your pee is yellow and your poo brown because it's broken down blood!!

Amazing ways evolution has made things look like bird sh*t:

Now, bird crap's a whole different story.  When amino acids, the building blocks of proteins, break down (for a number of reasons, like they were digested but not used by the body or just as the wearing out of protein) they can turn into urea directly or its nastier cousin ammonia which is quickly broken down by the kidneys into the safer urea.  Well, that's how it goes if you're a mammal, anyway.   Birds and reptiles do things a  little more complicated for two reasons.  One is to save water.  Urea is water soluble and therefore needs to be flushed out with large quantities of water in order to be disposed of.  This means, as mammals, we're constantly throwing away a relatively large amount of perfectly good water in order to get rid of urea.  Birds and reptiles solve this problem by expending a little more metabolic energy and turning broken down amino acids into uric acid which is relatively insoluble and can be concentrated as the white precipitate paste we see in bird poo (often mixed with brown since they poo/wee out of the same orifice and usually at the same time).

The other more interesting reason is to solve an egg problem.  When a fetus in utero makes waste, the mommy is nice enough to dispose of the babies waste through the placenta.  If you are a species that is isolated in eggs urine trouble! (Get it?  Get it?...)  They have to isolate their waste so it doesn't poison the entire egg.  One of the most effective ways of doing this is to make it mostly insoluble in water so it can't spread - make it into uric acid.  Genius.

It's interesting to note some exceptions.  Some turtles start off making uric acid while  in the egg to get its largely insoluble advantage, but then later in life switch to urea (like us) since it takes less metabolic energy to make (I assume the same is true for egg laying mammals like platypuses and echidnas which do use urea later in life).  The kangaroo rat, even though it's a mammal, is another exception that has evolved to survive in a harsh desert environment by using the water conserving uric acid method of amino acid disposal.

I hope you never think of using the bathroom the same!

Works Sighted [sic]:

http://www.strangequestions.com/question/583/Why-is-urine-yellow.html#ixzz1C69R7NKk
http://www.thenakedscientists.com/HTML/content/latest-questions/question/2213/
http://www.whycenter.com/why-is-urine-yellow/
http://scienceblogs.com/startswithabang/2009/04/putting_the_yellow_in_your_uri.php
http://xenophilius.wordpress.com/2010/08/04/ask-a-scientist-why-is-urine-yellow-is-it-gold-urobilin/
http://www.expasy.ch/cgi-bin/show_image?L5&down
http://sxxz.blogspot.com/2006/07/why-is-urine-yellow.html
http://www.primermagazine.com/2009/field-manual/why-is-bird-poop-white

Pictures from here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here, here and here.

Teeth - Why We Lose 'Em and How Other Animals Are Different

Why do we lose our teeth as kids?  It's such a common occurrence that unfortunately we don't really stop to think about the evolutionary implications.

The conventional wisdom is that we need bigger teeth as our jaw gets bigger.  Well, why don't our teeth just get bigger then?  They could certainly grow if they wanted to.  Doesn't it seem like an energetically profligate endeavor to build teeth just to toss them away?  Yes, it does.  So, why?

After having those thoughts my mind had an exciting jump: maybe it's an evolutionary tie over from when our ancestors teeth shed all the time!  

It's not quite that simple, but with a little web research found that there's lots to learn about teeth that I had no idea about!  Enjoy the below!

Different Teeth Growth Strategies:

1 Set of Teeth Called Monophyodonts - Rabbits/Lagomorphs and Toothed Whales

2 Sets of Teeth or Diphyodonts or Those with Deciduous Teeth - All Mammals, with a Few Exceptions

Polyphydonts or Those with Multiple Sets of Teeth - Reptiles, Amphibians, Fish, Manatees, Elephants

Alligators (croc above) can  have 80 teeth at once and go through 3000 teeth in a lifetime.

Manatee jaw with 'marching molars' that come in in the back and drop out towards the front.

Not the greatest picture, but this shows a mammoths 'marching molars' slowly emerging from deep within the jaw.

No Teeth! - Baleen Whales, Ant and Termite eating Pangolins and Anteaters

No teeth here!  Just baleen!

No teeth here!  Just tongue!

Toothless pangolin skull

Toothless anteater skull

The Most Neurologically Complex Tooth: the Narwal.  The tooth is multipurpose and is used in feeding, navigation and mating.

Longest Tooth - Straight-Tusked Elephant

Heaviest Tooth - Columbian Mammoth

Gene Found in Chickens that Inhibits Tooth Growth .  Best part of this: it can be turned off and chickens can have teeth.  

Pictures from here, here, here, here, here, here, here, here, here, here, here, here, and here.

Works sighted [sic]:

Multi-Row Teeth Gene Found: http://www.sciencedaily.com/releases/2009/02/090226160751.htm

http://today.msnbc.msn.com/id/29411799/ns/today-today_healthhttp://www.thedailyweird.com/miscellaneous/racist-toothpaste/
http://discovermagazine.com/2010/sep/17-20-things-you-didn2019t-know-about-teeth?utm_source=feedburner&utm_medium=feed&utm_campaign=Feed:+DiscoverMag+(Discover+Magazine)&utm_content=Google+Reader
http://www.earthlife.net/mammals/teeth.html
http://en.wikipedia.org/wiki/Tooth
http://en.wikipedia.org/wiki/Hyperdontia
http://www.answers.com/topic/dental-development
http://nimbleit.squarespace.com/simple-curiosity/2007/7/6/why-do-we-have-baby-teeth.html
http://www.npr.org/templates/story/story.php?storyId=5230538

http://www.bioone.org/doi/abs/10.2992/0145-9058(2004)36[159:EODRIM]2.0.CO;2

The Pinnacle (and Nadir) of Evolution - Asymmetry

This blog was born out of interesting argument had by biologist Lynn Rothschild and James Hanken (seen at the very end of the talk here: http://forum-network.org/lecture/origin-and-evolution-life-earth-i) about if bilateral symmetry was inevitable in multicellular life.  If so, and if other planets have multicellularity, it would have arisen elsewhere in the universe.  Let's do a quick review to make sure we understand what an awesome question that is and then talk about the how evolution even takes that complexity one step forward.

Arguably evolution follows a progression of body plans ascending in complexity, specificity, mobility, dexterity and difficulty of development.  

Undifferentiated:

Radial

Bilateral

James Hanken argued that this progression naturally goes toward bilaterality, because predation is inevitable.  It's always easier to steal energy than it is to make it.  Once that threshold of predation is crossed an arms race between predator and prey will follow and the most logical defense is physical separation by movement.  At that point speed becomes essential and once speed is essential the simplest form of body plan is bilaterality since it deals with both gravity and directionality (up/down, back/front).  It's just plain good for speed since it's easier to be really fast in one direction than many.

An interesting, almost compromise, between radiality and bilaterality is echinoderms.  As plankton they're bilateral, both showing a little family history coming from bilateral organisms, and quite possibly a desire to be more mobile in the water column.  Later in life they'll settle down on the ocean floor and live a much more sessile life adopting a near radial make up usually consisting of 5 radial quadrants, which has other advantages like having spare parts, etc.

I'll just mention that speed isn't the only advantage to bilateral make ups.  Having a one way gut and your anus away from your mouth is helpful, too.  Flowers seem to like the idea of bilaterality, too.  It's been shown that the simplicity of radial flower symmetry (in may cases) is being exchanged for the specificity of bilaterality.

There's another step beyond even bilaterality that I think is often not appreciated - asymmetry.  I'll show how wide spread it is before commenting on the function and developmental difficulties.

Trilobites or should I say, trilobites.  The fossil record for trilobites show something odd.  There's a two to one preference for bite marks to appear on the right side of trilobite.  Numerous explanations abound - left could be more lethal, could have to do with the way they curl up, could be sampling bias, etc.  But, more likely than not, this is a very interesting example of ancient asymmetry, handedness.

Univalves- you might find articles like this that show how one sided asymmetry has major effects on predator/prey (snake/snail) interactions.  Snails that were spiraled the opposite way as most of their relatives got away since one particular snail eating snake species seem to always tilt their head one way when feeding.

Crustaceans - it's exceedingly common for crustaceans to have asymmetrical claws that serve different functions.

Lobster

Giant Land crab

Flounders/Soles - Interesting picture showing how handedness and flounder/sole species go...hand in hand!!

Penises - animals with more than one penis (way more common than you'd think) have favorites.  See ear whigs.

Us!  Here's a short list of ways that we are intentionally asymmetrical (laying aside developmental abnormalities)

Heart - it's on the left side.  Why?  You tell me!

Lungs - the right lung has three lobes and the left lung has two.

Stomach/Spleen - left side.

Liver/Gallbladder - right side.

Colon/Intestines - the actors exit stage right.

Face - you've seen these, but they're cool and they do prove a point - our faces are asymmetrical and maybe for a good reason! (Or maybe not!)  People are also dominant eyed which has major ramifications for archers and sharp-shooters.

Testicles - many males typically have the left testicle hang lower.  Apparently, this has to do with the vascular attachment.

Brain - the asymmetry of the brain is so well known it even enters our vernacular with being 'right brained' or 'left brained'.  There is actually good science for this and the differences go much deeper than just functionality like many colloquial generalizations about the left brain's  logic and right's creativity and into actual structural differences like the planum temporale.  

It's also this brain sidedness that causes our hand handedness, which has considerable effects in our vocabulary (handedness, sinister, etc.), writing, instruments, desks, driving, guns, keyboards, video game controllers, scissors, catcher gloves, computer mouses, golf clubs, power tools, kitchen utensils, cameras...


It all comes down to specificity. It's better to have one really dominant hand that's extremely adept at tasks than two mediocre hands (since learning complex motions can take considerable time). Likewise, it's better two have two different kinds of claws as opposed to one. The body plan must fit the environment and tasks needed.

There's a reason asymmetry has taken so long evolutionarily to show up, though - it's difficult. You not only have to distinguish up and down like in radial symmetry, and front/back like bilateral symmetry, but also distinguish right and left - it's a whole other level of complexity.

Only as recent as 1995 have that we've started to get a grasp on the embryological steps that make it possible. The development of up/down in embryos could be something like gravity, front/back could be caused by a sperm entry point, but it takes something far more complex like chirality (handedness) of molecules to create intentional developmental asymmetry, which is much more difficult, hence why it's both the zenith of evolutionary complexity in body lay outs.

Pictures from here, here, here, here, here, here, here, here, here, here, here, and here.