[paul.fr]

the questions in the title. why is urine yellow?

[Bored chemist]

Because it contains the breakdown products of the bile pigments. Also so you know what snow not to thaw out for drinking water.

[Ben6789]

urine isn't always yellow, drink enough water and it's clear! It all depends on the consumption.

[Karen W.]

Agreed My doctor says that if your urine is strong(smelling and brightly colored you are not getting proper water intake. Ben is right I believe increase your water untill your urine becomes clearer, then continue with that amout of water to maintain the clarity!! I also agree that other things you eat may have a factor..

I think that one should also be ready to seek medical advice if the urine is unusually dark for a prolonged amount of time! It could contain other things protein, blood etc.. so always use good judgement and know what is the norm for you and seek help if that changes significantly without you having had a huge diet change.

I am no doctor, but it seems to make sennse to be aware of your own toileting habits and their colors!

[science_guy]

hypothetically, what if it's pink?

[Karen W.]

Well Then I would definitely go see the Doc.. LOL Although perhaps alein's urinations are pink, hypothetically of coarse!

[DrDick]

Remember that clarity and color are two different things.  Urine should be clear (i.e., not cloudy) regardless of the color.  Then, the intensity of the color of any colored solution is linearly dependent on the solute causing the color.  So, if you dilute your urine (by drinking enough water), the color should lessen significantly.  This is why it tends to be darkest when you wake up, since you haven't been drinking any water all night.

Dick

[paul.fr]

hypothetically, what if it's pink?

it can actually turn pink, and yes you should be seeing a doctor.

[paul.fr]

not wanting to start a new topic about wee, but when i eat sugar puffs my wee smells of them for a few days!

[drkev]

Urine is yellow so men know if they're coming or going hee hee

Seriously though mine is never yellow it's always clear unless I drink heavily on a friday or saturday night and then it's dark yellow the next day and stinks real bad!

[chris]

The answer to this question lies in the chemical we rely upon to transport oxygen around the body, which is haemoglobin.

This consists of four globular protein subunits linked together to form a tetramer. Associated with each protein subunit is a structure known as a haem group, the core of which contains an iron (Fe²⁺) atom. It's this iron that gives haemoglobin its red colour:

 [ Invalid Attachment ] . This is the haem group that sits in the centre of each of the four proteins making up one haemoglobin molecule.

Haemoglobin is ferried around the body in red blood cells, which are made in the bone marrow. There are about 2–3×10¹³ (ten million million of them) in circulation in the average human at any given time. The lifetime of these cells is about 120 days after which the immune system removes them from the blood in the same way that the Bank of England removes worn bank notes from circulation.

These aged cells are then broken down by other cells such as macrophages, which dismantle them up into their component parts - rather like a car wreckers yard. The protein is stripped away from the haemoglobin group for recycling, but the haem group is much harder to deal with.

It consists of four pyrrole rings linked into a larger structure known as a porphyrin ring. This is first broken open by an enzyme called microsomal haem oxygenase, which releases the iron. The result is a chain of four linked rings, known as biliverdin, which has a green colour. This is then modified by an enzyme called biliverdin reductase, which adds some hydrogen to the molecule to produce unconjugated bilirubin, the yellow chemical that makes people who have too much of it look jaundiced:

 [ Invalid Attachment ]

This is an organic molecule which will not willingly mix with water - in other words it's hydrophobic. In this form it cannot get out of the blood stream (to enter urine for example). Instead it locks onto proteins (albumin) in the blood and passes to the liver, which plucks it off the albumin. In the liver cells, to make it more readily excretable, it needs to be "conjugated" or linked to water-loving sugars which will help it to dissolve. This is done in the liver, which adds two glucoronic acid molecules (these are glucose molecules which bear an acid group).

This produces bilirubin diglucouronide, which is then excreted from the body in bile (which is why bile is yellow). Once in the intestine, bacteria metabolise the molecule and produce a derivative called urobilinogen, some of which gets re-absorbed by the intestine and enters the blood stream.

Unlike the unconjugated bilirubin, which remains bound to albumin, urobilinogen is water soluble and so as blood passes through the kidney its get filtered out into the urine. Within the urine urobilinogen oxidises to form urobilin, which is dark yellow / brown. That's why urine that's let sitting in the toilet bowl (or in your bladder) for a while turns dark.

Meanwhile, in the intestines, the urobilinogen that isn't reabsorbed passes into the colon (large intestine), where further bacterial modification produces a brown chemical called stercobilin. It's this stuff that's responsible for the fact that we do brown jobbies...

Chris