Poop (or feces) isn’t always brown, but it often
is. That’s because of pigments. Pigments are
molecules that give things color. If you’re
wondering why our bodies would make a pigment just
to waste it, good question! The answer is we
don’t. But some pigments do wind up as waste
This story really starts in our blood. One of
the big jobs of blood is to move oxygen from the
lungs to our cells. Our red blood cells are filled
with a molecule called hemoglobin, which
carries the oxygen. Hemoglobin is made of
iron and protein.
Our red blood cells are the only cells in our
bodies without nuclei (the plural of nucleus).
This allows them to carry more hemoglobin. It also
means that the red blood cells don’t have
instructions for repairing themselves, so they
don’t last long, only about 4 months. Our bodies
are good at recycling, so when blood cells are
old, they break down the hemoglobin and re-use
parts of it. The protein is broken down and used
to make new proteins. Some of the iron is recycled
directly to make new hemoglobin. Some of the
parts with iron are changed to something called
bilirubin (sounds like “Billy Rueben”).
The next stop for bilirubin could be the kidneys,
where it is put into urine. It makes urine yellow.
Another path it could take is to be turned into
bile by the liver. The liver helps to clean our
blood of toxic stuff. The bile that it makes is
stored in the gall bladder. When we eat fatty
foods, the gall bladder squirts bile into the
small intestine to help break up the fat. Some of
the bile and the bilirubin it contains keeps going
through the small intestine, then the large
intestine, and leaves in the feces, giving them
the brownish color. Some of the bile might get
absorbed into the blood vessels in the walls of
the intestines. When this blood goes to the liver
for cleaning, the bilirubin is put back into bile
for another trip.
What might a doctor learn by knowing about a
If questions like this interest you, you might be
interested in studying biology or medicine.