|MadSci Network: Microbiology|
Red blood cells are continually being broken down and replaced. The main byproduct of this process is called bilirubin, which is (specifically) a breakdown product of hemoglobin, the molecule in red blood cells that carries oxygen. Bilirubin (like many other biochemical wastes) is dumped by the liver into the large intestine via the gall bladder. Stuff that the gall bladder secretes (including bilirubin) is called 'bile', and it is the characteristic color of bile (the main component of which is bilirubin) that gives feces its typical color. Without bile, feces would probably be the color of vomit, as you might expect (but not really like to think about).
As you might expect, feces can change color in sickness.
Dave Featherstone Dept. of Biology University of Utah Salt Lake City, UT
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