Do red blood cells break down?
Yes they do, in about 3-4 months. They have no nucleus so they cannot reproduce. They loose their nucleus so they have room for hemoglobin molecules. They usually die in the spleen and get broken down in the liver. This is where they are turned into bilirubin. Some of the bilirubin is used to make bile, the rest is excreted in your feces, that is what gives it the brownish color.