The catabolism of hemoglobin is outlined in the graphic on
the left. Red blood cells are continuously undergoing a hemolysis
(breaking apart) process. The average life-time of a red blood
cell is 120 days. As the red blood cells disintegrate, the hemoglobin
is degraded or broken into globin, the protein part, iron (conserved
for latter use), and heme (see middle graphic). The heme initially
breaks apart into biliverdin, a green pigment which is rapidly
reduced to bilirubin, an orange-yellow pigment (see bottom graphic).
These processes all occur in the reticuloendothelial cells of
the liver, spleen, and bone marrow. The bilirubin is then transported
to the liver where it reacts with a solubilizing sugar called
glucuronic acid. This more soluble form of bilirubin (conjugated)
is excreted into the bile.
The bile goes through the gall bladder into the intestines
where the bilirubin is changed into a variety of pigments. The
most important ones are stercobilin, which is excreted in the
feces, and urobilinogen, which is reabsorbed back into the blood.
The blood transports the urobilinogen back to the liver where
it is either re-excreted into the bile or into the blood for
transport to the kidneys. Urobilinogen is finally excreted as
a normal component of the urine.