As a red cell ages, there will be
a decrease in many of its enzymes, a decrease in ATP, and the cell will tend to
become smaller and slightly more dense. Destruction of the red blood cell most
commonly occurs in the bone marrow, through phagocytosis by the
reticuloendothelial cells. As soon as this occurs, there is breakdown of the
hemoglobin. The iron is freed for reuse by new red cells, and the amino acids,
from the globin, are returned to the amino acid pool. The protoporphyring ring
is broken at one of the methene bridges and biliverdin is formed. The
biliverdin is reduced to bilirubin, which is carried by the plasma albumin to
the liver for eventual excretion. If hemoglobin is released directly into the
blood, it becomes attached to haptoglobin (a globulin), taken to the
reticuloendothelial cell, and processed in the normal way. Free hemoglobin in
the blood may be oxidized to methemoglobin. The heme groups will then combine
with hemopexin, leave the circulation, and be catabolized. If there is an excess
of heme groups, they will combine with albumin to form methemalbumin until
hemopexin becomes available.