Life Span and Destruction of Red
Blood Cells
When red blood cells are delivered from the bone marrow into the
circulatory system, they normally cir-culate an average of 120 days before
being destroyed. Even though mature red cells do not have a nucleus,
mitochondria, or endoplasmic reticulum, they do have cytoplasmic enzymes that
are capable of metabolizing glucose and forming small amounts of adenosine
triphosphate. These enzymes also (1) maintain pliabil-ity of the cell membrane,
(2) maintain membrane transport of ions, (3) keep the iron of the cells’
hemo-globin in the ferrous form rather than ferric form, and (4) prevent
oxidation of the proteins in the red cells. Even so, the metabolic systems of
old red cells become progressively less active, and the cells become more and more
fragile, presumably because their life processes wear out.
Once the red cell membrane becomes fragile, the cell ruptures
during passage through some tight spot of the circulation. Many of the red
cells self-destruct in the spleen, where they squeeze through the red pulp of
the spleen. There, the spaces between the structural trabeculae of the red
pulp, through which most of the cells must pass, are only 3 micrometers wide,
in com-parison with the 8-micrometer diameter of the red cell. When the spleen
is removed, the number of old abnormal red cells circulating in the blood
increases considerably.
Destruction
of Hemoglobin. When red blood cells burstand release their hemoglobin, the
hemoglobin is phagocytized almost immediately by macrophages in many parts of
the body, but especially by the Kupffer cells of the liver and macrophages of
the spleen and bone marrow. During the next few hours to days, the macrophages
release iron from the hemoglobin and pass it back into the blood, to be carried
by transfer-rin either to the bone marrow for the production of new red blood
cells or to the liver and other tissues for storage in the form of ferritin.
The porphyrin portion of the hemoglobin molecule is converted by the
macrophages, through a series of stages, into the bile pigment bilirubin, which is released into the
blood and later removed from the body by secretion through the liver into the
bile;.
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