426
Chapter 12
hemoglobin is 14 g/100 ml blood in women and 16 g/100
ml in men. Chapter 13 further describes the structure and
functions of hemoglobin in the section discussing the trans-
port of oxygen and carbon dioxide.
Erythrocytes have the shape of a biconcave disk—that is,
a disk thicker at the edges than in the middle, like a dough-
nut with a center depression on each side instead of a hole
(
Figure 12–67
). This shape and their small size (7 μm in
diameter) impart to the erythrocytes a high surface area-to-
volume ratio, so that oxygen and carbon dioxide can diffuse
rapidly to and from the interior of the cell. The erythrocyte
plasma membrane contains specifi c polysaccharides and pro-
teins that differ from person to person, and these confer
upon the blood its type, or group. Blood types are described
in Chapter 18, in the context of the immune responses that
occur in transfusion reactions.
The site of erythrocyte production is the soft interior
of bones called
bone marrow,
specifi cally the red bone mar-
row. With differentiation, the erythrocyte precursors produce
hemoglobin, but then they ultimately lose their nuclei and
organelles—their machinery for protein synthesis. Young eryth-
rocytes in the bone marrow still contain a few ribosomes, which
produce a weblike (reticular) appearance when treated with spe-
cial stains, an appearance that gives these young erythrocytes
the name
reticulocyte.
Normally, only mature erythrocytes,
which have lost these ribosomes, leave the bone marrow and
enter the general circulation. In the presence of unusually rapid
erythrocyte production, however, many reticulocytes do enter
the blood, a phenomenon of clinical diagnostic usefulness.
Because erythrocytes lack nuclei and most organelles,
they can neither reproduce themselves nor maintain their nor-
mal structure for very long. The average life span of an eryth-
rocyte is approximately 120 days, which means that almost
one percent of the body’s erythrocytes are destroyed and must
be replaced every day. This amounts to 250 billion cells per day!
Destruction of damaged or dying erythrocytes normally occurs
in the spleen and the liver. As we will later describe, most of the
iron released in the process is conserved. The major breakdown
product of hemoglobin is
bilirubin,
which is returned to the
circulation and gives plasma its characteristic yellowish color
(Chapter 15 will describe the fate of this substance).
The production of erythrocytes requires the usual nutri-
ents needed to synthesize any cell: amino acids, lipids, and car-
bohydrates. In addition, both iron and certain growth factors,
including the vitamins folic acid and vitamin B
12
, are essential.
Iron
As noted previously,
iron
is the element to which oxygen
binds on a hemoglobin molecule within an erythrocyte. Small
amounts of iron are lost from the body via the urine, feces,
sweat, and cells sloughed from the skin. Women lose an addi-
tional amount via menstrual blood. In order to remain in
iron balance, the amount of iron lost from the body must be
replaced by ingestion of iron-containing foods. Particularly
rich sources of iron are meat, liver, shellfi sh, egg yolk, beans,
nuts, and cereals. A signifi cant upset of iron balance can result
either in
iron defi
ciency,
leading to inadequate hemoglobin
production, or in an excess of iron in the body, with serious
toxic effects
(
hemochromatosis
).
The homeostatic control of iron balance resides primarily
in the intestinal epithelium, which actively absorbs iron from
ingested foods. Normally, only a small fraction of ingested iron
is absorbed. However, this fraction is increased or decreased
in a negative feedback manner, depending upon the state of
the body’s iron balance—the more iron in the body, the less
ingested iron is absorbed (the mechanism will be described in
Chapter 15).
The body has a considerable store of iron, mainly in the
liver, bound up in a protein called
ferritin.
Ferritin serves as
a buffer against iron defi ciency. About 50 percent of the total
body iron is in hemoglobin, 25 percent is in other heme-
containing proteins (mainly the cytochromes) in the cells of
the body, and 25 percent is in liver ferritin. Moreover, the
recycling of iron is very effi cient (
Figure 12–68
). As old
erythrocytes are destroyed in the spleen (and liver), their iron
is released into the plasma and bound to an iron-transport
plasma protein called
transferrin.
Transferrin delivers almost
all of this iron to the bone marrow to be incorporated into new
erythrocytes. Recirculation of erythrocyte iron is very impor-
tant because it involves 20 times more iron per day than the
body absorbs and excretes. On a much lesser scale, noneryth-
rocyte cells, some of which are continuously dying and being
replaced, release iron from their cytochromes into the plasma
and take up iron from it, transferrin serving as a carrier.
Folic Acid and Vitamin B
12
Folic acid,
a vitamin found in large amounts in leafy plants,
yeast, and liver, is required for synthesis of the nucleotide
base thymine. It is, therefore, essential for the formation of
DNA and thus for normal cell division. When this vitamin is
not present in adequate amounts, impairment of cell division
Figure 12–67
Electron micrograph of erythrocytes.
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