BLOOD CELLS
There are three kinds of blood cells: red blood cells,
white blood cells, and platelets. Blood cells are pro-
duced from stem cells in hemopoietic tissue. After
birth this is primarily the red bone marrow, found
in flat and irregular bones such as the sternum,
hip bone, and vertebrae. Lymphocytes mature and
divide in lymphatic tissue, found in the spleen,
lymph nodes, and thymus gland. The thymus contains
stem cells that produce T lymphocytes, and the stem
cells in other lymphatic tissue also produce lympho-
cytes.
RED BLOOD CELLS
Also called erythrocytes, red blood cells (RBCs)
are biconcave discs, which means their centers are
thinner than their edges. You may recall from Chapter
3 that red blood cells are the only human cells with-
out nuclei. Their nuclei disintegrate as the red blood
cells mature and are not needed for normal func-
tioning.
A normal RBC count ranges from 4.5 to 6.0 million
cells per microliter (L) of blood (1 microliter 1
mm^3 one millionth of a liter, a very small volume).
RBC counts for men are often toward the high end of
this range; those for women are often toward the low
end. Another way to measure the amount of RBCs is
the hematocrit. This test involves drawing blood into
a thin glass tube called a capillary tube, and centrifug-
ing the tube to force all the cells to one end. The per-
centages of cells and plasma can then be determined.
Because RBCs are by far the most abundant of the
blood cells, a normal hematocrit range is just like that
of the total blood cells: 38% to 48%. Both RBC count
and hematocrit (Hct) are part of a complete blood
count (CBC).
Function
Red blood cells contain the protein hemoglobin
(Hb), which gives them the ability to carry oxygen.
Each red blood cell contains approximately 300 mil-
lion hemoglobin molecules, each of which can bond to
four oxygen molecules (see Box Fig. 3–B). In the pul-
monary capillaries, RBCs pick up oxygen and oxyhe-
moglobin is formed. In the systemic capillaries,
hemoglobin gives up much of its oxygen and becomes
reduced hemoglobin. A determination of hemoglobin
level is also part of a CBC; the normal range is 12 to
18 grams per 100 mL of blood. Essential to the for-
mation of hemoglobin is the mineral iron; there are
four atoms of iron in each molecule of hemoglobin. It
is the iron that actually bonds to the oxygen and also
makes RBCs red.
Hemoglobin is also able to bond to carbon dioxide
(CO 2 ), and does transport some CO 2 from the tissues
to the lungs. But hemoglobin accounts for only about
10% of total CO 2 transport (most is carried in the
plasma as bicarbonate ions).Production and Maturation
Red blood cells are formed in red bone marrow (RBM)
in flat and irregular bones. Within red bone marrow
are precursor cells called stem cells. Recall from
Chapter 3 that stem cells are unspecialized cells that
may develop, or differentiate, in several ways. The
stem cells of the red bone marrow may also be called
hemocytoblasts(hemo “blood,” cyto “cell,” blast
“producing”), and they constantly undergo mitosis
to produce all the kinds of blood cells, many of which
are RBCs (Figs. 11–2 and 11–3). The rate of produc-
tion is very rapid (estimated at several million new
RBCs per second), and a major regulating factor is
oxygen. If the body is in a state of hypoxia, or lack of
oxygen, the kidneys produce a hormone called ery-
thropoietin, which stimulates the red bone marrow to
increase the rate of RBC production (that is, the rate
of stem cell mitosis). This will occur following hemor-
rhage or if a person stays for a time at a higher altitude.
As a result of the action of erythropoietin, more RBCs
will be available to carry oxygen and correct the
hypoxic state.
The stem cells that will become RBCs go through
a number of developmental stages, only the last two of
which we will mention (see Fig. 11–2). The nor-
moblastis the last stage with a nucleus, which then
disintegrates. The reticulocytehas fragments of the
endoplasmic reticulum, which are visible when blood
smears are stained for microscopic evaluation. These
immature cells are usually found in the red bone mar-
row, although a small number of reticulocytes in the
peripheral circulation is considered normal (up to
1.5% of the total RBCs). Large numbers of reticulo-
cytes or normoblasts in the circulating blood mean
that the number of mature RBCs is not sufficient to254 Blood