526
UNIT FOUR
Red blood cells have nuclei during their early stages of
development but extrude them as the cells mature, which
provides more space for hemoglobin. They lack nuclei, so
red blood cells cannot synthesize messenger RNA or divide.
Red blood cells produce ATP through glycolysis only and use
none of the oxygen they carry because they also lack mito-
chondria. As long as cytoplasmic enzymes function, these
cells can carry on vital energy-releasing processes. With
time, however, red blood cells become less and less active.
They become more rigid and more likely to be damaged or
worn and eventually removed by the spleen and liver.
As hematopoietic stem cells divide, the new cells, myel-
oid and lymphoid stem cells, respond to different secreted
growth factors, called
hematopoietic growth factors,
that
turn on some genes and turn off others. This exposure to
growth factors ultimately sculpts the distinctive formed ele-
ments of blood, including the cellular components of the
immune system. A protein called
thrombopoietin
(TPO)
stimulates large cells called
megakaryocytes
to proliferate.
These cells eventually come apart, yielding platelets.
RECONNECT
To Chapter 3, Stem and Progenitor Cells, page 104.
Many companies of
er to store a person’s bone marrow stem cells
or umbilical cord stem cells For possible Future use to treat cancers
oF the blood or immune system. In the Future, banks oF such cells
may be established that do not have to be matched to particular
patients. ±or example, one company is testing a “multipotent adult
progenitor cell” From bone marrow that can be easily expanded in
number in the laboratory, does not have cell-surFace molecules that
trigger an immune response in a recipient, dampens in²
ammation,
and is pluripotent—can be coaxed into giving rise to any oF several
cell types.
Characteristics of Red Blood Cells
Red blood cells,
or
erythrocytes
(e
˘-rith
ro-sı¯tz), are tiny,
approximately 7.5 µm in diameter. They are biconcave
discs, thin near their centers and thicker around their rims
(f g. 14.5)
. This distinctive shape is an adaptation for the red
blood cell’s function of transporting gases; it increases the
surface area through which gases can diffuse. The shape also
places the cell membrane closer to oxygen-carrying
hemo-
globin
(he
mo-glo
bin) molecules in the cell. A red blood
cell’s shape enables it to readily squeeze through the narrow
capillaries.
The volume of each red blood cell is about one-third
hemoglobin. This protein is responsible for the color of the
blood. The rest of the cell mainly consists of membrane,
water, electrolytes, and enzymes. When hemoglobin com-
bines with oxygen, the resulting
oxyhemoglobin
is bright red;
when the oxygen is released, the resulting
deoxyhemoglobin
is darker. Blood rich in deoxyhemoglobin may appear bluish
when it is viewed through blood vessel walls.
A person experiencing prolonged oxygen de³
ciency (hypoxia) may
become
cyanotic.
The skin and mucous membranes appear bluish
due to an abnormally high blood concentration oF deoxyhemoglo-
bin. Exposure to low temperature may also result in cyanosis. Such
exposure constricts super³
cial blood vessels, which slows blood ²
ow,
allowing removal oF more oxygen than usual From blood Flowing
through the vessels.
Top view
2.0 micrometers
7.5 micrometers
Sectional view
(a)
(b)
FIGURE 14.5
Red blood cells. (
a
) The biconcave shape oF a red
blood cell makes possible its Function oF transporting oxygen.
(
b
) ±alsely colored scanning electron micrograph oF human red blood
cells (5,000
×
).
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