634
UNIT 4
Maintenance of the Body
17
Formed Elements
Te
formed elements
of blood—
erythrocytes, leukocytes
, and
platelets
—have some unusual features.
±wo of the three are not even true cells: Erythrocytes have
no nuclei or organelles, and platelets are cell fragments. Only
leukocytes are complete cells.
Most of the formed elements survive in the bloodstream for
only a few days.
Most blood cells do not divide. Instead, stem cells divide con-
tinuously in red bone marrow to replace them.
If you examine a stained smear of human blood under the
light microscope, you will see disc-shaped red blood cells, a va-
riety of gaudily stained spherical white blood cells, and some
scattered platelets that look like debris
(Figure 17.2)
. Eryth-
rocytes vastly outnumber the other types of formed elements.
±able 17.2 on p. 644 summarizes the important characteristics
of the formed elements.
Erythrocytes (Red Blood Cells)
Describe the structure, function, and production of
erythrocytes.
Describe the chemical composition of hemoglobin.
Give examples of disorders caused by abnormalities of
erythrocytes. Explain what goes wrong in each disorder.
Structural Characteristics
Erythrocytes
or
red blood cells (RBCs)
are small cells, about
7.5 μm in diameter
(Figure 17.3)
. Shaped like biconcave
discs—flattened discs with depressed centers—they appear
lighter in color at their thin centers than at their edges. Con-
sequently, erythrocytes look like miniature doughnuts when
viewed with a microscope.
Mature erythrocytes are bound by a plasma membrane, but
lack a nucleus (are
anucleate
) and have essentially no organelles.
In fact, they are little more than “bags” of
hemoglobin
(
Hb
), the
RBC protein that functions in gas transport. Other proteins are
present, such as antioxidant enzymes that rid the body of harm-
ful oxygen radicals, but most function as structural proteins,
allowing the RBC to deform yet spring back into shape.
For example, a network of proteins, especially one called
spec-
trin
, attached to the cytoplasmic face of RBC plasma membranes
maintains the biconcave shape of an erythrocyte. Te spectrin
net is deformable, allowing erythrocytes to change shape as
necessary—to twist, turn, and become cup shaped as they are
carried passively through capillaries with diameters smaller than
themselves—and then to resume their biconcave shape.
Te erythrocyte is a superb example of complementarity of
structure and function. It picks up oxygen in the capillaries of
the lungs and releases it to tissue cells across other capillaries
throughout the body. It also transports some 20% of the carbon
dioxide released by tissue cells back to the lungs. Tree struc-
tural characteristics contribute to erythrocyte gas transport
functions:
Its small size and biconcave shape provide a huge surface
area relative to volume (about 30% more surface area than
comparable spherical cells). Te biconcave disc shape is ide-
ally suited for gas exchange because no point within the cy-
toplasm is far from the surface.
Discounting water content, an erythrocyte is over 97% he-
moglobin, the molecule that binds to and transports respira-
tory gases.
Platelets
Neutrophils
Lymphocyte
Erythrocytes
Monocyte
Figure 17.2
Photomicrograph of a human blood smear
stained with Wright’s stain.
(640
3
)
2.5
u
m
7.5
u
m
Side view (cut)
Top view
Figure 17.3
Structure of erythrocytes (red blood cells).
Notice
the distinctive biconcave shape.
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