644
UNIT 4
Maintenance of the Body
17
Table 17.2
Summary of Formed Elements of the Blood
CELL TYPE
ILLUSTRATION
DESCRIPTION*
CELLS/µL (mm
3
)
OF BLOOD
DURATION OF
DEVELOPMENT (D)
AND LIFE SPAN (LS)
FUNCTION
Erythrocytes
(red
blood cells, RBCs)
Biconcave, anucleate
disc; salmon-colored;
diameter 7–8 μm
4–6 million
D: about 15 days
LS: 100–120 days
Transport oxygen
and carbon dioxide
Leukocytes
(white
blood cells, WBCs)
Spherical, nucleated
cells
4800–10,800
Granulocytes
Neutrophil
Multilobed nucleus;
inconspicuous
cytoplasmic granules;
diameter 10–12 μm
3000–7000
D: about 14 days
LS: 6 hours to a few
days
Phagocytize bacteria
Eosinophil
Bilobed nucleus; red
cytoplasmic granules;
diameter 10–14 μm
100–400
D: about 14 days
LS: about 5 days
Kill parasitic worms;
complex role in
allergy and asthma
Basophil
Bilobed nucleus;
large purplish-black
cytoplasmic granules;
diameter 10–14 μm
20–50
D: 1–7 days
LS: a few hours to a
few days
Release histamine
and other mediators
of inflammation;
contain heparin, an
anticoagulant
Agranulocytes
Lymphocyte
Spherical or indented
nucleus; pale blue
cytoplasm; diameter
5–17 μm
1500–3000
D: days to weeks
LS: hours to years
Mount immune
response by direct
cell attack or via
antibodies
Monocyte
U- or kidney-shaped
nucleus; gray-blue
cytoplasm; diameter
14–24 μm
100–700
D: 2–3 days
LS: months
Phagocytosis;
develop into
macrophages in the
tissues
Platelets
Discoid cytoplasmic
fragments containing
granules; stain deep
purple; diameter
2–4 μm
150,000–400,000
D: 4–5 days
LS: 5–10 days
Seal small tears
in blood vessels;
instrumental in
blood clotting
*Appearance when stained with Wright’s stain.
Despite their similar appearance, the two types of agranulo-
cytes have very different lineages.
Monocytes are derived from myeloid stem cells, and share a
common precursor with neutrophils that is not shared with
the other granulocytes. Cells following the monocyte line pass
through the
monoblast
and
promonocyte
stages before leaving
the bone marrow and becoming monocytes (Figure 17.11d).
T and B lymphocytes are derived from
T
and
B lymphocyte
precursors
, which arise from the lymphoid stem cell. ±e T
lymphocyte precursors leave the bone marrow and travel to
the thymus, where their further differentiation occurs (as we
describe in Chapter 21). B lymphocyte precursors remain
and mature in the bone marrow.
Monocytes may live for several months, whereas the life span of
lymphocytes varies from a few hours to decades.
Leukocyte Disorders
Overproduction of abnormal leukocytes occurs in leukemia
and infectious mononucleosis. At the opposite pole,
leukopenia
(loo
0
ko-pe
9
ne-ah) is an abnormally low white blood cell count
(
penia
5
poverty), commonly induced by drugs, particularly
glucocorticoids and anticancer agents.
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