534
UNIT FOUR
Each platelet lacks a nucleus and is less than half the
size of a red blood cell. It is capable of ameboid movement
and may live for about ten days. In normal blood, the
platelet
count
varies from 130,000 to 360,000 platelets per microliter.
Platelets help repair damaged blood vessels by sticking
to broken surfaces. They release
serotonin,
which contracts
smooth muscles in the vessel walls, reducing blood fl ow.
Table 14.5
summarizes the characteristics of blood cells and
platelets.
PRACTICE
26
What is the normal human blood platelet count?
27
What is the function of blood platelets?
may change in particular diseases. The number of neutro-
phils, for instance, usually increases during bacterial infec-
tions, and eosinophils may become more abundant during
certain parasitic infections and allergic reactions. In HIV
infection and AIDS, the numbers of a type of lymphocyte
called helper T cells plummet.
Table 14.4
lists some disorders that alter the numbers
of particular types of white blood cells. Clinical Application
14.2 examines leukemia, cancer of white blood cells.
PRACTICE
23
What is the normal human white blood cell count?
24
Distinguish between leukocytosis and leukopenia.
25
What is a diF
erential white blood cell count?
Blood Platelets
Platelets
(pla
¯t
letz), also called
thrombocytes
(throm
bo-sı¯tz),
are not complete cells. They arise from very large cells in the
red bone marrow, called
megakaryocytes
(meg
ah-kar
o-sı¯tz),
that fragment like a shattered plate, releasing small sections
of cytoplasm—platelets—into the circulation. The larger frag-
ments of the megakaryocytes shrink and become platelets
as they pass through the blood vessels of the lungs (see F g.
14.13). Megakaryocytes, and therefore platelets, develop from
hematopoietic stem cells (see F g. 14.4) in response to the hor-
mone
thrombopoietin
(throm
bo-poi
e
˘-tin).
TABLE
14.4
|
Abnormal White Blood
Cell Numbers
White Blood Cell
Population Change
Illness
Elevated lymphocytes
Hairy cell leukemia, whooping cough,
mononucleosis
Elevated eosinophils
Tapeworm infestation, hookworm infestation,
allergic reactions
Elevated monocytes
Typhoid fever, malaria, tuberculosis
Elevated neutrophils
Bacterial infections
Too few helper T cells
(lymphocytes)
AIDS
Epidermis
Dermis
Blood vessels
1
Splinter
punctures
epidermis
5
Neutrophils move through
blood vessel walls and
migrate toward bacteria
6
Neutrophils destroy
bacteria by phagocytosis
2
Bacteria are introduced
into the dermis
3
Bacteria
multiply
4
Injured cells
release histamine,
causing blood
vessels to dilate
FIGURE 14.16
When bacteria invade the tissues, leukocytes migrate into the region and destroy the microbes by phagocytosis.
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