640
UNIT 4
Maintenance of the Body
17
Leukocytes (White Blood Cells)
List the classes, structural characteristics, and functions of
leukocytes.
Describe how leukocytes are produced.
Give examples of leukocyte disorders, and explain what
goes wrong in each disorder.
General Structural and Functional Characteristics
Leukocytes
(
leuko
5
white), or
white blood cells (WBCs)
, are
the only formed elements that are complete cells, with nuclei
and the usual organelles. Accounting for less than 1% of total
blood volume, leukocytes are far less numerous than red blood
cells. On average, there are 4800–10,800 WBCs/μl of blood.
Leukocytes are crucial to our defense against disease. Tey
form a mobile army that helps protect the body from damage
by bacteria, viruses, parasites, toxins, and tumor cells. As such,
they have special functional characteristics. Red blood cells are
confined to the bloodstream, and they carry out their functions
in the blood. But white blood cells are able to slip out of the
capillary blood vessels—a process called
diapedesis
(di
0
ah-pĕ-
de
9
sis; “leaping across”)—and the circulatory system is simply
their means of transport to areas of the body (mostly loose con-
nective tissues or lymphoid tissues) where they mount inflam-
matory or immune responses.
As we explain in more detail in Chapter 21, the signals
that prompt WBCs to leave the bloodstream at specific loca-
tions are cell adhesion molecules displayed by endothelial cells
forming the capillary walls at sites of inflammation. Once out
of the bloodstream, leukocytes move through the tissue spaces
by
amoeboid motion
(they form flowing cytoplasmic exten-
sions that move them along). By following the chemical trail of
molecules released by damaged cells or other leukocytes, a phe-
nomenon called
positive
chemotaxis
, they pinpoint areas of
tissue damage and infection and gather there in large numbers
to destroy foreign substances and dead cells.
Whenever white blood cells are mobilized for action, the
body speeds up their production and their numbers may dou-
ble within a few hours. A
white blood cell count
of over 11,000
cells/μl is
leukocytosis
. Tis condition is a normal homeostatic
response to an infection in the body.
Leukocytes are grouped into two major categories on the
basis of structural and chemical characteristics.
Granulocytes
contain obvious membrane-bound cytoplasmic granules, and
agranulocytes
lack obvious granules. We provide general infor-
mation about the various leukocytes next. More details appear
in
Figure 17.9
and
Table 17.2
on p. 644.
Students are oFen asked to list the leukocytes in order from
most abundant to least abundant. Te following phrase may
help you with this task:
N
ever
l
et
m
onkeys
e
at
b
ananas (neu-
trophils, lymphocytes, monocytes, eosinophils, basophils).
Granulocytes
Granulocytes
(gran
9
u-lo-sīts), which include neutrophils,
eosinophils, and basophils, are all roughly spherical in shape.
Tey are larger and much shorter lived (in most cases) than
anemia, individuals with only one copy of the gene (sickle-cell
trait) have a better chance of surviving malaria. Teir cells
only sickle under abnormal circumstances, most importantly
when they are infected with malaria. Sickling reduces the ma-
laria parasites’ ability to survive and enhances macrophages’
ability to destroy infected RBCs and the parasites they contain.
Several treatment approaches focus on preventing RBCs
from sickling. ±etal hemoglobin (Hb±) does not “sickle,” even
in those destined to have sickle-cell anemia.
Hydroxyurea
, a
drug used to treat chronic leukemia, switches the fetal he-
moglobin gene back on. Tis drug dramatically reduces the
excruciating pain and overall severity and complications of
sickle-cell anemia (by 50%). Another class of drugs reduces
sickling by blocking ion channels in the RBC membrane,
keeping ions and water inside the cell. Other approaches being
tested include oral arginine to stimulate nitric oxide produc-
tion and dilate blood vessels, stem cell transplants, and gene
therapy to deliver genes for synthesizing normal beta chains.
Polycythemia
Polycythemia
(pol
0
e-si-the
9
me-ah;
“many
blood cells”) is an abnormal excess of erythrocytes that in-
creases blood viscosity, causing it to sludge, or flow sluggishly.
Polycythemia vera
, a bone marrow cancer, is characterized by
dizziness and an exceptionally high RBC count (8–11 million
cells/μl). Te hematocrit may be as high as 80% and blood vol-
ume may double, causing the vascular system to become en-
gorged with blood and severely impairing circulation. Severe
polycythemia is treated by diluting blood—removing some
blood and replacing it with saline.
Secondary polycythemias
result when less oxygen is avail-
able or EPO production increases. Te secondary polycythemia
that appears in individuals living at high altitudes is a normal
physiological response to the reduced atmospheric pressure and
lower oxygen content of the air in such areas. RBC counts of 6–8
million/μl are common in such people.
Blood doping
, practiced by some athletes competing in
aerobic events, is artificially induced polycythemia. Some of
the athlete’s red blood cells are drawn off and stored. Te body
quickly replaces these erythrocytes because removing blood
triggers the erythropoietin mechanism. Ten, when the stored
blood is reinfused a few days before the athletic event, a tempo-
rary polycythemia results.
Since red blood cells carry oxygen, the additional infusion
should translate into increased oxygen-carrying capacity due to
a higher hematocrit, and hence greater endurance and speed.
Other than the risk of stroke and heart failure due to high
hematocrit and high blood viscosity described earlier, blood
doping seems to work. However, the practice is considered
unethical and has been banned from the Olympic Games.
Check Your Understanding
4.
How many molecules of oxygen can each hemoglobin
molecule transport? What part of the hemoglobin binds the
oxygen?
5.
Patients with advanced kidney disease often have anemia.
Explain the connection.
For answers, see Appendix H.
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