Chapter 17
Blood
639
17
stiff rods so that hemoglobin S becomes spiky and sharp. Tis,
in turn, causes the red blood cells to become crescent shaped
when they unload oxygen molecules or when the oxygen con-
tent of the blood is lower than normal, as during vigorous
exercise and other activities that increase metabolic rate.
Te stiff, deformed erythrocytes rupture easily and tend
to dam up in small blood vessels. Tese events interfere with
oxygen delivery, leaving the victims gasping for air and in ex-
treme pain. Bone and chest pain are particularly severe, and
infection and stroke are common sequels. Blood transfusion
is still the standard treatment for an acute sickle-cell crisis, but
preliminary results using inhaled nitric oxide to dilate blood
vessels are promising.
Sickle-cell anemia occurs chiefly in black people who live
in the malaria belt of Africa and among their descendants. It
strikes nearly one of every 500 black newborns in the United
States.
Why would such a dangerous genetic trait persist in a
population? Globally, about 250 million people are infected
with malaria and about a million die each year. While indi-
viduals with two copies of the sickle-cell gene have sickle-cell
intake of iron-containing foods and impaired iron absorp-
tion. Te erythrocytes produced, called
microcytes
, are small
and pale because they cannot synthesize their normal com-
plement of hemoglobin. Te obvious treatment is to increase
iron intake in diet or through iron supplements.
Pernicious anemia
is an autoimmune disease that most
oFen affects the elderly. Te immune system of these indi-
viduals destroys cells of their own stomach mucosa. Tese
cells produce a substance called
intrinsic factor
that must
be present for vitamin B
12
to be absorbed by intestinal cells.
Without vitamin B
12
, the developing erythrocytes grow but
cannot divide, and large, pale cells called
macrocytes
result.
±reatment involves regular intramuscular injections of vita-
min B
12
or application of a B
12
-containing gel to the nasal
lining once a week.
As you might expect, lack of vitamin B
12
in the diet also
leads to anemia. However, this is usually a problem only in
strict vegetarians because meats, poultry, and fish provide
ample vitamin B
12
in the diet of nonvegetarians.
Renal anemia
is caused by the lack of EPO, the hormone
that controls red blood cell production. Renal anemia fre-
quently accompanies renal disease because damaged or dis-
eased kidneys cannot produce enough EPO. ²ortunately, it
can be treated with synthetic EPO.
Aplastic anemia
may result from destruction or inhibition
of the red marrow by certain drugs and chemicals, ionizing
radiation, or viruses. In most cases, though, the cause is un-
known. Because marrow destruction impairs formation of
all
formed elements, anemia is just one of its signs. Defects in
blood clotting and immunity are also present. Blood transfu-
sions provide a stopgap treatment until stem cells harvested
from a donor’s blood, bone marrow, or umbilical cord blood
can be transplanted.
Too many red blood cells destroyed.
In
hemolytic anemias
(he
0
mo-lit
9
ik), erythrocytes rupture, or lyse, prematurely. He-
moglobin abnormalities, transfusion of mismatched blood,
and certain bacterial and parasitic infections are possible
causes. Here we focus on the hemoglobin abnormalities.
Production of abnormal hemoglobin usually has a genetic
basis. ±wo such examples, thalassemia and sickle-cell anemia,
can be serious, incurable, and sometimes fatal diseases. In
both diseases the globin part of hemoglobin is abnormal and
the erythrocytes produced are fragile and rupture prematurely.
Talassemias
(thal
0
ah-se
9
me-ahs; “sea blood”) typically
occur in people of Mediterranean ancestry, such as Greeks
and Italians. One of the globin chains is absent or faulty, and
the erythrocytes are thin, delicate, and deficient in hemo-
globin. Tere are many subtypes of thalassemia, classified ac-
cording to which hemoglobin chain is affected and where.
Tey range in severity from mild to so severe that monthly
blood transfusions are required.
In
sickle-cell anemia
, the havoc caused by the abnormal
hemoglobin,
hemoglobin S
(
HbS
), results from a change in
just one of the 146 amino acids in a beta chain of the globin
molecule! (See
Figure 17.8
.) Tis alteration causes the beta
chains to link together under low-oxygen conditions, forming
Val His Leu Thr Pro Glu Glu
1
2
3
4
5
6
7
146
...
Val His Leu Thr Pro Val Glu
1
2
3
4
5
6
7
146
...
(a)
(b) Sickled erythrocyte results from a single
amino acid change in the beta chain of
hemoglobin.
Normal erythrocyte has normal
hemoglobin amino acid sequence
in the beta chain.
Figure 17.8
Sickle-cell anemia.
Scanning electron micrographs
(4950
3
).
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