Maintenance of the Body
Blood cells develop from collections of mesenchymal cells,
blood islands
, derived from the mesoderm germ layer.
Te fetus forms a unique hemoglobin,
hemoglobin F
, that has
a higher affinity for oxygen than does adult hemoglobin (hemo-
globin A). It contains two alpha and two gamma (γ) polypeptide
chains per globin molecule, instead of the paired alpha and beta
chains typical of hemoglobin A. AFer birth, the liver rapidly de-
stroys fetal erythrocytes carrying hemoglobin ±, and the baby’s
erythroblasts begin producing hemoglobin A.
Te most common blood diseases that appear during aging
are chronic leukemias, anemias, and clotting disorders. However,
these and most other age-related blood disorders are usually pre-
cipitated by disorders of the heart, blood vessels, or immune sys-
tem. ±or example, the increased incidence of leukemias in old age
is believed to result from the waning efficiency of the immune
system, and abnormal thrombus and embolus formation reflects
atherosclerosis, which roughens the blood vessel walls.
Check Your Understanding
Emily Wong, 17, is brought to the ER with a fever, headache,
and stiff neck. You suspect bacterial meningitis. Would you
expect to see an elevated neutrophil count in a differential
WBC count? Explain.
How is hemoglobin F different from adult hemoglobin?
For answers, see Appendix H.
Blood serves as the vehicle that the cardiovascular system uses
to transport substances throughout the body, so it could be con-
sidered the servant of the cardiovascular system. On the other
hand, without blood, the normal functions of the heart and blood
vessels are impossible. So perhaps the organs of the cardiovascular
system, described in Chapters 18 and 19, are subservient to blood.
Te point of this circular thinking is that blood and the cardiovas-
cular system are vitally intertwined in their common functions: to
ensure that nutrients, oxygen, and other vital substances reach all
tissue cells of the body and to relieve the cells of their wastes.
determines the relative proportions of individual leukocyte
types, is a valuable diagnostic tool. ±or example, a high eosin-
ophil count may indicate a parasitic infection or an allergic re-
sponse somewhere in the body.
A number of tests provide information on the status of the
hemostasis system. ±or example, clinicians determine the
thrombin time
to assess the ability of blood to clot, or do a
platelet count
when thrombocytopenia is suspected.
²wo batteries of tests—a SMAC (SMA24, CHEM-20, or simi-
lar series) and a
complete blood count (CBC)
—are routinely
ordered during physical examinations and before hospital admis-
sions. SMAC is a blood
profile that measures various
electrolytes, glucose, and markers of liver and kidney disorders.
Te CBC includes counts of the different types of formed ele-
ments, the hematocrit, measurements of hemoglobin content, and
size of RBCs. ²ogether these tests provide a comprehensive picture
of a person’s general health in relation to normal blood values.
Appendix ± lists normal values for selected blood tests.
Developmental Aspects
of Blood
Describe changes in the sites of blood production and in
the type of hemoglobin produced after birth.
Name some blood disorders that become more common
with age.
Early in fetal development, blood cells form at many sites—the
fetal yolk sac, liver, and spleen, among others—but by the sev-
enth month, the red marrow has become the primary hemato-
poietic area and remains so (barring serious illness) throughout
life. If there is a severe need for blood cell production, however,
the liver and spleen may resume their fetal blood-forming roles.
Additionally, inactive yellow bone marrow regions (essentially
fatty tissue) may reconvert to active red marrow.
Chapter Summary
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Overview: Blood Composition and Functions
(pp. 632–633)
(p. 632)
Blood is composed of formed elements and plasma. Te
hematocrit is a measure of one formed element, erythrocytes, as a
percentage of total blood volume.
Physical Characteristics and Volume
(p. 632)
Blood is a viscous, slightly alkaline fluid representing about 8% of
total body weight. Blood volume of a normal adult is about 5 L.
(pp. 632–633)
Distribution functions include delivering oxygen and nutrients
to body tissues, removing metabolic wastes, and transporting
Regulation functions include maintaining body temperature,
constant blood pH, and adequate fluid volume.
Protective functions include hemostasis and prevention of
Blood Plasma
(p. 633)
Plasma is a straw-colored, viscous fluid and is 90% water. Te re-
maining 10% is solutes, such as nutrients, respiratory gases, elec-
trolytes, hormones, and proteins. Plasma makes up 55% of whole
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