523
CHAPTER FOURTEEN
Blood
UNIVERSAL PRECAUTIONS
family members become infected while tending their loved ones. In the 2005
outbreak, contaminated medical equipment caused the rapid and deadly spread
of the infection. Nontrained clinic workers reused needles, and some people
reused needles and intravenous equipment in their homes. Universal precau-
tions could not contain the virus, which spreads in vomit, sweat, and saliva, as
well as the huge volumes of blood. Another situation in which circumstances
may overwhelm attempts at universal precautions is in treating patients with
combat-related blast injuries.
B
lood can contain more than cells, nutrients, proteins, and
water—a single drop from an infected individual can harbor bil-
lions of viruses. In the wake of the AIDS epidemic, in 1988 the U.S.
Centers for Disease Control and Prevention (CDC) devised “uni-
versal precautions,” which are speciF
c measures that health-care
workers should take to prevent transmission of bloodborne infectious agents in
the workplace. The CDC singled out HIV and the hepatitis B virus. The guidelines
grew out of earlier suggestions for handling patients suspected to have been
exposed to viruses. The term
universal
refers to the assumption that
any
patient
may have been exposed to a pathogen that can be transmitted in a body ±
uid.
Attention to safety in the health-care setting can prevent transmission
of infectious diseases. The World Health Organization estimates that 4%
to 7% of new infections worldwide are transmitted via unsafe injections.
SpeciF
c recommendations include:
Use of personal protection equipment, such as gloves, goggles, and masks.
Engineering controls, such as fume hoods and sharps containers.
Work-practice controls, such as enforcing hand washing before and
after performing procedures.
Universal precautions were designed for, and work well in, prevent-
ing transmission of viral illnesses in settings already relatively safe, such as
clinics. This isn’t the case for outbreaks, natural disasters, and combat zones.
²or example, several pediatric nurses who aided neighbors infected with
the Marburg virus in the isolated town of Uige in Angola, South Africa, in
2005 and hundreds of others died from this hemorrhagic fever. Headache,
fever, vomiting, and diarrhea begin three to nine days after exposure to the
virus. Then the person bleeds from all body openings, internally and under
the skin. Plummeting blood pressure kills most infected individuals within a
week, and anyone contacting their blood is in danger of infection. Victims
must be isolated and not touched, but the scourge spreads because many
This health-care worker is practicing safe handling of a patient’s blood sample.
14.1
INTRODUCTION
Blood signif
es liFe, and For good reason—it has many vital
Functions. This complex mixture oF cells, cell Fragments, and
dissolved biochemicals transports nutrients, oxygen, wastes,
and hormones; helps maintain the stability oF the intersti-
tial fl
uid; and distributes heat. The blood, heart, and blood
vessels Form the cardiovascular system and link the body’s
internal and external environments.
Blood is a type oF connective tissue whose cells are
suspended in a liquid extracellular matrix. Blood is vital in
transporting substances between body cells and the external
environment, thereby promoting homeostasis.
Whole blood is slightly heavier and three to Four times
more viscous than water. Its cells, which Form mostly in red
bone marrow, include red blood cells and white blood cells.
Blood also contains cellular Fragments called blood platelets
(f g. 14.1)
. The cells and platelets are termed “Formed ele-
ments” oF the blood, in contrast to the liquid portion.
Blood volume varies with body size, changes in fl uid and
electrolyte concentrations, and the amount oF adipose tissue.
Blood volume is typically about 8% oF body weight. An aver-
age-sized adult has a blood volume oF about 5 liters.
IF a blood sample stands in a tube For awhile and is pre-
vented From clotting, the cells separate From the liquid por-
tion oF the blood and settle to the bottom. CentriFuging the
sample quickly packs the cells into the lower part oF the cen-
triFuge tube, as
f
gure 14.2
shows. The percentage oF cells
and liquid in the blood sample can then be calculated.
A blood sample is usually about 45% red blood cells by
volume. This percentage is called the
hematocrit
(HCT), or
packed cell volume
(PCV). The white blood cells and plate-
lets account For less than 1%. The remaining blood sample,
about 55%, is the clear, straw-colored
plasma
(plaz
mah).
Plasma is a complex mixture that includes water, amino
acids, proteins, carbohydrates, lipids, vitamins, hormones,
electrolytes, and cellular wastes
(f
g. 14.3)
.
Appendix B,
Laboratory Tests o± Clinical Signif
cance
(pp. 940–942),
lists values For the hematocrit and other blood tests com-
monly perFormed on healthy individuals.
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