632
UNIT 4
Maintenance of the Body
17
blood volume. Plasma makes up most of the remaining 55% of
whole blood.
Physical Characteristics and Volume
Blood is a sticky, opaque fluid with a characteristic metallic
taste. As children, we discover its saltiness the first time we stick
a cut finger into our mouth. Depending on the amount of oxy-
gen it is carrying, the color of blood varies from scarlet (oxygen
rich) to dark red (oxygen poor). Blood is more dense than water
and about five times more viscous, largely because of its formed
elements. It is slightly alkaline, with a pH between 7.35 and 7.45.
Blood accounts for approximately 8% of body weight. Its av-
erage volume in healthy adult males is 5–6 L (about 1.5 gallons),
somewhat greater than in healthy adult females (4–5 L).
Functions
Blood performs a number of functions, all concerned in one
way or another with distributing substances, regulating blood
levels of particular substances, or protecting the body.
Distribution
Distribution functions
of blood include
Delivering oxygen from the lungs and nutrients from the di-
gestive tract to all body cells.
Transporting metabolic waste products from cells to elimi-
nation sites (to the lungs to eliminate carbon dioxide, and to
the kidneys to dispose of nitrogenous wastes in urine).
Transporting hormones from the endocrine organs to their
target organs.
Regulation
Regulatory functions
of blood include
Maintaining appropriate body temperature by absorbing and
distributing heat throughout the body and to the skin surface
to encourage heat loss.
blood then flows from the heart to the lungs, where it picks up
oxygen and then returns to the heart to be pumped throughout
the body once again. Now let us look more closely at the nature
of blood.
Overview: Blood Composition
and Functions
Describe the composition and physical characteristics of
whole blood. Explain why it is classified as a connective
tissue.
List eight functions of blood.
Components
Blood is the only fluid tissue in the body. It appears to be a thick,
homogeneous liquid, but the microscope reveals that it has both
cellular and liquid components. Blood is a specialized connec-
tive tissue in which living blood cells, called the
formed ele-
ments
, are suspended in a nonliving fluid matrix called
plasma
(plaz
9
mah). Blood lacks the collagen and elastic fibers typical of
other connective tissues, but dissolved fibrous proteins become
visible as fibrin strands during blood clotting.
If we spin a sample of blood in a centrifuge, centrifugal force
packs down the heavier formed elements and the less dense
plasma remains at the top
(Figure 17.1)
. Most of the reddish
mass at the bottom of the tube is
erythrocytes
(ĕ-rith
9
ro-sīts;
erythro
5
red), the red blood cells that transport oxygen. A thin,
whitish layer called the
buffy coat
is present at the erythrocyte-
plasma junction. ±is layer contains
leukocytes
(
leuko
5
white),
the white blood cells that act in various ways to protect the body,
and
platelets
, cell fragments that help stop bleeding.
Erythrocytes normally constitute about 45% of the total vol-
ume of a blood sample, a percentage known as the
hematocrit
(he-mat
9
o-krit; “blood fraction”). Normal hematocrit values
vary. In healthy males the norm is 47%
6
5%; in females it is
42%
6
5%. Leukocytes and platelets contribute less than 1% of
Plasma
• 55% of whole blood
• Least dense component
Buffy coat
• Leukocytes and platelets
• <1% of whole blood
Erythrocytes
• 45% of whole blood
(hematocrit)
• Most dense component
Wi
thdraw blood
and place in tube
.
2
1
C
entrifuge the
blood sample
.
Formed
elements
Figure 17.1
The major components of whole blood.
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