Maintenance of the Body
blood volume. Plasma makes up most of the remaining 55% of
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).
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.
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
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
Overview: Blood Composition
Describe the composition and physical characteristics of
whole blood. Explain why it is classiﬁed as a connective
List eight functions of blood.
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
, are suspended in a nonliving ﬂuid matrix called
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
. Most of the reddish
mass at the bottom of the tube is
red), the red blood cells that transport oxygen. A thin,
whitish layer called the
is present at the erythrocyte-
plasma junction. ±is layer contains
the white blood cells that act in various ways to protect the body,
, 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
o-krit; “blood fraction”). Normal hematocrit values
vary. In healthy males the norm is 47%
5%; in females it is
5%. Leukocytes and platelets contribute less than 1% of
• 55% of whole blood
• Least dense component
• Leukocytes and platelets
• <1% of whole blood
• 45% of whole blood
• Most dense component
and place in tube
The major components of whole blood.