360
Chapter 12
b
eyond a distance of a few cell diameters, diffusion—the random movement of substances
from a region of higher concentration to one of lower concentration—is too slow to meet the
metabolic requirements of cells. In large, multicellular organisms, therefore, some mechanism
other than diffusion is needed to transport molecules rapidly over the long distances between internal
cells and the body’s surface, and between the various specialized tissues and organs. This purpose is
achieved by the
circulatory system,
which includes a pump (the
heart
), a set of interconnected tubes
(
blood vessels,
or
vascular system
), and a mixture of extracellular fl uid and cells that fi lls the tubes
(the
blood
). This body-wide transport system is often termed the
cardiovascular system.
SECTION A
Overall Design of the Circulatory System
System Overview
The three principal components that make up the circulatory
system are the heart, blood vessels, and the blood itself. Each
will be discussed in greater detail in subsequent sections, but
we will begin with an overview of the anatomical design of
the system and a discussion of some of the physical factors that
determine its function.
Blood is composed of
formed elements
(cells and cell
fragments) suspended in a liquid called
plasma.
Dissolved in
the plasma are a large number of proteins, nutrients, metabolic
wastes, and other molecules being transported between organ
systems. The cells are the
erythrocytes
(red blood cells) and
the
leukocytes
(white blood cells), and the cell fragments are
the
platelets.
More than 99 percent of blood cells are eryth-
rocytes, which carry oxygen. The leukocytes protect against
infection and cancer, and the platelets function in blood clot-
ting. The constant motion of the blood keeps all the cells dis-
persed throughout the plasma.
The
hematocrit
is defi ned as the percentage of blood vol-
ume that is erythrocytes. It is measured by centrifuging (spin-
ning at high speed) a sample of blood. The erythrocytes are
forced to the bottom of the centrifuge tube, the plasma remains
on top, and the leukocytes and platelets form a very thin
layer between them (
Figure 12–1
). The normal hematocrit is
approximately 45 percent in men and 42 percent in women.
The volume of blood in a 70-kg (154-pound) person is
approximately 5.5 L. If we take the hematocrit to be 45 per-
cent, then:
Erythrocyte volume = 0.45
×
5.5 L = 2.5 L
Because the volume occupied by leukocytes and platelets is
normally negligible, the plasma volume equals the difference
between blood volume and erythrocyte volume; therefore, in
our 70-kg person:
Plasma volume = 5.5 L – 2.5 L = 3.0 L
The rapid fl
ow of blood throughout the body is pro-
duced by pressures created by the pumping action of the heart.
This type of fl ow is known as
bulk fl
ow
because all con-
stituents of the blood move in one direction together. The
extraordinary degree of branching of blood vessels ensures
that almost all cells in the body are within a few cell diam-
eters of at least one of the smallest branches, the capillaries.
Nutrients and metabolic end products move between capil-
lary blood and the interstitial fl uid by diffusion. Movements
Plasma = 55%
“buffy coat”
Leukocytes
and
platelets
Erythrocytes = 45%
(hematocrit = 45%)
Figure 12–1
Measurement of the hematocrit by centrifugation. The values shown
are typical for a healthy male. Due to the presence of a thin layer of
leukocytes and platelets between the plasma and red cells, the value
for plasma is actually very slightly less than 55 percent.
Figure 12–1
physiological
inquiry
Estimate the hematocrit of a person with a plasma volume of 3 L
and total blood volume of 4.5 L.
Answer can be found at end of chapter.
previous page 388 Vander's Human Physiology The Mechanisms of Body Function read online next page 390 Vander's Human Physiology The Mechanisms of Body Function read online Home Toggle text on/off