Cardiovascular Physiology
361
between the interstitial fl uid and the cell interior are accom-
plished by both diffusion and mediated transport across the
plasma membrane.
At any given moment, only about 5 percent of the total
circulating blood is actually in the capillaries. Yet it is this
5 percent that is performing the ultimate functions of the
entire cardiovascular system: the supplying of nutrients and
the removal of metabolic end products and other cell secre-
tions. All other components of the system serve the over-
all function of getting adequate blood fl
ow through the
capillaries.
As British physiologist William Harvey reported in
1628, the cardiovascular system forms a closed loop, so that
blood pumped out of the heart through one set of vessels
returns to the heart by a different set. There are actually two
circuits (
Figure 12–2
), both originating and terminating
in the heart, which is divided longitudinally into two func-
tional halves. Each half of the heart contains two chambers:
an upper chamber—the
atrium
—and a lower chamber—the
ventricle.
The atrium on each side empties into the ventricle
on that side, but there is no direct blood fl ow between the two
atria or the two ventricles in the adult heart.
The
pulmonary circulation
includes blood pumped
from the right ventricle through the lungs and then to the
left atrium. It is then pumped through the
systemic circula-
tion
from the left ventricle through all the organs and tissues
of the body except the lungs, and then to the right atrium. In
both circuits, the vessels carrying blood away from the heart
are called
arteries,
and those carrying blood from body
organs and tissues back toward the heart are called
veins.
In the systemic circuit, blood leaves the left ventricle via
a single large artery, the
aorta
(see Figure 12–2). The arter-
ies of the systemic circulation branch off the aorta, divid-
ing into progressively smaller vessels. The smallest arteries
branch into
arterioles,
which branch into a huge number
(estimated at 10 billion) of very small vessels, the
capillaries,
which unite to form larger-diameter vessels, the
venules.
The
arterioles, capillaries, and venules are collectively termed the
microcirculation.
The venules in the systemic circulation then unite to form
larger vessels, the veins. The veins from the various peripheral
organs and tissues unite to produce two large veins, the
infe-
rior vena cava,
which collects blood from below the heart,
and the
superior vena cava,
which collects blood from above
the heart (for simplicity, these are depicted as a single vessel in
Figure 12–2). These two veins return the blood to the right
atrium.
The pulmonary circulation is composed of a similar cir-
cuit. Blood leaves the right ventricle via a single large artery,
the
pulmonary trunk,
which divides into the two
pulmo-
nary arteries,
one supplying the right lung and the other the
left. In the lungs, the arteries continue to branch, ultimately
forming capillaries that unite into venules and then veins.
The blood leaves the lungs via four
pulmonary veins,
which
empty into the left atrium.
As blood fl ows through the lung capillaries, it picks up
oxygen supplied to the lungs by breathing. Therefore, the blood
in the pulmonary veins, left side of the heart, and systemic
Lungs
Pulmonary
veins
Systemic
arteries
Systemic
veins
Pulmonary trunk
and arteries
Right
atrium
Vena
cava
Left
atrium
Aorta
Left
ventricle
Right
ventricle
Systemic
circulation
Pulmonary capillaries
Pulmonary
circulation
Systemic arterioles,
capillaries, and venules
in all organs and tissues
except the lungs
Figure 12–2
The systemic and pulmonary circulations. As depicted by the color
change from blue to red, blood becomes fully oxygenated (red) as
it fl ows through the lungs and then loses some oxygen (red to blue)
as it fl ows through the other organs and tissues. For simplicity,
the arteries and veins leaving and entering the heart are depicted
as single vessels; in reality, this is true for the arteries but there are
multiple pulmonary veins and two venae cavae (see Figure 12–6).
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