4
CARDIOVASCULAR PHYSIOLOGY CONCEPTS
THE FUNCTIONS OF THE HEART
AND BLOOD VESSELS
Heart
The heart sometimes is thought of as an organ
that pumps blood through the organs of the
body. While this is true, it is more accurate to
view the heart as a pump that receives blood
from venous blood vessels at a low pressure,
imparts energy to the blood (raises it to a
higher pressure) by contracting around the
blood within the cardiac chambers, and then
ejects the blood into the arterial blood vessels.
It is important to understand that organ
blood flow is not driven by the output of the
heart per se, but rather by the pressure gen-
erated within the arterial system as the heart
pumps
blood
into
the
vasculature, which
serves as a resistance network.
Organ blood flow
is determined by the arterial pressure minus the
venous pressure, divided by the vascular resistance
of the organ
(see Chapters 5 and 7). Pressures
in the cardiovascular system are expressed in
millimeters of mercury (mm Hg) above atmos-
pheric pressure. One millimeter of mercury is
the pressure exerted by a 1-mm vertical col-
umn of mercury (1 mm Hg is the equivalent
of 1.36 cm H2O hydrostatic pressure). Vascular
resistance is determined by the size of blood
vessels, the anatomical arrangement of the vas-
cular network, and the viscosity of the blood
flowing within the vasculature.
The right atrium receives systemic venous
blood (venous return) at very low pressures
(near 0 mm Hg) (Fig. 1.3). This venous return
then passes through the right atrium and fills
the right ventricle; atrial contraction also
contributes to the ventricular filling. Right
ventricular
contraction
ejects
blood
from
the right ventricle into the pulmonary artery.
This generates a maximal pressure (systolic
pressure) that ranges from 20 to 30 mm Hg
within the pulmonary artery. As the blood
passes through the pulmonary circulation, the
blood pressure falls to about 10 mm Hg. The
left atrium receives the pulmonary venous
blood, which then flows passively into the
left ventricle; atrial contraction provides a
small amount of additional filling of the left
■ FIGURE 1.3 Blood flow within the heart. Venous
blood returns to the right atrium (
RA
) via the supe-
rior
(SVC)
and inferior vena cava (/VC). Blood passes
from the RA into the right ventricle
(RV),
which
ejects the blood into the pulmonary artery
(PA).
After passing through the lungs, the blood flows
into the left atrium
(LA)
and then fills the left ventri-
cle
(LV),
which ejects the blood into the aorta (Ao)
for distribution to the different organs of the body.
ventricle. As the left ventricle contracts and
ejects blood into the systemic arterial system,
a relatively high pressure is generated (100
to 140 mm Hg maximal or systolic pressure).
Therefore,
the left ventricle is a high-pressure
pump, in contrast to the right ventricle, which is
a low-pressure pump
. Details of the pumping
action of the heart are found in Chapter 4.
The
pumping
activity
of the
heart is
usually expressed in terms of its cardiac out-
put, which is the amount of blood ejected
with each contraction (i.e., stroke volume)
multiplied by the heart rate. Any factor that
alters heart rate or stroke volume will alter the
cardiac output. The heart rate is determined
by specialized cells within the heart that act
as electrical pacemakers, and their activity is
increased or decreased by autonomic nerves
and hormones (see Chapter 2). The action
potentials
generated
by
these
pacemaker
cells are conducted throughout the heart and
trigger contraction of cardiac myocytes (see
Chapter 3). This results in ventricular con-
traction and ejection of blood. The force of
previous page 17 Cardiovascular Physiology Concepts  2nd Edition read online next page 19 Cardiovascular Physiology Concepts  2nd Edition read online Home Toggle text on/off