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An Outline of Cardiovascular Structure and Function
1-5
Brachiocephalic
(innominate)
artery
Aortic
arch
Left
common carotid artery
Left
subclavian artery
Descending aorta
(No valves)
Pulmonary
Veins (4)
Left
atrium
Bicuspid
valve
Direction of
blood flow
Interventricular
septum
Inferior
vena cava
(eustachian valve)
Right ventricle
Pulmonary
semi-lunar valve
Right atrium
Pulmonary artery
Tricuspid valve
Aortic semilunar
valve
Sinoatrial node
Superior vena cava
(no valve)
Ascending
aorta
Base of the heart
Atrioventricular
node
Left
ventricle
Thoracic aorta
Apex of
heart
FIGURE 1.1
Anterior view of the human heart showing the four chambers, the inlet and outlet valves, the inlet and
outlet major blood vessels, the wall separating the right side from the left side, and the two cardiac pacing centers —
the
sinoatrial node and the
atrioventricular node. Boldface arrows show the direction of flow through the heart
chambers, the valves, and the major vessels.
blood through the pulmonary circulation, so it functions as a low-pressure (
P
≤
40 mmHg gauge) pump
compared with the left side of the heart, which does most of its work at a high pressure (up to 140 mmHg
gauge or more) to drive blood through the entire systemic circulation to the furthest extremes of
the organism.
Each cardiac (heart) pump is further divided into two chambers: a small upper receiving chamber,
or atrium (auricle), separated by a one-way valve from a lower discharging chamber, or ventricle, which
is about twice the size of its corresponding atrium. In order of size, the somewhat spherically shaped
left atrium is the smallest chamber — holding about 45 ml of blood (at rest), operating at pressures
on the order of 0 to 25 mmHg gauge, and having a wall thickness of about 3 mm. The pouch-shaped
right atrium is next (63 ml of blood, 0 to 10 mmHg gauge of pressure, 2-mm wall thickness), followed
by the conical/cylindrically shaped left ventricle (100 ml of blood, up to 140 mmHg gauge of pressure,
variable wall thickness up to 12 mm) and the crescent-shaped right ventricle (about 130 ml of blood,
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1-6
Biomedical Engineering Fundamentals
up to 40 mmHg gauge of pressure, and a wall thickness on the order of one-third that of the left ventricle,
up to about 4 mm). All together, then, the heart chambers collectively have a capacity of some 325
to 350 ml, or about 6.5% of the total blood volume in a “typical” individual — but these values are
nominal, since the organ alternately fills and expands, contracts, and then empties as it generates a
cardiac
output.
During the 480-msec or so filling phase — diastole — of the average 750-msec cardiac cycle, the
inlet valves of the two ventricles (3.8-cm-diameter tricuspid valve from right atrium to right vent-
ricle; 3.1-cm-diameter bicuspid or mitral valve from left atrium to left ventricle) are open, and the
outlet valves (2.4-cm-diameter pulmonary valve and 2.25-cm-diameter aortic semilunar valve, respect-
ively) are closed — the heart ultimately expanding to its end-diastolic-volume (EDV), which is on the
order of 140 ml of blood for the left ventricle. During the 270-msec emptying phase — systole —
electrically induced vigorous contraction of
cardiac muscle drives the intraventricular pressure up, for-
cing the one-way inlet valves closed and the unidirectional outlet valves open as the heart contracts
to its end-systolic-volume (ESV), which is typically on the order of 70 ml of blood for the left vent-
ricle. Thus the ventricles normally empty about half their contained volume with each heart beat, the
remainder being termed the
cardiac reserve volume. More generally, the difference between the
actual
EDV and the
actual ESV, called the
stroke volume (SV), is the volume of blood expelled from the heart
during each systolic interval, and the ratio of SV to EDV is called the
cardiac ejection fraction, or
ejec-
tion ratio (0.5–0.75 is normal, 0.4–0.5 signifies mild cardiac damage, 0.25–0.40 implies moderate heart
damage, and <0.25 warms of severe damage to the heart’s pumping ability). If the stroke volume is
multiplied by the number of systolic intervals per minute, or heart (HR), one obtains the total cardiac
output (CO):
CO
=
HR
×
(
EDV
−
ESV
)
(1.1)
Dawson [1991] has suggested that the cardiac output (in milliliters per minute) is proportional to the
weight
W (in kilograms) of an individual according to the equation
CO
−
224
W
3
/
4
(1.2)
and that “normal” heart rate obeys very closely the relation
HR
=
229
W
−
1
/
4
(1.3)
For a “typical” 68.7-kg individual (blood volume = 5200 ml), Equation 1.1, Equation 1.2, and Equa-
tion 1.3 yield CO
=
5345 ml/min, HR
=
80 beats/min (cardiac cycle period
=
754 msec) and
SV
=
CO/HR
=
224
W
3
/
4
/
229
W
−
1
/
4
=
0.978
W
=
67.2 ml/beat, which are very reasonable values.
Furthermore, assuming this individual lives about 75 years, his or her heart will have cycled over 3.1536
billion times, pumping a total of 0.2107 billion liters of blood (55.665 million gallons, or 8134 quarts per
day) — all of it emptying into the circulatory pathways that constitute the vascular system.
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