564
UNIT FOUR
even lower than venous pressure. As a result, blood fl
ows
into the atria from the large, attached veins. That is, as the
ventricles are contracting, the atria are F lling, already pre-
paring for the next cardiac cycle
(f
g. 15.16)
.
As ventricular systole progresses, ventricular pressure
continues to increase until it exceeds the pressure in the pul-
monary trunk (right side) and aorta (left side). At this point,
the pressure differences across the semilunar valves open
the pulmonary and aortic valves, and blood is ejected from
each valve’s ventricle into these arteries.
As blood fl ows out of the ventricles, ventricular pres-
sure begins to drop, and it falls even farther as the ventricles
relax. When ventricular pressure is lower than the blood
pressure in the aorta and pulmonary trunk, the pressure dif-
ference is reversed, and the semilunar valves close. The ven-
tricles continue to relax, and as soon as ventricular pressure
is less than atrial pressure, the AV valves open, and the ven-
tricles begin to F
ll once more. Atria and ventricles relax for a
brief interval.
Heart Sounds
A heartbeat heard through a stethoscope sounds like “lubb-
dupp.” These sounds are due to vibrations in the heart tis-
sues produced as the blood fl ow is suddenly slowed with the
contraction and relaxation of the heart chambers and with
the closing of the valves.
The F rst part of a heart sound (
lubb
) occurs during the
ventricular systole, when the AV valves are closing. The sec-
ond part (
dupp
) occurs during ventricular diastole, when the
pulmonary and aortic valves are closing.
Sometimes during inspiration, the interval between
the closure of the pulmonary and the aortic valves is long
enough that a sound comes from each of these events. In
this case, the second heart sound is said to be
split.
Heart sounds are of particular interest because they
can indicate the condition of the heart valves. ±or exam-
ple, infl
ammation of the endocardium (endocarditis) may
PRACTICE
12
Which structures make up the skeleton of the heart?
13
Review the path of blood through the heart.
14
How does blood composition diF
er in the right and left ventricle?
15
Which vessels supply blood to the myocardium?
16
How does blood return from the cardiac tissues to the right atrium?
15.3
HEART ACTIONS
The heart chambers function in coordinated fashion. Their
actions are regulated so that atria contract, called atrial
sys-
tole
(sis
to-le), while ventricles relax, called ventricular
dias-
tole
(di-as
to-le); then ventricles contract (ventricular systole)
while atria relax (atrial diastole). Then the atria and ventricles
both relax for a brief interval. This series of events constitutes
a complete heartbeat, or
cardiac cycle
(kar
de-ak si
kl).
Cardiac Cycle
During a cardiac cycle, the pressure in the heart chambers
rises and falls. These changes open and close the valves,
much like a door being blown open or closed by the wind.
Pressure in the ventricles is low early in diastole, and the
pressure difference between atria and ventricles opens the
AV valves. The ventricles F ll. About 70% of the returning
blood enters the ventricles prior to contraction, and ventric-
ular pressure gradually increases. During atrial systole, the
remaining 30% of returning blood is pushed into the ven-
tricles, and ventricular pressure increases. Then, as the ven-
tricles contract, ventricular pressure rises sharply. As soon
as the ventricular pressure exceeds atrial pressure, the AV
valves close. At the same time, the papillary muscles con-
tract. By pulling on the chordae tendineae, they prevent the
cusps of the AV valves from bulging too far into the atria.
During ventricular systole, the AV valves remain closed.
The atria are now relaxed, and pressure in the atria is low,
Pulmonary
valve closed
Tricuspid
and mitral
valves open
(a)
(b)
Aortic
valve closed
Atrial systole
LA
LV
RV
RA
Ventricular
diastole
Pulmonary
valve open
Tricuspid
and mitral
valves closed
Aortic
valve open
Atrial diastole
Ventricular
systole
FIGURE 15.16
The atria (
a
) empty during atrial systole and (
b
) ±
ll with blood during atrial diastole.
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