Page 351 - Anatomy and Physiology of Farm Animals, 8th Edition

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(ventricular). The first sound marks the the same. If these are not equal, blood
tends to accumulate in either the systemic
beginning of systole, and the second sound
VetBooks.ir marks the beginning of diastole (Fig. 18‐3). or pulmonary circulation.

Systole
Diastole
As the ventricles begin their contraction, At the beginning of diastole, both the semi­
the pressure within the ventricle increases. lunar and A‐V valves are closed, so the initial
Almost immediately, the pressure within phase is termed isovolumetric relaxation.
each ventricle exceeds the pressure within When ventricular relaxation reaches the
their respective atria, and the pressure dif­ point that atrial blood pressures exceed ven­
ferences force the atrioventricular (A‐V) tricular blood pressures, the pressure differ­
valves closed. The first heart sound is asso­ ences open the A‐V valves. While the A‐V
ciated with closure of the right and left valves are closed during systole and early
A‐V valves (Fig. 18‐3). diastole, blood continues to flow into the
The ventricles continue to contract and
pressure continues to increase during the right and left atria from the systemic and
pulmonary circulations, respectively. The
early part of systole. At this point in the accumulation of blood within the atria
cycle, all four heart valves are closed, and all increases atrial blood pressure. When A‐V
remain closed until pressure in the ventri­ valves open, much of the accumulated blood
cles exceeds that in the arterial vessels that flows rapidly into the ventricles. Most ven­
they supply (aorta for left ventricle and pul­ tricular filling occurs during this period
monary trunk for right ventricle). The prior to any atrial contraction (Fig. 18‐3).
period of systole during which all valves are Blood continues to flow into the atria
closed is the isovolumetric contraction throughout diastole, and because the A‐V
period, because during it the volume of valves are open, blood flows directly
each ventricle remains constant (Fig. 18‐3). through the atria into the ventricles. As
When ventricular pressures exceed those
in their respective arterial vessels, the semi­ mentioned earlier, diastole is the phase of
the cardiac cycle that lengthens most with
lunar valves open to permit ejection of slow heart rates, so slow heart rates pro­
blood. There is no sound associated with the vide a long period for ventricular filling.
opening of the semilunar valves. An initial Atrial contraction (atrial systole) occurs
rapid ejection phase of systole is followed by during ventricular diastole, forcing an
a reduced ejection phase, during which ven­ additional volume of blood into the ventri­
tricular and arterial pressures fall (Fig. 18‐3). cles, but this amount is relatively small
The elasticity of the aorta and pulmo­
nary trunk maintains blood pressure in (perhaps 15%) compared to the volume
already in the ventricles.
these vessels even though the ventricles The volume of blood in each ventricle at
begin to relax. When the blood pressures in
these vessels are greater than the pressures the end of diastole is the end‐diastolic
volume (EDV). During systole, each ven­
in their associated ventricles, the pressure tricle ejects only a percentage of its EDV
differences close the semilunar valves. The (Fig. 18‐2), typically 40 to 60%. The per­
second heart sound is associated with centage of the EDV that is ejected is the
closure of the aortic and pulmonary valves, ejection fraction.
and this sound is used to mark the end of
systole (ventricular) and the beginning of
diastole (ventricular) (Fig. 18‐3). Heart Sounds and Murmurs
Stroke volume is the volume of blood
ejected from each ventricle during a single The first and second heart sounds are asso­
cardiac cycle. Normally, the stroke vol­ ciated with valve closures, but turbulent
umes for the right and left ventricles are blood flow and vibrations of large vessels