Page 754 - Fundamentals of anatomy physiology

Chapter 20 The Heart 741

Hormones on the heart rate: The faster the heart rate, the shorter is the time
available for filling. Venous return is variable over this period. It
Epinephrine, norepinephrine, and thyroid hormone increase varies in response to changes in cardiac output, blood volume,
heart rate by their effect on the SA node. The effects of epineph- patterns of peripheral circulation, skeletal muscle activity, and
rine on the SA node are similar to those of norepinephrine. other factors that affect the rate of blood flow back to the heart.
Epinephrine also affects the contractile cells. After massive sym- (We explore these factors in Chapter 21.)
pathetic stimulation of the adrenal medullae, the myocardium
may become so excitable that abnormal contractions occur. Preload. The degree of stretching in ventricular muscle cells
during ventricular diastole is called the preload. The preload is
Venous Return directly proportional to the EDV: The greater the EDV, the larger
the preload. Preload matters because it affects the ability of
In addition to its indirect effect on heart rate by the atrial reflex, ve- muscle cells to produce tension. As sarcomere length increases
nous return also directly affects nodal cells. When venous return past resting length, the amount of force produced during sys-
increases, the atria receive more blood and the walls are stretched. tole increases.
Stretching of the cardiac muscle fibers of the SA node leads to
more rapid depolarization and an increase in the heart rate. The amount of preload, and hence the degree of myocar-
dial stretching, varies with the demands on the heart. When you
Factors Affecting the Stroke Volume are standing at rest, your EDV is low. The ventricular muscle
is stretched very little, and the sarcomeres are relatively short.
The stroke volume is the difference between the end-diastolic During ventricular systole, the cardiac muscle cells develop
volume and the end-systolic volume. Changes in either EDV little power, and the ESV (the amount of blood in the ventricle
or ESV can change the stroke volume, and thus cardiac output. after contraction) is relatively high because the muscle cells
The factors involved in the regulation of stroke volume are in- contracted only a short distance. If you begin exercising, venous
dicated in Figure 20–23. return increases and more blood flows into your heart. Your
EDV increases, and the myocardium stretches further. As the
The EDV sarcomeres approach optimal lengths, the ventricular muscle
cells can contract more efficiently and produce more force-
Recall that the EDV is the amount of blood in a ventricle at the ful contractions. They also shorten more, and more blood is
end of diastole, just before a contraction begins. Two factors af- pumped out of your heart.
fect this volume: the filling time and the venous return. Filling
time is the duration of ventricular diastole. It depends entirely 20

Figure 20–23 Factors Affecting Stroke Volume. The arrows indicate the nature of the effects:

↑ 5 increases, ↓ 5 decreases.

Factors Affecting Stroke Volume (SV)

Venous return (VR) Filling time (FT) Increased by Decreased by Increased by E, NE,
VR = EDV FT = EDV sympathetic parasympathetic glucagon,
stimulation
VR = EDV FT = EDV stimulation thyroid hormones

Preload Contractility (Cont) Increased by Decreased by
End-diastolic of muscle cells vasoconstriction vasodilation
volume (EDV) Cont = ESV
Cont = ESV

End-systolic Afterload (AL)
volume (ESV) AL = ESV
AL = ESV

STROKE VOLUME (SV) ESV = SV
EDV = SV ESV = SV
EDV = SV