682
UNIT 4
Maintenance of the Body
18
a normal heart, the higher the preload, the higher the stroke
volume will be. Tis relationship between preload and stroke
volume is called the
Frank-Starling law of the heart
. Recall that
at an
optimal length
of muscle fibers (and sarcomeres) (1) the
maximum number of active cross bridge attachments is possible
between actin and myosin, and (2) the force of contraction is
maximal (see Figure 9.22, p. 302). Cardiac muscle, like skeletal
muscle, exhibits a
length-tension relationship
.
Resting skeletal muscle fibers are kept near optimal length
for developing maximal tension while resting cardiac cells are
normally
shorter
than optimal length. As a result, stretching car-
diac cells can produce dramatic increases in contractile force.
Te most important factor stretching cardiac muscle is
venous
return
, the amount of blood returning to the heart and distend-
ing its ventricles.
Anything that increases the volume or speed of venous re-
turn, such as a slow heart rate or exercise, increases EDV and,
consequently, SV and contraction force (Figure 18.22). A slow
heartbeat allows more time for ventricular filling. Exercise
speeds venous return because both increased sympathetic ner-
vous system activity and the squeezing action of the skeletal
muscles compress the veins, decreasing the volume of blood
they contain and returning more blood to the heart. During
vigorous exercise, SV may double as a result of increased venous
return. Conversely, low venous return, such as might result from
severe blood loss or an extremely rapid heart rate, decreases
EDV, causing the heart to beat less forcefully and lowering SV.
Because the systemic and pulmonary circulations are in se-
ries, the intrinsic mechanism we just described ensures equal
outputs of the two ventricles and proper distribution of blood
volume between the two circuits. If one side of the heart suddenly
and heart rate are regulated. As you read the next sections, refer
to
Figure 18.22
for an overview of the factors that affect stroke
volume and heart rate, and consequently, cardiac output.
Regulation of Stroke Volume
Mathematically, stroke volume (SV) represents the difference
between
end diastolic volume (EDV)
, the amount of blood that
collects in a ventricle during diastole, and
end systolic volume
(ESV)
, the volume of blood remaining in a ventricle
afer
it has
contracted. Te EDV, determined by how long ventricular dias-
tole lasts and by venous pressure, is normally about 120 ml. (An
increase in either factor
raises
EDV.) Te ESV, determined by
arterial blood pressure and the force of ventricular contraction,
is approximately 50 ml. (Te higher the arterial blood pressure,
the higher the ESV.) ±o figure normal stroke volume, simply
plug these values into this equation:
As you can see, each ventricle pumps out about 70 ml of
blood with each beat, which is about 60% of the blood in its
chambers.
So what is important here—how do we make sense out of this
alphabet soup (SV, ESV, EDV)? Although many factors affect SV
by altering EDV or ESV, the three most important are
preload,
contractility
, and
aferload
. As we describe in detail next, preload
affects EDV, whereas contractility and a²erload affect the ESV.
Preload: Degree of Stretch of Heart Muscle
Te degree to
which cardiac muscle cells are stretched just before they con-
tract, which is called the
preload
, controls stroke volume. In
Venous
return
Contractility
Sympathetic
activity
Parasympathetic
activity
EDV
(preload)
Stroke
volume
Heart
rate
Cardiac
output
ESV
Exercise (by
sympathetic activity,
skeletal muscle and
respiratory pumps;
see Chapter 19)
Heart rate
(allows more
time for
ventricular
filling)
Bloodborne
epinephrine,
thyroxine,
excess Ca
2
+
Exercise,
fright, anxiety
Initial stimulus
Result
Physiological response
Figure 18.22
Factors involved in determining cardiac output.
SV
5
EDV
2
ESV
5
120 ml
2
50 ml
5
70 ml
beat
beat
beat
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