1 dr ismail ibrahim almustansirya college of

1 Dr. Ismail Ibrahim / Almustansirya college of /physiology department / C.V.S.
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Regulation of the cardiac output.
The regulation of cardiac output
depends on the control of both
heart rate and stroke volume (figure
7.23).
[A] 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 at the
end of diastole, and end systolic
volume ( ESV ), the volume of blood
remaining in a ventricle after it has
contracted.
Therefore
, SV = EDV – ESV.
Although many factors affect SV by altering EDV or ESV, the three most
important are preload , contractility , and afterload .
[1] Preload: Degree of stretch of heart
muscle (EDV) (intrinsic control): The law
states that the stroke volume of the heart
increases in response to an increase in the
volume of blood in the ventricles, before
contraction (the end diastolic volume),
when all other factors remain constant
This relationship between preload and
stroke volume is called the Frank-Starling law
of the heart (figure 7.24). the force of
contraction is directly proportional to the
initial length of muscle fiber. So that the
greater the stretch of the ventricle the greater
the contraction. As preload increases, this
results in more active tension by muscle fibers
and also results in a higher velocity of fiber
shortening. As sarcomere length increases, the
calcium sensitivity of troponin C rises, and this increases the amount of tension
developed by the muscle fiber
. Outputs from the left and right sides of the heart are also equalized as a result of the
Frank-Starling principle because the output of one side becomes the venous return of
the other side.
Figure 7.23: Factors affect cardiac output.
Figure 7.24: The Frank-Starling principle of
the relationship between SV and EDV.

1 Dr. Ismail Ibrahim / Almustansirya college of /physiology department / C.V.S.
the most important two factors that determine the degree of stretching of cardiac
muscle [ EDV] are the venous return and ventricular filling time .
 Venous return is the amount of blood returning to the heart and distending
its ventricles. It is an intrinsic control of stroke volume. Anything that
increases the volume or speed of venous return, increases EDV and
consequently SV. An example of that is exercise . Exercise speeds venous
return because both increased sympathetic nervous system activity (causes
venoconstriction and shifting more blood to the right atrium) and the
squeezing action of the skeletal muscles compress the veins (decreasing
the volume of blood they contain) . As we inhale, abdominal pressure
increases, squeezing local veins and forcing blood toward the heart. At the
same time, the pressure in the chest decreases, allowing thoracic veins to
expand and speeding blood entry into the right atrium.