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Cardiac Output, Venous Return
and Their Regulation
It is the quantity of blood pumped into the aorta
each minute by the heart or the quantity of blood that flows
through the circulation each minute.
Cardiac output = Heart Rate Stroke Volume
It is the quantity of blood flowing from the veins into
the right atrium each minute.
Venous Return = Cardiac Output
Increase in Cardiac Output
The factors which increases the
cardiac output are:
Mean Systematic Filling Pressure
Size of the body (Cardiac
Role Of Frank-Starling Mechanism
It states that “ Within physiologic limits, the
heart pumps all the blood that returns to it by the
way of veins.”
Increased Venous Return
Cardiac muscles stretches to greater length
Ventricular muscle contracts with greater force
Increased Cardiac Output
Increased heart volume stretches muscles and
causes stronger contraction.
Stretch increases heart rate as well.
Direct effect on rhythmicity of the node to increase
heart rate as much as 10-15%.
It gives reflexes to the vasomotor center and then
back to the heart by the way of sympathetic nerves and
vagi, increases the heart rate.
Cardiac Output Regulation
The venous return to the
heart is the sum of all the
local blood flows through
all the individual tissue
segments of the peripheral
At each increasing level of
work output during
exercise, the oxygen
consumption and the
cardiac output increase in
parallel to each other.
To summarize, cardiac output is
determined by the sum of all the various
factors throughout the body that control
local blood flow. All the local blood flows
summate to form the venous return, and
the heart automatically pumps this
returning blood back into the arteries to
flow around the system again.
Relation of Cardiac Output
and Stroke Volume
Volume of blood that is
ejected by each ventricles during
systole is called Stroke Volume
As the stroke volume increases the
cardiac output also increases.
Stroke Volume depends upon
1. End diastolic Volume
Limitations For The Cardiac Output
There are definite limits to the amount of blood that
the heart can pump, which is expressed quantitatively
in the form of cardiac output curves.
The plateau level of this normal cardiac
output curve is about 13 L/min.
The lowermost curves are for hypoeffective
hearts that are pumping at levels below normal.
Hypereffective heart plateau – 5 L/min
The uppermost curves are for
Hypereffective hearts that are pumping better
than normal. Hypoeffective heart plateau – 20
In this the heart is pumping at a very slow
rate and the cardiac output decreases.
Factors that can cause hypoeffectivity are following:
1. Increased Arterial Pressure
2. Inhibition of Nervous Excitation
3. Abnormal Heart Rhythm
4. Coronary Artery Blockage
5. Valvular Heart Disease
6. Congenital Heart Disease
8. Cardiac Hypoxia
In this the heart is pumping at a
high rate and the cardiac output
Two type of factors can make the heart a
better pump than normal:
1. Nervous Stimulation:
It involves Sympathetic Stimulation and
2. Hypertrophy of The Heart
It involves the increased mass and
contractile strength due to exercise and
causing hypertrophy which allows increased
Pathologically High or Low
In healthy humans, the
average Cardiac Outputs are
surprisingly constant from one
person to another. However.
Multiple clinical abnormalities
can cause either high or low
High Cardiac Output
High cardiac output is mostly caused by
reduced total peripheral Resistance.
Following are some of the conditions that
can decrease the Peripheral Resistance
and at the same time increase the Cardiac
Output to above the normal.
Low Cardiac Output
There are two
factors that cause
that cause the
Venous Return to
fall too low
effectiveness of the
Heart to fall too low
A. Cardiac Factors:
Whenever the heart becomes severely
damaged, regardless of the cause, its limited level of
pumping may fall below that needed for adequate
blood flow to the tissues. Some examples of this include:
1. Severe Coronary Blood
2. Myocardial Infarction
3. Severe Valvular Heart Disease
5. Cardiac Temponade
6. Cardiac Metabolic Derangements
B. Decreased Venous Return:
Anything that interferes with venous return also can
lead to decreased cardiac output. Some of these factors
are the following:
1.Decreased Blood Volume
2. Acute Venous Dilation
3. Obstruction of the Large Veins
4. Decreased Muscle Mass
5. Decreased Metabolic Rate
of the Tissues