Cardiac output is the volume of blood pumped by each ventricle per minute. It is calculated as stroke volume multiplied by heart rate. Normal cardiac output is 5 liters per minute. Cardiac output is regulated by factors that influence stroke volume and heart rate. Stroke volume depends on end diastolic volume and end systolic volume. Heart rate is controlled by the autonomic nervous system, including the parasympathetic and sympathetic nerves, as well as the vasomotor center in the medulla. Parasympathetic stimulation decreases heart rate while sympathetic stimulation increases it.

1. Cardiac Output
Maryam Fida (o-1827)
Source: The Guyton and Hall Physiology

2. Cardiac Output
 The volume of blood pumped by each ventricle per
minute is called cardiac output
 Cardiac output = Stroke Volume X Heart Rate
 Normal value = 5 Liters /Minute

3. Regulation of Cardiac Output
 Cardiac output = Stroke Volume X Heart Rate
 The factors which regulate stroke volume and Heart
rate are basically regulating Cardiac output

4. Factors Regulating Stroke
Volume
 Volume of blood ejected by each ventricle in single systole; Normal Value
= 70 ml/beat
 Stroke Volume = End diastolic Volume – End Systolic Volume
 So stroke volume is mainly controlled by
 EDV
 ESV

5. Factors Regulating EDV
 VENOUS RETURN: What ever blood volume returns to the heart,
same is pumped forward through the Frank’s Starlings Law.
According to this law 13- 15 liters of blood volume can be pumped
out without cardiac stimulation.
 DURATION OF DIASTOLE OR FILLING TIME: ventricular filling occurs
during diastole, so there must be adequate ventricular filling time.
 DISTENSIBILITY OF THE VENTRICLES: Normally ventricles are
distensible to accommodate adequate blood volume. Infarction
decreases the distensibility which decreases the EDV.
 ATRIAL CONTRACTION: There must be adequate atrial contraction
to have adequate EDV. If atrial function is not adequate then EDV
will decrease.

6. Factors Regulating ESV
 E.S.V is basically CONTROLLED BY MYOCARDIAL CONTRACTION
 FORCE OF MYOCARDIAL CONTRACTION: It depends upon the
initial length of muscle fibers according to frank’s starlings
law.
 PRELOAD: The effect of EDV on initial length is called
preload. So EDV also effects the ESV.
 AFTER LOAD: Force of contraction is also dependant upon the
resistance against which the ventricles have to pump
 CONDITION OF THE MYOCARDIUM : It also effects the force of
contraction.
 AUTONOMIC NERVES : Sympathetic stimulation increases and
parasympathetic stimulation decreases force of contraction
 HORMONES: Catecholamines, thyroxine, glucagon, digitalis,
calcium, increased temp, caffeine, theophyline increase the
force.
 Force decreases by hypoxia, acidosis, barniturates,
procainamide and quinidine decrease the force of contraction.

7. Factors regulating Heart Rate
 Number of Heart Beats per minute is called Heart Rate.
 Normal Value = 70-80 beats / minute
 Generally with increase in heart rate, C.O increases.
 But this is upto 150 beats / minute.
 In the range of 150 – 180 beats / min there is no increase in C.O
with increased HR.
 When heart rate is increased above 180 beats / min there is
decrease in C.O because diastole is not adequate for filling.

8. Factors regulating Heart Rate
 Basically the heart rate is autonomous.
 In the body heart rate is modified by nervous mechanisms which
include the autonomic nerves and vasomotor centre.
 We can also include the afferent nerves which carry impulses to
vasomotor centre.
 Sympathetic stimualtion increases heart rate
 Parasympatehtic stimulation decreases heart rate

9. Factors Regulating Heart Rate
 In the body heart rate is modified by nervous mechanisms
which include the autonomic nerves and vasomotor centre.
We can also include the afferent nerves which carry
impulses to vasomotor centre.
 So heart rate is controlled by
 Parasympathetic nerves – Vagi
 Sympathetic nerves – Sympathetic chain
 Vasomotor centre

11. Effects of Parasympathetic
stimulation on Heart Rate
 Vagi i.e. right vagus mailnly controls the SA node and left vagus
mainly controls the AV node.
 Vagal fibers arise form Dorsal nucleus of Vagus and supply the atrial
muscle, SA NODE, AV NODE PURKINGE FIBER but not ventricular
muscle.
 PARASYMPATHETIC STIMULATION leads to :

12. Effects of Parasympathetic
stimulation on Heart Rate
 NEGATIVE CHRONOTROPIC EFFECT: decrease frequency of
heart beat
 NEGATIVE DROMOTROPIC EFFECT: Velocity of conduction is
slowed down. AV nodal delay is prolonged.
 NEGATIVE BATHMOTROPIC EFFECT: Decreased excitability of
heart.
 SLIGHT NEGATIVE IONOTROPIC EFFECT: decrease force of
contraction of heart.

13. Effects of Parasympathetic
stimulation on Heart Rate
 The effect of vagi on the heart is continuous. This is called the vagal
tone. Vagi don’t allow the heart rate to increase.
 Vagal escape: When vagi are strongly stimulated, heart stops beating
but when strong vagal stimulation is continued, heart starts beating
again called vagal escape. The possible mechanisms of vagal escape
are:
 Exhaustion of acetylcholine stores
 Cardiac muscle becomes refractory to the action of acetyl choline
 Heart stops beating, blood pressure falls which stimulates the
baroreceptor reflex.
 Idioventricular rhythm, a focus in the ventricle starts the generation of
impulses.

14. Effects of Sympathetic
stimulation on Heart Rate
 Sympathetic nerves are called accelerator nerves.
 Effect of sympathetic is also continuous but not as
powerful as vagal tone.
 Preganglionic sympathetic fibers arise form T1 to T5.
These fibers go to the inferior cervical ganglion. And then
supply the heart.
 Sympathetic supply is mainly to ventricle musculature.
 POSITIVE IONOTROPIC EFFECT
 POSITIVE DROMTROPIC EFFECT
 POSITIVE BATHMOTROPIC EFFECT
 POSITIVE CHRONOTROPIC EFFECT

15. Vasomotor Centre
 The vasomotor center is a portion of the medulla oblongata that
regulates blood pressure and other homeostatic processes.
 The actions of the sympathetic and parasympathetic nerves is
coordinated by vasomotor centre.
 The medial part of vasomotor centre is cardioinhibitory through the
vagi supplying the heart
 The lateral part is cardioacceleratory through the sympathetic nerve
supplying the heart.
 The vasomotor centre is affected by impulses from different parts of
the heart to modify the heart rate.