The cardiovascular system consists of the heart and blood vessels. The heart contains three types of muscle fibers: nodal, conducting, and contractile. Nodal fibers initiate electrical impulses and are located in the sinoatrial and atrioventricular nodes. Conducting fibers transmit impulses through bundles like the bundle of His. Contractile fibers make up the bulk of the heart and contract in response to electrical signals. The heart has four main properties: rhythmicity, excitability, conductivity, and contractility. Rhythmicity allows the heart to beat regularly, while excitability, conductivity, and contractility enable electrical signals and contractions to propagate through the heart.

1. Cardiovascular System
The heart and blood vessels
Dr. Asaad Ahmed Elmileegy1

2. Cardiac Muscle Fibers
Dr. Asaad Ahmed Elmileegy2
The heart is
composed of 3
types of muscle
fibers;
1) Nodal muscle fibers.
2) Conducting muscle
fibers.
3) Contractile muscle
fibers.

3. 1) Nodal Fibers
Dr. Asaad Ahmed Elmileegy3
Sinoatrial
node (SAN)
Atrioventricular
node (AVN)
Site Posterior wall
of the right
atrium
immediately
beneath the
opening of the
S.V.C. (superior
vena cava)
Right side of the
interatrial septum
at the junction of
the atria and
ventricles.
Function It is the region
where cardiac
impulse arises
(pacemaker of
the heart)
It conducts the
impulse to the
atrio-ventricular
bundle (bundle
of His)

4. 2) Specialized conducting fibers
Dr. Asaad Ahmed Elmileegy4
A) Bundle of His (AV
bundle):
 It is continuous with the AV
node.
 It is the only connection
between the atria and
ventricles.
 It breaks into two branches:
right and left, that run down
the right and left sides of the
interventricular septum.
B) Purkinje network
(fibers):
 These are the terminal fine
branches of the bundle system
which blend with the ordinary
ventricular muscle fibers.

5. Cardiac Valves
Dr. Asaad Ahmed Elmileegy5
 Atrioventricular valves:
 Tricuspid valve
 Mitral valve
 Semilunar valves:
 Pulmonary valve
 Aortic valve

6. Properties of Cardiac Muscle
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4 basic properties:
I- Rhythmicity
II- Excitability
Ill- Conductivity
IV- Contractility

7. I- Rhythmicity
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1-Def:
 It is the ability of the heart to beat
regularly.
2-Origin or Nature:
 It is an inherent property of the cardiac
muscle i.e. myogenic in origin, not
neurogenic.
3-Rate:
 SAN: 110 /min.
 AVN: 90 /min.
 Bundle tissues: 45 /min
 Purkinje fibers: 35 /min
 Ventricles: 25 /min
4-Mechansim of self-excitation
of SAN:
 A natural leakiness of the SAN cell-
membrane to sodium ions causes a
low resting membrane potential (-55
millivolts), and causes self-excitation
of SAN fibers.

8. Factors affecting rhythmicity:
Dr. Asaad Ahmed Elmileegy8
I- Nervous factors
1- Parasympathetic (vagus n.):
 –ve chronotropic effect.
 The basal level of vagal
discharge i.e. vagal tone,
inhibits the sinus rhythm from
110 to 75 /min.
 Mild vagal stimulation inhibits
the sinus rhythm while strong
stimulation can stop the SAN
rhythm, but not the ventricular
rhythm because the vagus
does not supply the
ventricles.
2- Sympathetic:
 Sympathetic stimulation has a
+ve chronotropic effect

9. Dr. Asaad Ahmed Elmileegy9
II- Physical factors:
 Moderate warming: increases rhythmicity
 Increases the speed of ionic fluxes across the membrane
during the action potential.
 This explains tachycardia in fevers
 Moderate cooling: decreases rhythmicity.
 Excessive warming (45oC) causes cardiac
damage.
 Excessive cooling depresses the intracellular
metabolism and stops rhythmicity.

10. Dr. Asaad Ahmed Elmileegy10
III. Chemical factors:
 Catecholamines have +ve chronotropic effect (as
sympathetic).
 Thyroxine has a +ve chronotropic effect as it
stimulates the metabolism of the SAN.
 Cholinergic drugs (as methacholine) have –ve
chronotropic effect (as parasympathetic).
 Digitalis depresses nodal tissue.

11. II- EXCITABILITY
Dr. Asaad Ahmed Elmileegy11
Definition:
 The ability of the
cardiac muscle to
respond to a stimulus

12. Excitability changes during action potential:
Dr. Asaad Ahmed Elmileegy12
Absolute
refractory
period (ARP)
Relative
refractory
period
(RRP)
Supernormal
phase
Excitability Completely
abolished
Gradually
recovers
Rise above
normal
Stimulus
needed to
excite
No response,
whatever the
strength of
the stimulus
Strong
stimulus 
weaker
systole
Weaker stimulus
 Strong
systole
Coincide
with:
- Systole. - 1st half of
diastole.
- 2nd half of
diastole.
Importance Prevent
tetanus
(continuous
contraction)
- Stimuli in SNP
 extrasystole

13. III- CONDUCTIVITY
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Def:
 The ability of cardiac
muscle to conduct
excitation wave from
one part of the heart to
another.

14. Dr. Asaad Ahmed Elmileegy14
A- Transmission of cardiac impulse through the atria:
 The impulse arise from SAN is propagated to the AVN and the atria
 Velocity of conduction: 1 meter/second.
B- Transmission of cardiac impulse in AV node:
 A delay in the conduction of the excitation wave occurs at AV node.
 Velocity of conduction: 0.05 meter/second
 Significance of the slow conductivity in the AVN:
 Allows sufficient time for the atria to empty their blood into the
ventricles before ventricular contraction begins.
 Protects the ventricles from pathological high atrial rhythms.

15. Dr. Asaad Ahmed Elmileegy15
C-Transmission of cardiac
impulse in Purkinje fibers:
 Transmit impulses into the
ventricles.
 Velocity of conduction: 4.0
m/sec.
 The rapid transmission of
action potential by Purkinje
fibers is caused by high level
of the gap junctions.
D- Transmission of cardiac
impulse in ventricular
muscles:
 Velocity of conduction: 0.4 -
meter/second.

16. IV- CONTRACTILITY
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Def:
 The ability of the muscles to convert the potential
energy of fuel into mechanical energy or work.

17. Starling's law of cardiac muscle:
Dr. Asaad Ahmed Elmileegy17
 States that: “Within certain limit,
the greater the initial length of the
cardiac muscle fiber, the greater
the force of contraction”.
 The initial length of cardiac muscle
is determined by the degree of
diastolic filling i.e. end diastolic
volume (EDV).
 Excess venous return (e.g. during
muscle exercise) increases the
initial length of muscle (EDV) 
increase the force of ventricular
contraction (prevents stagnation of
blood in venous side)
 Mechanism: myogenic.
 Overstretching of the muscle
causes marked decrease in
contractility

18. Dr. Asaad Ahmed Elmileegy18