3. The Conducting System
• Heartbeat
– A single contraction of the heart
– The entire heart contracts in series
• First the atria
• Then the ventricles
4. The Conducting System
• Structures of the Conducting System
– Sinoatrial (SA) node - wall of right atrium
– Atrioventricular (AV) node - junction between
atria and ventricles
– Conducting cells - throughout myocardium
5. The Conducting System
• Conducting Cells
– Interconnect SA and AV nodes
– Distribute stimulus through myocardium
– In the atrium
• Internodal pathways
– In the ventricles
• AV bundle and the bundle branches
7. The Conducting System
• Heart Rate
– SA node generates 80–100 action potentials per
minute
– AV node generates 40–60 action potentials per
minute
8. The Conducting System
• The Sinoatrial (SA) Node
– In posterior wall of right atrium
– Contains pacemaker cells
– Connected to AV node by internodal pathways
– Begins atrial activation (Step 1)
10. The Conducting System
• The Atrioventricular (AV) Node
– In floor of right atrium
– Receives impulse from SA node (Step 2)
– Delays impulse (Step 3)
– Atrial contraction begins
13. The Conducting System
• The AV Bundle
– In the septum
– Carries impulse to left and right bundle branches
• Which conduct to Purkinje fibers (Step 4)
– And to the moderator band
• Which conducts to papillary muscles
18. Cardiac Cycle
The cardiac events that occur from the
beginning of one heartbeat to the beginning
of the next are called the cardiac cycle.
19. • The cardiac cycle consists:
– Period of relaxation - diastole,
• during which the heart fills with blood,
– Followed by a period of contraction called systole.
• during which heart ejects blood
20. Cardiac Cycle
• During systole there is contraction of the cardiac
muscle and pumping of blood from the heart
through arteries.
• During diastole, there is relaxation of cardiac muscle
and filling of blood.
• Various changes occur in different chambers of heart
during each heartbeat. These changes are repeated
during every heartbeat in a cyclic manner.
• Each cycle is initiated by spontaneous generation of
action potential in the S.A node.
21. Divisions of cardiac cycle
• The cardiac cycle consists of a period of relaxation
called diastole, during which the heart fills with blood
this period is followed by a period of contraction called
systole.
• The contraction and relaxation of atria are called atrial
systole and atrial diastole respectively.
• The contraction and relaxation of ventricles are called
ventricular systole and ventricular diastole
respectively.
22.
23. Requirements for Efficient Cardiac
Contraction
1. Atrial excitation and contraction need to be
complete before ventricular contraction
occurs.
2. Excitation of cardiac muscle fibers should be
coordinated so that each chamber contracts
as a unit.
3. Pair of atria and pair of ventricles should be
coordinated so that both members of the
pair contract simultaneously.
24. Cardiac cycle
• First assembled by Lewis in 1920 but first conceived by Wiggers in
1915
• It is named after Dr. Carl J. Wiggers, M.D.
• A Wiggers diagram is a standard diagram used in cardiac physiology
• The X axis is used to plot time, while the Y axis contains all of the
following on a single grid:
• Blood pressure
Aortic pressure
Ventricular pressure
Atrial pressure
• Ventricular volume
• Electrocardiogram
• Arterial flow (optional)
• Heart sounds (optional)
28. The Cardiac Cycle
• Cardiac Cycle and Heart Rate
– At 75 beats per minute
• Cardiac cycle lasts about 800 msecs
– When heart rate increases
• All phases of cardiac cycle shorten, particularly diastole
30. The Cardiac Cycle
Eight Steps in the Cardiac Cycle
1. Atrial systole
•
Atrial contraction begins
•
Right and left AV valves are open
1. Atria eject blood into ventricles
•
Filling ventricles
3. Atrial systole ends
•
AV valves close
•
Ventricles contain maximum blood volume
•
Known as end-diastolic volume (EDV)
31. The Cardiac Cycle
Eight Steps in the Cardiac Cycle
4. Ventricular systole
•
Isovolumetric ventricular contraction
•
Pressure in ventricles rises
•
AV valves shut
5. Ventricular ejection
•
Semilunar valves open
•
Blood flows into pulmonary and aortic trunks
•
Stroke volume (SV) = 60% of end-diastolic volume
32. The Cardiac Cycle
Eight Steps in the Cardiac Cycle
6. Ventricular pressure falls
•
Semilunar valves close
•
Ventricles contain end-systolic volume (ESV), about 40% of enddiastolic volume
7. Ventricular diastole
•
Ventricular pressure is higher than atrial pressure
•
All heart valves are closed
•
Ventricles relax (isovolumetric relaxation)
33. The Cardiac Cycle
Eight Steps in the Cardiac Cycle
8. Atrial pressure is higher than ventricular
pressure
•
AV valves open
•
Passive atrial filling
•
Passive ventricular filling
•
Cardiac cycle ends
34. Pressure changes in the atria
•
a, c and v atrial pressure waves are noted in atria
•
The ‘a’ wave is caused by the atrial contraction, during right atrial
contraction pressure increases 4-6 mm of Hg, while in left atrium it is 7-8
mm of Hg.
•
The ‘c’ wave occur when the ventricles begin to contract, it is caused
partially by slight back flow of blood when there is ventricular
contraction, but mainly by bulging the A-V valves backward toward the
atria because of increasing pressure in ventricles
•
The ‘v’ wave occur toward the end of ventricular contraction, this is
caused by the slow flow of blood from great veins while the A-V valves
are closed, when A-V valves opens allowing the store blood into
ventricle this ‘v’ wave disappear.
36. Summary: Pathway of Heartbeat
• Begins in the sinoatrial (S-A)
node
• Internodal pathway to
atrioventricular (A-V) node
• Impulse delayed in A-V node
(allows atria to contract before
ventricles)
• A-V bundle takes impulse into
ventricles
• Left and right bundles of
Purkinje fibers take
impulses to all parts of
ventricles
KEY
Red = specialized cells;
all else = contractile cells
