The Cardiac Cycle

Cardiac Cycle & Cardiac Conduction
Catalyst #2
• What is the outer covering of the heart called?
• What cavity is the heart in?
• What are the functions of the cardiovascular
13 - 2
The cardiac cycle
The left and right sides of the heart contract
together in a coordinated fashion
Systole – contraction
Diastole – relaxation
Ventricular Systole
• Ventricles contract to expel blood
• Atria are in diastole during ventricular systole, filling with blood
• Semilunar valves are opened, while AV valves are closed
Ventricular Diastole
• Ventricles are relaxed, filling with blood
• Ventricles are 70% full before atria contract
• Atrial systole pushes the remaining 30% of blood into ventricles
• AV valves are opened while semilunar valves are closed
Heart Sounds
The heart valves produce a distinct sound as they
close, which can be heard through a stethoscope.
• Lubb = sound of AV valves closing
• Occurs during ventricular systole
• Dupp = sound of semilunar valve closing
• Occurs during ventricular diastole
murmur = abnormal sound from the cusps
not closing completely
Heart Sounds - Ausculation
Aortic valve (A)
heard at 2nd intercostal space,
right of the sternum
Pulmonary valve (P)
heard at 2nd intercostal space,
left of the sternum
Tricuspid Valve (T)
heard at 5th intercostal space,
either right or left of the sternum
Mitral Valve (M)
heard at 5th intercostal space,
below left nipple
Image from Grant’s Atlas of Anatomy. Each heart valve is indicated by a colored oval and the area of
auscultation of the valve is indicated as a circle of the same color containing the first letter of the valve
Cardiac Conduction of the Heart
The heart is autorhythmic:
• Specialized cardiac tissue initiate and distribute electrical
impulses that generate heart contractions.
Syncytium – mass of interconnected cells that function as a unit
• Intercalated discs allow cardiac muscles to contract as a syncytium
• Atrial Syncytium – Left and right atria contract together
• Ventricular Syncytium – Left and right ventricles contract together
Cardiac Conduction of the Heart
Sinoatrial (SA) node
• Pacemaker of heart
• Initiates atrial syncytium
• Fires 80 impulses per minute
• Parasympathetic inhibition keeps
heart rate at about 72 beats per
Junctional Fibers
• Conduct impulses towards both
atria and towards AV node
Figure 15.18 Illustrates the
cardiac conduction system.
Cardiac Conduction of the Heart
Atrioventricular (AV) node
• Located within inferior wall of
interatrial septum
• Provides a junction between atrial
and ventricular syncytia
AV Bundle (Bundle of His)
• Only known conduction pathway
between atria and ventricles
• Divides into left and right bundle
Figure 15.18 Illustrates the
cardiac conduction system.
Cardiac Conduction of the Heart
Bundle Branches (Left and Right)
• Runs down in left and right ventricles
along either side of interventricular
• Gives rise to Purkinje Fibers
Purkinje Fibers
• Carries impulses to ventricular
myocardium and papillary
• Initiates ventricular syncytium at
apex of heart
Figure 15.19 Summarizes the
cardiac conduction system
Figure 15.20 Muscle fibers of the ventricles
are whorled shape, which increases the
blood output during ventricular systole.
End of Section 2, Chapter 15