Section 2, Chapter 15 Cardiac Cycle & Cardiac Conduction

The cardiac cycle

Systole – contraction

Diastole – relaxation

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

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

The left and right sides of the heart contract together in a coordinated fashion

Heart SoundsThe heart valves produce a distinct sound as they close, which can be heard through a stethoscope.

Lubb-Dupp• 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

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 name.

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

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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 HeartSinoatrial (SA) node• Pacemaker of heart

• Initiates atrial syncytium

• Fires 80 impulses per minute

• Parasympathetic inhibition keeps heart rate at about 72 beats per minute

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 branches

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 septum

• Gives rise to Purkinje Fibers

Purkinje Fibers• Carries impulses to ventricular

myocardium and papillary muscles

• 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