Section 2, Chapter 15

The cardiac cycle
The left and right sides of the heart contract together in a coordinated fashion
Systole – phase during contraction of a heart chamber

Diastole – phase during the relaxation of a heart chamber
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, filling with blood
Semilunar valves are opened, while AV valves are closed

Heart Sounds
The 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

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 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 15.18 Illustrates the cardiac conduction system.

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.