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 Cardiac muscle cells contract:  Spontaneously  Independently  Two systems regulate heart rhythm:  Intrinsic Conduction System  Uses the specialized cardiac muscle cells (nodal tissue)  75 beats per minute  Autonomic nervous system  Speed up or slow down

 Sinoatrial (SA) node  Atrioventricular (AV) node  Atrioventricular (AV) bundle (bundle of His)  Discovered by Wilhelm His in 1893  Bundle branches  Purkinje fibers © 2015 Pearson Education, Inc.

Superior vena cava Sinoatrial (SA) node (pacemaker) Atrioventricular (AV) node Right atrium Bundle branches Purkinje fibers Left atrium Atrioventricular (AV) bundle (bundle of His) Purkinje fibers Interventricular septum

 Sinoatrial Node = the heart’s natural pacemaker  The “SA Node”  Located in the right atrium  Starts each heartbeat; sets the pace  From the SA node the impulse spreads:  To the AV node – atria contract …  Then the impulse travels to the:  AV bundle > Bundle branches > Purkinje fibers  Ventricles contract toward atria, ejecting blood  dent_view0/chapter22/animation__conducting_system_of_ the_heart.html dent_view0/chapter22/animation__conducting_system_of_ the_heart.html

 Ischemia —lack of adequate oxygen supply to heart muscle results in fibrillation  Fibrillation—a rapid, uncoordinated shuddering of the heart muscle  Major cause of death from heart attacks  Use of defibrillator can save life by restoring natural rhythm using electrical current  Tachycardia —rapid heart rate over 100 bpm  Prolonged can progress to fibrillation  Bradycardia —slow heart rate less than 60 bpm © 2015 Pearson Education, Inc.

 Cardiac cycle refers to one complete heartbeat  Refers to the sounds of the ventricles  Systole  contraction  Diastole  relaxation  beats about 75 times per minute  length of beat is about 0.8 seconds © 2015 Pearson Education, Inc.

 Two distinct sounds  “Lub” – closing of the AV valves-  ventricular systole (systolic blood pressure)  “Dup” – closing of the semilunar valves  Ventricular diastole (diastolic blood pressure)  Faulty valves reduce efficiency of the heart  Can lead to abnormal heart sounds (murmurs)  Sounds like a whooshing or swishing

1. Autonomic Nervous System  Sympathetic nervous system  Parasympathetic nervous system 2. Hormones and ions via Endocrine System  Epinephrine and thyroxine increase heart rate  Imbalances of Ca⁺⁺ or K⁺ can depress, stop, or increase heart rhythms 3. Physical factors  Age : faster in young  Gender : faster in females  Exercise  Body temperature © 2015 Pearson Education, Inc.