1. ANATOMY OF A HEART BEAT
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2. Cardiac Muscle Cells
• The myocardium is composed of cardiac
myocytes
• They contract similar to skeletal muscle cells.
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3. Cardiac Muscle Cells
• Features:
• Striations
• Intercalated disks
• Gap junctions
• Cell communications
• Branched
• Anchored to endomysium
• desmosomes
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4. Cardiac Muscle Contraction
• Contraction of the heart is rhythmic and
spontaneous
• Similar to skeletal muscle cells, an action
potential is generated
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5. Steps of an Action Potential (REVIEW)
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6. Role of the Sodium-Potassium Pump
1. Resting state
•
Gates closed
1. Depolarization
•
Stimulus causes Na+ gates to open
1. Repolarization
•
Na+ gates close / K+ gates open
1. Hyperpolarization
•
K+ gates close / Sodium-Potassium pumps activate
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7. Cardiac Muscle Contraction
• Two chambers contract at different times
• Both generate action potentials
1. Atrium
•
Contraction initiated by sinoarterial node (SA
node)
1. Ventricles
•
Contraction initiated when signal from SA node
reaches atrioventricular node (AV node)
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8. Electrocardiography
•
Electrocardiogram (ECG or EKG): a composite
of all the action potentials generated by the heart
•
Three waves
1. P wave: depolarization of SA node
2. QRS complex: ventricular depolarization
3. T wave: ventricular repolarization
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9. QRS complex
Sinoatrial
node
Atrial
depolarization
Ventricular
depolarization
Ventricular
repolarization
Atrioventricular
node
P-Q
Interval
S-T
Segment
Q-T
Interval
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Figure 17.16

10. SA node
Depolarization
R
Repolarization
R
T
P
Q
S
1 Atrial depolarization, initiated
by the SA node, causes the
P wave.
R
AV node
T
P
Q
S
4 Ventricular depolarization
is complete.
R
T
P
T
P
Q
S
2 With atrial depolarization
complete, the impulse is
delayed at the AV node.
R
Q
S
5 Ventricular repolarization
begins at apex, causing the
T wave.
R
T
P
T
P
Q
S
3 Ventricular depolarization
begins at apex, causing the
QRS complex. Atrial
repolarization occurs.
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Q
S
6 Ventricular repolarization
is complete.
Figure 17.17

11. (a) Normal sinus rhythm.
(b) Junctional rhythm. The SA
node is nonfunctional, P waves
are absent, and heart is paced by
the AV node at 40 - 60 beats/min.
(c) Second-degree heart block. (d) Ventricular fibrillation. These
chaotic, grossly irregular ECG
Some P waves are not conducted
deflections are seen in acute
through the AV node; hence more
heart attack and electrical shock.
P than QRS waves are seen. In
this tracing, the ratio of P waves
to QRS waves is mostly 2:1.
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Figure 17.18

12. Heart Physiology: Sequence of Excitation
1. Sinoatrial (SA) node (pacemaker)
•
Generates impulse
1. Atrioventricular (AV) node
2. Atrioventricular bundle
•
Connects atria to ventricles
1. Bundle branches
•
Connects the ventricles
1. Purkinji fibers
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13. Superior vena cava
Right atrium
1 The sinoatrial (SA)
node (pacemaker)
generates impulses.
Internodal pathway
2 The impulses
pause (0.1 s) at the
atrioventricular
(AV) node.
3 The atrioventricular
(AV) bundle
connects the atria
to the ventricles.
4 The bundle branches
conduct the impulses
through the
interventricular septum.
5 The Purkinje fibers
Left atrium
Purkinje
fibers
Interventricular
septum
depolarize the contractile
cells of both ventricles.
(a) Anatomy of the intrinsic conduction system showing the
sequence of electrical excitation
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Figure 17.14a

14. Heart Sounds
• Two sounds (lub-dup)
• closing of heart valves
1. AV valves close
• Beginning of systole
1. SL valves close
• Beginning of diastole
• Heart murmurs: abnormal heart sounds
• Valve problems
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15. Mechanical Events: The Cardiac Cycle
• Cardiac cycle: blood flow through the heart
during one heartbeat
• Systole —contraction
• Diastole —relaxation
• Generally refers to activity of ventricles
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16. Steps of the Cardiac Cycle
1. Ventricles fill – mid diastole
•
AV valves are open
1. Ventricular systole - contraction
•
AV valves close
•
SL valves open
1. Ventricles relax –begin diastole
•
SL valves close
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17. Steps of the Cardiac Cycle
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18. Cardiac Output (CO)
• Volume of blood pumped by each ventricle in
one minute
• CO = heart rate (HR) x stroke volume (SV)
• HR = beats / minute
• SV = volume of blood pumped per beat
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19. Regulation of HR
• Sympathetic nervous system
• emotional or physical stress
• Increases HR
• Parasympathetic nervous system
• Depresses HR
• Utilizes vagus nerve
• Hormones
• Epinephrine increases heart rate
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20. Other Factors that Influence Heart Rate
• Age
• Gender
• Exercise
• Body temperature
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21. HEART DISEASES
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22. Homeostatic Defects
Defects in contractions:
1.Arrhythmias:
•
irregular heart rhythms
1.Fibrillation:
•
rapid, irregular contractions
•
cannot pump blood
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23. Heart Disease
• Angina pectoris
• Chest pain
• Low coronary blood flow
• Myocardial infarction (heart attack)
• Prolonged coronary blockage
• Cardiac cell death
• repaired with scar tissue
• (not cardiac myocytes)
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24. Abnormal Heart Rate
• Tachycardia:
• fast heart rate (>100 bpm)
• may lead to fibrillation
• Bradycardia:
• slow heart rate (< 60 bpm)
• Inadequate blood flow
• endurance training
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25. Congestive Heart Failure (CHF)
• Cardiac output too low
• Caused by:
• Coronary atherosclerosis
• high blood pressure
• myocardial infarcts
• Dilated cardiomyopathy (DCM)
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