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Copyright © 2011 Pearson Education, Inc.

ANATOMY OF A HEART BEAT


Cardiac Muscle Cells

• The myocardium is composed of cardiac myocytes

• They contract similar to skeletal muscle cells.


Cardiac Muscle Cells

• Features:

• Striations

• Intercalated disks

• Gap junctions

• Cell communications

• Branched

• Anchored to endomysium

• desmosomes


Cardiac Muscle Contraction

• Contraction of the heart is rhythmic and spontaneous

• Similar to skeletal muscle cells, an action potential is generated


Steps of an Action Potential (REVIEW)


Role of the Sodium-Potassium Pump

1. Resting state

• Gates closed

2. Depolarization

• Stimulus causes Na+ gates to open

3. Repolarization

• Na+ gates close / K+ gates open

4. Hyperpolarization

• K+ gates close / Sodium-Potassium pumps activate


Cardiac Muscle Contraction

• Two chambers contract at different times

• Both generate action potentials

1. Atrium

• Contraction initiated by sinoarterial node (SA node)

2. Ventricles

• Contraction initiated when signal from SA node reaches atrioventricular node (AV node)


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

Copyright © 2011 Pearson Education, Inc. Figure 17.16

Sinoatrialnode

Atrioventricularnode

Atrialdepolarization

QRS complex

Ventriculardepolarization

Ventricularrepolarization

P-QInterval

S-TSegment

Q-TInterval


Atrial depolarization, initiatedby the SA node, causes theP wave.

P

R

T

QS

SA node

AV node

With atrial depolarizationcomplete, the impulse isdelayed at the AV node.

Ventricular depolarizationbegins at apex, causing theQRS complex. Atrialrepolarization occurs.

P

R

T

QS

P

R

T

QS

Ventricular depolarizationis complete.

Ventricular repolarizationbegins at apex, causing theT wave.

Ventricular repolarizationis complete.

P

R

T

QS

P

R

T

QS

P

R

T

QS

Depolarization Repolarization

1

2

3

4

5

6


(a) Normal sinus rhythm.

(c) Second-degree heart block. Some P waves are not conducted through the AV node; hence more P than QRS waves are seen. In this tracing, the ratio of P waves to QRS waves is mostly 2:1.

(d) Ventricular fibrillation. These chaotic, grossly irregular ECG deflections are seen in acute heart attack and electrical shock.

(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.


Heart Physiology: Sequence of Excitation

1. Sinoatrial (SA) node (pacemaker)

• Generates impulse

2. Atrioventricular (AV) node

3. Atrioventricular bundle

• Connects atria to ventricles

4. Bundle branches

• Connects the ventricles

5. Purkinji fibers

Copyright © 2011 Pearson Education, Inc. Figure 17.14a

(a) Anatomy of the intrinsic conduction system showing the sequence of electrical excitation

Internodal pathway

Superior vena cavaRight atrium

Left atrium

Purkinje fibers

Inter-ventricularseptum

1 The sinoatrial (SA) node (pacemaker)generates impulses.

2 The impulsespause (0.1 s) at theatrioventricular(AV) node. The atrioventricular(AV) bundleconnects the atriato the ventricles.4 The bundle branches conduct the impulses through the interventricular septum.

3

The Purkinje fibersdepolarize the contractilecells of both ventricles.

5


Heart Sounds

• Two sounds (lub-dup)

• closing of heart valves

1. AV valves close

• Beginning of systole

2. SL valves close

• Beginning of diastole

• Heart murmurs: abnormal heart sounds

• Valve problems


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


Steps of the Cardiac Cycle

1. Ventricles fill – mid diastole

• AV valves are open

2. Ventricular systole - contraction

• AV valves close

• SL valves open

3. Ventricles relax –begin diastole

• SL valves close


Steps of the Cardiac Cycle


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


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


Other Factors that Influence Heart Rate

• Age

• Gender

• Exercise

• Body temperature


HEART DISEASES


Homeostatic Defects

Defects in contractions:

1.Arrhythmias:

• irregular heart rhythms

2.Fibrillation:

• rapid, irregular contractions

• cannot pump blood


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)


Abnormal Heart Rate

• Tachycardia:

• fast heart rate (>100 bpm)

• may lead to fibrillation

• Bradycardia:

• slow heart rate (< 60 bpm)

• Inadequate blood flow

• endurance training


Congestive Heart Failure (CHF)

• Cardiac output too low

• Caused by:

• Coronary atherosclerosis

• high blood pressure

• myocardial infarcts

• Dilated cardiomyopathy (DCM)

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