THE HEARTBEAT

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THE HEARTBEAT

• HEART ANATOMY

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Heart Physiology

• The heart rate is the number of times the heart
  beats per minute.
• Regulated by the autonomic nerve supply,
  sympathetic fibers to the heart, located in the
  superior, middle and inferior cardiac nerves.
• Impulses traveling over them insure that the
  heart beats fast enough to maintain good
  circulation during any activity.

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• Parasympathetic fibers, traveling in the vagus
  nerves, tend to slow the heart, and serve as
  cardioinhibitor nerves.
• This insures the heart does not beat too fast or
  too slow when the individual is resting or being
  active.
• The heart is composed of a fused mass of
  contractile cell, and obeys the all-or-nothing
  law of conductivity.

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• Under any conditions, the heart, when stimulated
  will contract maximally or not at all. That is
  not good!!
• The heart cannot tetanize ( a sustained
  contraction due to repeated stimulus, such as
  skeletal muscle).
• The heart has an extra long refractory period
  (0.3 sec. 65 times as long as a skeletal muscle.

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• Stimuli applied during systole will not produce
  an additional response.
• An extrasystole can be demonstrated.
• These are premature beats and are always followed
  by a long compensatory pause.

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The Cardiac Cycle

• Events that occur during one heartbeat, or
  contraction and subsequent relaxation.
• Average of 72 beats per minute.
• Must be completed in 0.8 sec.
• The amount of blood pumped is about 70 cc of
  blood per stroke. This is called stroke volume

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Cardiac Cycle
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Figuring Stroke Volume

• Cardiac Output Heart Rate x Stroke Vol.
• Heart Rate of 70 x Stroke Volume of 70 4900 cc
  of blood which is normal base line.
• 5 liters of blood are pumped each minute by the
  heart when physical activity is no more strenuous
  than sitting in a chair.
• Loss of blood, shock, cardiogenic shock, etc.

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• ALL OF THESE FACTORS AFFECT THE HEART !!
• THE END

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HOW DOES THE HEART BEAT?

• Two general classes of cardiac muscle cells
  involved in the normal heartbeat contractile
  cells and the conducting system.
• Cardiac muscle cells have a long refractory
  period (resting).
• Rapid stimulation produces isolated rather than
  tetanic (abnormally prolonged contractions that
  results from disturbances in the electrolyte
  balance) contractions.

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Action Potential of Cardiac Cells

• Rapid Depolarization Sodium channels open, and
  the membrane suddenly becomes permeable to
  sodium. Causes rapid depolarization. The channels
  are called fast channels because they are open
  only a few milliseconds.
• The Plateau As sodium channels close, calcium
  channels open. The channels are called slow
  channels because they open slowly and remain open
  for a longer period (175 milliseconds).
• Repolarization As the plateau continues, calcium
  channels begin closing and slow potassium
  channels begin opening. Causes rapid
  repolarization and resting potential.

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Refractory Period

• For sometime after an action potential begins,
  the membrane will not respond to a second
  stimulus.
• This period is called the refractory period.
• Absolute refractory period The membrane cannot
  respond at all.
• Relative refractory period Sodium channels are
  closed but capable of opening.

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Plateau
Rapid depolarization
Repolarization
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That is why you need to have dairy products,
small amounts of salt, and fresh fruits and
vegetables in your diet. Your heart depends on
calcium, sodium, and potassium to cause a
heartbeat !!
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Conducting System

• Cardiac muscle contracts on its own, in absence
  of neural or hormonal stimulation.
• This is called automaticity.
• The conducting system is a network of specialized
  cardiac cells that initiates and distributes
  electrical impulses.

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The conduction system consists of the
following Sinoatrial (SA) node, located in the
wall of the right atrium. Atrioventricular (AV)
node, located at the junction between the atria
and ventricles. Conducting cells, which
interconnect the two nodes and distribute the
impulse throughout the myocardium. The AV
bundle, bundle branches, and Purkinje
fibers, which distribute the stimulation to the
ventricular myocardium.
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Sinoatrial Node

• Embedded in the posterior wall of the right
  atrium near the superior vena cava.
• Contains pacemaker cells that establish heart
  rate.
• SA node is also known as the cardiac pacemaker.
• The contractile stimulus is passed to the right
  and left atria.

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Atrioventricular Node

• Sits within the floor of the right atrium
• The rate of propagation impulse slows as it
  leaves the internodal pathways and enters the AV
  node.
• This delay is important because the atria must
  contract before the ventricles.
• When the atrial completes its contraction the
  ventricular contraction begins.

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Bundle of His, Bundle Branches, Purkinje Fibers

• The AV node and Bundle of His is the only
  electrical connection between the atria and
  ventricles.
• The impulse travels to the right and left bundle
  branches.
• The left branch supplies the left ventricle and
  the right branch supplies the right ventricle.

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• Both branches extend toward the apex and fan out
  beneath the endocardial surface.
• The Purkinje fibers relay action potentials to
  the ventricles.
• Ectopic pacemakers bypass the conducting system
  disrupting the timing of the contraction.

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SA node impulse
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Signal 2, contraction of the atria
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Impulse to the AV node
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Impulse travels down the bundle of His, to the
right and left bundle branches