Physiology of the Cardiovascular System

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Physiology of the Cardiovascular System
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The Conduction System of the Heart

• Modified cardiac muscle that specializes in
  contraction
• There are four main structures that compose the
  conduction system of the heart
• Sinoatrial (SA) node
• Atrioventricular (AV) node
• AV bundle
• Purkinje system
  

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Sinoatrial (SA) Node

• Initiates the mechanical contraction of the heart
  (known as the pacemaker)
• Located in the right atrium just below the
  junction of the superior vena cava
• Possesses an intrinsic rhythm. This means that
  without any stimulation by nerve impulses from
  the brain and cord, they themselves initiate
  impulses at regular intervals.

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Atrioventricular (AV) Node

• The impulse generated in the SA node travels
  swiftly throughout the muscle fibers of each
  atria.
• The stimulated atria begin to contract they will
  completely contract before the impulse reaches
  the ventricles
• The action potential enters the AV node by way of
  three internodal bundles of conducting fibers

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AV Bundle and Purkinje Fibers

• The impulse slowly passes through the AV node
  then speeds up as the impulse is relayed through
  the AV bundle (bundle of His) into the ventricles
  
• It is the right and left bundle branches and the
  Purkinje fibers that conduct the impule through
  the ventricles, causing them to contract
  simultaneously.

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Conduction System of the Heart
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Artificial Pacemakers

• Devices that electrically stimulate the heart
• Electrodes sewn directly into the epicardium or
  directly inserted into the heart chamber
• Inferior to the hearts natural pacemaker

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Electrocardiogram (ECG)

• Impulse conduction generates tiny electrical
  currents in the heart that spread through
  surrounding tissues to the surface of the body.
• An electrocardiogram (ECG OR EKG) is a graphic
  record of the hearts electrical activitythe
  impules that preced the actual contraction
• Electrodes are attached to a voltmeter and to the
  limbs and chest of the subject.

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Theory of Electrocardiography

1. At rest (baseline)
2. Action potential reaches the first electrode
   (relatively negative)
3. Action potential reaches the second electrode
   (pen returns to baseline)
4. End of action potential passes the first
   electrode (relatively positive)
5. Action potential passes second electrode (returns
   to baseline)

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ECG Waves

• Represent the dynamic events that happen during
  the contraction of the heart
• Letters are arbitrary and do not stand for any
  words

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P Wave

1. Heart wall is completely relaxedno change in
   electrical activity
2. P wave represents the depolarization of the atria

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QRS Complex

1. Atrial walls are completely depolarizedno change
   recorded
2. QRS Complex occurs as atrial walls repolarize and
   ventricular walls depolarize (massive
   depolarization of ventricles overshadows atrial
   repolarization)

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T Wave

• E. Atrial walls are completely repolarized and
  ventricular walls are completely depolarized
• T wave appears as the ventricular walls replarize

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Back to Baseline

• Once the ventricles are completely repolarized it
  is back to the baseline ECG

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Bradycardia

• Slow heart rhythmbelow 60 beats per minute
• Slight bradycardia is normal during sleep and in
  conditioned athletes when awake
• Can be caused by damage to the SA node

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Tachycardia

• Very rapid heart rhythmmore than 100 beats per
  minute
• Normal during and after exercise and during the
  stress response
• Can result from improper autonomic control of
  the heart, blood loss, shock and a host of other
  factors

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Ventricular Fibrillation

• Complete disruption of the normal heart rhythm
• Death may occur within minutes if the heart beat
  is not corrected by defibrillation or other means

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