The Cardiac Cycle

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The Cardiac Cycle

• Relate the events during the cardiac cycle to the
  maintenance of blood flow through the heart
• Describe the relationship between pressure and
  volume changes in the heart and aorta and events
  in the cardiac cycle
• Discuss the roles of the sinoatrial node,
  atrioventricular node and bundle of His

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Cardiac Muscle

• Cardiac muscle is able to contract spontaneously
  without nervous or hormonal stimulation (if
  provided woth oxygen, nutrients and the right
  mixture of mineral salts)
• These contractions are called myogenic.

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The Pacemaker the SAN

• The cycle starts at the sinoatrial node (SA node
  or SAN). This is a small patch of tissue that has
  its own natural rhythm of contraction. The rhythm
  is made faster or slower by nervous impulses and
  hormones. The SAN determines the rate of
  contraction of the rest of the cardiac muscle.

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Cardiac Impulses

• The SAN generates waves of electrical impulses
  over the atria.
• When these reach the junction between the atria
  and ventricles they are delayed by the
  atrioventricular septum. This delay gives time
  for the waves of contraction to spread over the
  atria.

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Atrioventricular node (AV node)

• The atrioventricular septum is a layer of
  non-conducting connective tissue. It completely
  separates the atria from the ventricles except
  for a region in the right atrium called the
  atrioventricular node (AV node)
• The AV node is the only route of transmission for
  the cardiac impulse. It acts as a second
  pacemaker region, relaying the cardiac impulse to
  the ventricles

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What if the SAN misfunctions?

• The AV node will then take over but it is slower
  than the inherent rhythm

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Systole (contraction)

• The cardiac cycle is relayed from the AV node
  over the ventricles through a bundle of
  specialised muscle fibres called the bundle of
  His.
• This bundle branches into other fibres called the
  Purkyne fibres (or Purkinje fibres)

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Ventricular systole (contraction)

• The cardiac impulse passing through the Purkyne
  fibres causes a wave of contraction.
• It starts at the apex of the heart (bottom) and
  passes rapidly over the ventricles (ventricular
  systole).

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• Regions close to the AV node have thin Purkyne
  fibres which carry impulses more slowly than the
  thick fibres that supply the more distant parts
  of the ventricles.
• This ensures that all parts of the ventricles
  contract more or less simultaneously.

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Diastole (relaxation)

• The whole heart realxes after ventricular
  systole, allowing blood from the veins to fill up
  the heart. This is called diastole.

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Pressure changes in the heart

• The diagram shows the pressure changes during a
  cardiac cycle on the left side of the heart
• Note that the maximal pressure in the left
  ventricle is far higher than in the atrium
• This is because the ventricle has to work hard to
  pump blood to all parts of the body (except the
  lungs)
• The atrium only pumps blood into the ventricle.

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Electrical change in the heart the ECG

• Cardiac muscle contracts as a result of
  electrical stimulation.
• A wave of electrical charge is initiated in the
  pacemaker region and spreads across the heart
• This generates an electric current in the body
  fluids around the heart
• The currents can be detected ob the body surface
  using recording electrodes.

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Electrocardiogram ECG

• A cathode ray oscilloscope or chart recorder is
  used to show the ECG
• The P wave is caused by atrial systole
• The QRS wave is caused by ventricular systole
• The T wave coincides with ventricular diastole

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Heart Rate

• This can be calculated from the interval between
  one P wave and the next.

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Check your understanding

• 1. List the sequence of main events during the
  cardiac cycle
• 2. Why does the pressure in the left ventricle
  reach a higher peak than the blood pressure in
  the left atrium?
• 3. Which part of the heart normally initiates the
  cardiac cycle?

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