The Cardiac Cycle

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  • Created by: Lois
  • Created on: 12-12-12 11:56
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  • The Cardiac Cycle
    • Definition:the sequence of events in one heartbeat
    • Three phases in the sequence of contraction
      • 1) Filling phase - atria and ventricles are relaxing, internal volume increases and blood flows into the heart.
        • Into the atria, through the  open atroventricular valves and into the ventricles. This is callled diastole.
    • The sinoatrial node (SAN) is the heart's pacemaker. It is a small patch of tissue that sends out waves
      • of electrical excitation at regular intervals to initiate contractions.
    • Purkyne tissue is specially adapted muscle fibres that conduct the wave of excitation from the AVN down the septum to the ventricles.
    • The need for coordination
      • Cardiac muscle initiates its own contraction. So is described as myogenic. The muscle will contract and relax rhythmically.
        • The muscles from the atria and the ventricles each have they're own natural frequency of contraction.
          • The atrial muscle tends to contract at a higher freguency than the ventricular muscle.
            • This property of the muscle could cause innefficient puping (fibrillation) if the contractions of the chambers are not synchronised.
              • So the heart needs a mechabism that can coordinate the contractions of all four chambers.
    • How the heart starts
      • At the top of the right atrium, near the point where the vena cava enpties blood into tge atrium is the sinoatrial node (SAN).
        • This samll patch of tissue generates electrical activity.
          • The SAN initiates a wave of excitation at regukar intervals.
            • Ib a human, this occur approximately 55-80 times a minute.
    • Contraction of the atria
      • The wave of excitations quickly spreads over the walls of both atria.
        • It travels along th membranes of the msucle tissue. As the wave of excitation passes,
          • it causes the cariac muscle cells to contract. This is atrial systole.
      • At the base of the atria is a disc of tissue thar cannot conduct the wave of excitation.
        • So the excitation cannot spread directly to the ventricle walls.
          • At the top of the inter-ventricular septum is another node - the atrioventricular node (AVN).
            • This is the only route through the disc of non-conducting tissue. The wave of excitation is delayed in the node.
              • This allows time for the atria to finish contracting and for the blood to flow down the ventricles before they begin to contract.
    • Contraction of the ventricles
      • After this delay, the wave of excitation is carried awway from the AVN and dpwm specialised conducting tissue.
        • This is the Purkyne tissue and it runs down the inter-ventricukar septum.
          • At the base of the septum, the wave of excitation spreads out over the walls of the ventricles.
            • As the excitation spreads upwards from the base (apex) of the ventricles, it causes the muscles to contract.
              • This means that the ventricles contract from the base upwards, pushing blood u[p to the major arteries at the top of the heart.
  • 2) Atrial contraction - Heartbeat starts when atria contracts. Both right and left atria contract together.
    • Three phases in the sequence of contraction
      • 1) Filling phase - atria and ventricles are relaxing, internal volume increases and blood flows into the heart.
        • Into the atria, through the  open atroventricular valves and into the ventricles. This is callled diastole.
    • Small increase in pressure created helps push blood into the ventricles.
      • This stretches the walls of the ventricles and ensures they are full of blood.
        • Contraction of the artia is called atrial systole. Once the ventricles are full they begin to contract.
          • Blood fills the atroventricular valve flaps casuing them to snap shut. Preventing blood returning to the atria.
  • 3) Ventricular contraction - All four heart valves are closed. The walls of the ventricles contract. This is called ventricular systole.
    • This raises the pressure in the ventricles very quickly.
      • The contraction starts at the apex (base) of the heart so this pushes the blood upwards towards the arteries.
        • The semilunar valves open and blood is pushed out of the heart. The contraction only last for a short time.
          • Then the ventricle walls relax allowing the heart to fill up again.
  • The sound of the heart
    • The Cardiac Cycle
      • Definition:the sequence of events in one heartbeat
      • The sinoatrial node (SAN) is the heart's pacemaker. It is a small patch of tissue that sends out waves
        • of electrical excitation at regular intervals to initiate contractions.
      • Purkyne tissue is specially adapted muscle fibres that conduct the wave of excitation from the AVN down the septum to the ventricles.
      • The need for coordination
        • Cardiac muscle initiates its own contraction. So is described as myogenic. The muscle will contract and relax rhythmically.
          • The muscles from the atria and the ventricles each have they're own natural frequency of contraction.
            • The atrial muscle tends to contract at a higher freguency than the ventricular muscle.
              • This property of the muscle could cause innefficient puping (fibrillation) if the contractions of the chambers are not synchronised.
                • So the heart needs a mechabism that can coordinate the contractions of all four chambers.
      • How the heart starts
        • At the top of the right atrium, near the point where the vena cava enpties blood into tge atrium is the sinoatrial node (SAN).
          • This samll patch of tissue generates electrical activity.
            • The SAN initiates a wave of excitation at regukar intervals.
              • Ib a human, this occur approximately 55-80 times a minute.
      • Contraction of the atria
        • The wave of excitations quickly spreads over the walls of both atria.
          • It travels along th membranes of the msucle tissue. As the wave of excitation passes,
            • it causes the cariac muscle cells to contract. This is atrial systole.
        • At the base of the atria is a disc of tissue thar cannot conduct the wave of excitation.
          • So the excitation cannot spread directly to the ventricle walls.
            • At the top of the inter-ventricular septum is another node - the atrioventricular node (AVN).
              • This is the only route through the disc of non-conducting tissue. The wave of excitation is delayed in the node.
                • This allows time for the atria to finish contracting and for the blood to flow down the ventricles before they begin to contract.
      • Contraction of the ventricles
        • After this delay, the wave of excitation is carried awway from the AVN and dpwm specialised conducting tissue.
          • This is the Purkyne tissue and it runs down the inter-ventricukar septum.
            • At the base of the septum, the wave of excitation spreads out over the walls of the ventricles.
              • As the excitation spreads upwards from the base (apex) of the ventricles, it causes the muscles to contract.
                • This means that the ventricles contract from the base upwards, pushing blood u[p to the major arteries at the top of the heart.
  • The lup-dup sound made by the heart is made by the valves closing: -the first sound, lub, is made by the atrioventricukar valves closing as the ventricles start to contract.
    • The sound of the heart
      • The second sound, dub, is made by the semilunar valves closing as the ventricles start to relax.
        • The atrioventricular valves snap shut, so this noise is louder than the closing semilunar valves, which shut because blood is accumulating in their pockets.

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