Control of the cardiac cycle 

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  • Control of the cardiac cycle
    • The need for coordination
      • The heart is myogenic.
      • Muscles from the atria and the ventricles each have their own natural frequency of coordination.
        • Atria tends to have a higher frequency than the ventricles.
      • The property of the muscle could cause fibrillation (inefficent pumping) if the contractions of the chambers are not synchronised.
    • How the heartbeat starts
      • Near the top of the right atrium and near the vena cave is the SAN.
        • Small patch of tissue that generates electrical activity.
          • SAN initates a wave of excitation at regular intervals. SAN is also known as the pacemaker.
    • Contraction of the atria
      • The wave of excitation spreads over the walls of both atria. Travels along the membrane of the muscle tissue. As the wave of excitation passes, the muscle cells contract - atrial systole.
      • The tissue at the base of the atria cannot conduct the wave of excitation, so the excitation cannot spread to the ventricles.
        • At the top of the inter-ventricular septum is the AVN.
          • Wave of excitation is delayed in this node.
            • This allows time for the atria to finish contracting and for the blood to flow down into the ventricles before they contract.
    • Contraction of the ventricles
      • After the pause, the wave of excitation is carried away from the AVN and down specialised conducting tissue (purkyne tissue). This runs down the interventricular septum.
        • At the base of the septum, the wave of excitation spreads upwards from the base of the ventricles causing them to contract from the bottum upwards, pushing the blood out of the chamber.
    • Electrocardiograms
      • Monitors the electrical activity of the heart.
        • Attaches sensors to the skin and it picks up some of the electrical excitation created by the heart and converts this to a trace.
        • Used to find out whether the heart is healthy or not.
      • P = excitation of the atria.
      • QRS = excitation of ventricles.
      • T = diastole.
      • If the beat is irregular, this can be because of myocardinal infection or fibrillation.
      • Can also show if the heart is enlarged, or the purkyne system is not functioning properly.


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