The Cardiac Cycle

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  • The Cardiac Cycle
    • Sequence of Contraction
      • Artrial Systole - atrial contraction
        • left and right atria contract together (heart beat starts) and a small increase in pressure created pushes the blood into the ventricles
          • Ventricle walls stretch to ensure they are full of blood. When the pressure in the ventricles is above that in the atria, the valves snap shut
      • Diastole - Filling phase
        • whilst both the atria and the ventricles are relaxing, the internal volume increases and blood flows into the heart from the major veins
          • blood flows into the atria, through the open artioventricular valves and into the ventricles
      • Ventricular Systole - Ventricular contraction
        • Short period where all 4 valves are shut, the walls of the ventricles contract and the ventricular pressure increases quickly
          • Contraction starts in the apex of the heart so blood is pushed up towards the arteries
            • Semilunar valves open and blood is pushed out of the heart - contraction only lasts for a short time before the walls relax and the heart can start filling again
    • Control
      • Fibrillation
        • when the heart is un-co-ordinated, the contractions of the chambers are unsynchronised
        • leads to inefficient pumping, ventricles may ontract before full
      • Initiation
        • Sinoatrial Node - patch of tissues that generates electrical activity ( a wave of excitation)about 55-80 times aminute (in a human)
          • known as the pacemaker
          • heart is myogenic - it can initiate its own contraction
      • Ventricular Systole
        • the wave of excitation is carried away from the AV Node and down specialised, Purkyne tissue, that is conductive
          • at the base of the septum the wave spreads out over the walls of the ventricles causing the muslce cells to contract upwards, pushing the blood up to the major arteries
      • Artial Systole
        • wave of excitation quickly spreads over both atria, travelling along the membranes of the muscle tissue, causing the muscle cells to contract
        • at base of atria is a disc of tissue that cannot conduct the wave of excitation, so that the wave cannot spread directly over the ventricle walls
        • at the top of the inter-ventricular septum is the atrioventricular node - the only route through to the ventricles
        • the AV Node delays the wave of excitation, allowing the atria to fully contracandfor all the blood to be in the ventricles
    • Electrocardiograms (ECGs)
      • used to monitor the electrical activity of the heart by attaching sensors to the skin
      • the electrical activity generated by the heart can spread through the tissues next to the heart and onwards to the skin. Here it can be picked up and converted to a trace
      • a healthy trace has a certain shape - it consists of a series of waves - P, Q, R, S and T


Faith Thompson


Very detailed!!

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