The Cardiac Cycle

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  • The Cardiac Cycle
    • Sequence of contraction -  is important that the chambers of the heart contract in a coordinated fashion.
      • If contract out of sequence there will be inefficient pumping.
      • Filling phase - while the atria and ventricles are relaxing, the internal volume increases and blood flows into the heart
        • Blood flows into the atria then through the open atrioventricular valves and into the ventricles. Called diastole
      • Atrial contraction - atria contracts (left and right together), increases pressure to help push the blood into the ventricles.
        • Stretches the walls of the ventricles and ensures they are full of blood. Called the atrial systole.
        • Once the ventricles are full they begin to contract. Blood fills the atrioventricular valve flaps causing them to snap shut - stops blood returning to the atria
      • Ventricular contraction - there is a short period where all four heart valves are closed
        • Walls of the ventricles contract. Called ventricular systole. 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
    • Valves
      • The valves ensure that blood flows in the correct direction. They are opened and closed by changes in the blood pressure in the chambers in the heart
      • Atrioventricular valves
        • When the ventricular walls relax and recoil after contracting, the pressure in the ventricles drops below the pressure in the atria. causes the atrioventricular valves to open
          • Blood entering the heart flows straight through the atria and into the ventricles.
            • The pressure in the atria and the ventricles slowly rises as they fill with blood. The valves remain open while the atria contract.
        • As the ventricles begin to contract, the pressure of the blood in the ventricles rises.
          • When the pressure rises above that in the atria, the blood starts to move upwards. This movement fills the valve pockets and keeps them closed -  stops blood flowing back into the atria
      • Semilunar valves
        • When the ventricles start to contract, the pressure in the major arteries is higher than the pressure in the ventricles, means that the semilunar valves are closed
          • As the ventricles start to contract, the pressure inside rises very quickly because the blood cannot escape.
            • Once the pressure in the ventricles rises above the pressure in the aorta and pulmonary arteries, the semilunar valves are pushed open. The blood is under very high pressure, so it is forced out of the ventricles in a powerful spurt
        • Once the ventricle wall has finished contracting, elastic tissue in the walls of the ventricles recoils to stretch the muscle out again and return the ventricle to its original size -  causes the pressure in the ventricle to drop quickly.
          • As it drops below the pressure in the major arteries, the semilunar valves are pushed closed by blood starting to flow back towards the ventricles and collecting in the pockets of the valves -  stops blood returning to the ventricles
    • Controls of the cardiac cycle
      • The need for coordination
        • Cardiac muscle is able to initiate its own contraction described as  myogenic.
          • The muscle will contract and relax rhythmically even if its not connected to the body
        • This property of the muscle could cause fibrillation (inefficient pumping) if the contractions are not synchronised
      • How the heartbeat starts
        • At the top of the right atrium near the point where the vena cava empties the blood into the atrium is the sinoatrial node (SAN)
          • This is a small patch of tissue that generates electrical activity
      • Contraction of the atria
        • The wave of excitation quickly spreads over the walls of the atria. It travels along the membranes of the muscle tissue
          • As the wave passes, it causes the cardiac muscle cells to contract. Called atrial systole
        • At the base of the atria is a disc of tissue that cannot conduct the wave of excitation. so the excitation  cannot spread directly to the ventricle walls.
          • At the top of the septum is the atrioventricular node (AVN). Only route for the wave of excitation. The AVN delays the wave so that the atria can finish contracting and for the blood to flow down into the ventricles.
      • Contraction of the ventricles
        • The wave of excitation is carried away from the AVN and down the Purkyne tissue and it  runs down the septum. At the  apex, the wave spreads out over the walls of the ventricles. As the excitation spreads upwards it causes the ventricles to contract.
          • This means that the ventricles contract upwards  pushing the blood up to the major arteries at the tip of the heart
      • Electrocardiograms (ECG)
        • We can monitor the electrical activity of the heart using ECG. The sensors on the skin pick up the electrical excitation created by the heart and convert this into a trace


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