The cardiac cycle

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  • The cardiac cycle
    • Sequence of contraction
      • Filing phrase
        • The blood flows into the heart from the major veins when the atria and ventricles are relaxing.
          • The blood flows into the atria, then through the atrioventricular valves and into the ventricles
            • This is known as diastole.
      • Atrial contraction
        • Heart beat starts when the atria contract.
          • Both the left and right atria contract together which causes an increase in blood pressure which pushes the blood into the ventricles.
            • This stretches the walls of the ventricles, maximising blood intake.
              • Contraction of the atria is called atrial systole.
                • One the ventricles are full, they begin to contract. Blood fills the atrioventricular valve flaps causing them to shut which prevents blood flowing back to the atria.
      • Ventricular contraction
        • There's a short period where all four heart valves are closed.
          • The walls of the ventricles contract, and this is known as ventriuclar systole.
            • This raises the pressure in the ventricles.
              • Contraction starts at the apex of heart so this pushes the blood upwards towards the arteries.
                • Semilunar valves open and blood is pushed out of the heart. The ventricle walls then relax so they cam be filled up with blood again.
    • How valves work
      • Atrioventricular valves
        • When the ventricle walls relax and expand after contracting, the pressure in the ventricles is less than of the atria.
          • This causes the atrioventricular valves to open.
            • They remain open whilst the atria contract.
              • When the pressure increases, these valves shut to stop blood flowing back to the atria.
      • Semilunar valves
        • They are closed when the pressure in the major arteries is higher than the pressure in the ventricles.
          • The semilunar valves open when the ventricles contract, a the pressure in the ventricles is higher than the pressure in the aorta and pulmonary arteries.
            • They close again when the pressure in the ventricles decreases as the blood is starting to flow into the ventricles again.
      • Valves ensure blood flows in the right direction.
    • The sound of the heart
      • The heart beat sound is made by the valves closing.
      • The first sound, lub, is made by the atrioventricular valves closing as the ventricles start to contract.
      • The second sound, dup, is made when the semilunar valves close as the ventricles start to relax.
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