Cardiac Cycle

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  • Cardiac Cycle
    • The heart has two areas of specialised cells which are important in controlling the heart rate
      • The sinoatrial node (SAN)
      • The atrioventricular node (AVN)
      • The process starts in the SAN, located in the wall of the right atrium
        • The SAN acts like a pacemaker, setting the rhythm of the heart by sending out regular waves of electrical stimulation to the atrial walls
          • This causes the right and left atria to contract at the same time, pushing blood into the ventricles (Atrial systole)
            • The electrical stimulation is prevented from spreading to the ventricles by a thin layer of connective tissue which acts as an insulator
              • There is a slight delay before the AVN, which lies between the two atria, reacts which causes the ventricles to contract after the atria (giving the ventricles time to fill)
                • The AVN picks up the wave of excitation and passes it on to a strand of specialised tissues, called the bundle of His, which transmits it to the apex of the ventricles
                  • Here, the bundle of His branches into purkinje fibres, located in the ventricle walls. These fibres carry the wave of excitation upwards through the ventricle muscle
                    • The impulses cause the cardiac muscle in each ventricle to contract simultaneously (ventricular systole) from the apex upwards, which forces the blood from the ventricles into the aorta and pulmonary artery
                      • There is a short delay (ventricular and atrial diastole) before the next wave of excitation begins
    • One complete contraction and relaxation of the heart is called a heart beat
      • In adult humans the heart beats approximately 70 times/minute
    • Cardiac muscle is myogenic
      • This means that rather than having to depend on receiving signals from nerves telling it what to do, it can initiate it's own heartbeat
        • However, even though the heart muscle is myogenic, it can still be modified by certain hormones or the nervous system
    • Cardiac Cycle: 'The sequence of events that take place during one heart beat'
    • Can be divided into;
      • Atrial systole
        • Ventricles relax
          • Atria contract which decreases volume
            • Pressure in the atria increases and is higher than the pressure of the ventricles
              • Atrioventricular valves are forces open and blood flows into the ventricles from the atria down a pressure gradient
      • Ventricular systole
        • Atria relax
          • The ventricles contract which decreases their volume
            • The pressure in the ventricles increases and is higher than the pressure in the atria
              • This closes the atrioventricular valves to prevent backflow (Heart sound - LUB)
                • The high pressure opens the semi-lunar valves and blood flows into the pulmonary artery and aorta down the pressure gradient
      • Diastole
        • The atria and ventricles both relax
          • This increases volume and lowers the pressure in the heart chambers
            • There is new, higher pressure in the aorta and pulmonary artery than in the heart chambers
              • This closes the semi-lunar valve to prevent backflow (Heart sound - DUB)
                • Blood flows into the atria from the vena cava and pulmonary artery down a pressure gradient
                  • The cycle starts again
                    • Passive filling of the ventricles happens as there is not enough blood in the atrium to fill the ventricles


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