Unit 1 Section 5.3 Electrical Activity of The Heart

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  • Electrical Activity of The Heart
    • Control of Heartbeat
      • cardiac muscle is 'myogenic' - this means that it can contract and relax without receiving signals from nerves
      • the pattern of contraction controls the regular heartbeat
      • the process starts in the SINO - ATRIAL NODE which is in the wall of the right atrium
        • the SAN is like a pacemaker - it sets the rhythm of the heartbeat by sending regular waves of electrical activity over the atrial walls
          • this causes the right and left atria to contract at the same time
            • a band of non conducting collagen tissue prevents the waves of electrical activity from being passed directly from the atria to the ventricles
              • instead these waves of electrical activity are transferred from the SAN to the ATRIO-VENTRICULAR NODE
                • The AVN is responsible for passing the waves of electrical activity onto the bundle of His.
                  • but there is a slight delay before the AVN reacts, to make sure the ventricles contract after the atria have emptied
                    • The BUNDLE OF His is a group of muscle fibres responsible for conducting the waves of electrical activity to the finer muscle fibres  in the right and left ventricle walls, called PURKYNE TISSUE
                      • the purkyne tissue carries the waves of electrical activity into the muscular walls of the right and left ventricles causing them to contract simultaneously from the bottom up
    • Electro-cardiographs
      • an electrocardiograph is a machine that records the electrical activity of the heart
      • the heart muscle depolarises when it contracts and repolarises when it relaxes
        • an electrocardiograph records these changes in electrical charge using electrodes placed on tthe chest
      • electrocardiograms
        • the trace produced by an electrocardiograph is called an electrocardiogram, or ECG
          • QRS complex - its caused by contraction of the ventricles
          • the P wave is caused by contraction of the atria. the main peak of the heartbeat, together with the dips at either side
          • The T wave is due to relaxation of the ventricles
    • Calculating Heart Rate
      • your heart rate is the number of beats per unit of time
      • you can use an ECG to work out a persons heart rate by using the following equation
        • heart rate = 60 / time taken for one heart beat (s)
    • Diagnosing heart Problems
      • Doctors compare their patients ECG's with a normal trace, this helps the doctors diagnose any heart problems
      • The Heart beat is too fast - around 120 beats per minute. that might be OK during exercise but at rest it shows that the heart isn't pumping blood efficiently
      • Here, the atria are contracting but sometimes the ventricles are not . This might mean there's a problem with the AVN - impulses aren't travelling from the atria through the ventricles
      • This is Fibrilation - a really irregular heartbeat. the atria or ventricles completely lose their rhythm and stop contracting properly. it can result in anything from chest pain and fainting to lack of pulse and death


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