The Rhythm of the Heart

  • Cardiac muscles are myogenic.

  • Myogenic: Inititiase own contarction, not dependent on nervous system unless changing the rate.

  • The basic rhythm of the heart is maintained by a wave of electrical excitation.

The need for coordination:

  • Atria and ventricles have their own natural frequency of contraction.

  • Muscles from atria tend to contract at a higher frequency than ventricular muscle

  • To avoid inefficient pumping, contractions of the chambers have to be synchronised - otherwise the atria will contract more often than the ventricle

  • Coordination maximises blood flow and so oxygen/digestive product delivery

COORDINATION

  • SAN (sinoatrial node/pacemaker) sends out waves of excitation through the walls of both atria and causes atrial systole

    • Blood is forced from atira into ventricles

  • AVN delays wave of excitation until atria have finished contacting and ventricles are full of blood

    • the impulse cannot pass directly into the ventricles as tehre is a band of non-conducting tissue that separates the atria from the ventricles and slows down the passage of the excitation wave.

  • AVN sends wave down the septum via Purkyne fibres to heart apex.

  • A wave of excitation spreads up from apex through ventricle walls which leads to ventricular systole

  • Blood is pushed upwards to arteries

  • Heart goes into diastole

  • Blood passes form wins into artia and ventricles.

ECG:

  • Tachycardia:

    • Very fast > 100bpm

    • normal when exercise/fever/scared/angry

    • If it is abnormal it may be caused by problems in the elctrocla control of th heart

      • needs to be treated by medication or by surgery

  • Bradycardia:

    • slows down < 60bpm

    • Training makes heart beat more efficiently and slowly

    • Severe bradycardia can be serious

      • need to be treated w an artificial pacemaker to keep the heart beating slowly.

  • Ectopic heartbeat:

    • extra heartbeat that are out of normal rhythm.

    • one a day for most people

    • serious is too many

  • Atrial Fibrillation:

    • Example of arrythmia

    • Abnormal rhythm

    • Up to 400 bpm

    • Contract very fast but not properly and only some of the impulses are passed to the ventricles

      • So the blood isn't pumped efficiently