Coordination of the cardiac cycle

  • The heart muscle is myogenic - can initiate its own contraction.
  • Atrial muscle tends to contract at a higher frequency than the ventricular muscle.
  • Fibrillation - Uncoordinated contraction of atria and ventricles.

Initiation and control of the heartbeat

  • Sino-atrial node (SAN) at top of right atrium initiates a wave of excitation at regular intervals (pacemaker).
  • Wave of excitation spreads over atrial walls and along membranes of muscle tissue, causing cardiac muscle cells to contract - this is an atrial systole.
  • The tissue at the base of the atria cannot conduct the wave of excitation, so the wave travels via the atrio-ventricular node (at the top of the interventricular septum), which can conduct the wave through to the ventricles.


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