Cardiac cycle

Cardiac muscle

  • myogenic

    • does not need to be stimulated by a nerve impulse to make it contract


Cardiac cycle


  • cardiac cycle


Diastole

  • atria and ventricles are relaxed

  • blood flows into the heart from the vena cava and the pulmonary evein

  • atrioventricular valves are open

  • semilunar valves shut as blood falls downwards due to gravity


  • diastole


Atrial systole

  • right and left atria contract together

  • increase in pressure pushes blood into ventricles

  • AV valves are open


  • atrial systole


Ventricular systole

  • ventricles contract

  • blood is pushed against AV valves causing them to shut

  • blood is pushed upwards towards the arteries

  • semilunar valves open and blood is pumped out of the heart


  • ventricular systole


cardiac cycle

atria

ventricles

AV valves

semilunar valves

blood flow

diastole

relax

relax

open

shut

vena cava and pulmonary vein → atria

atrial systole

contract

relax

open

shut

atria → ventricles

ventricular systole

relax

contract

shut

open

ventricles → aorta and pulmonary artery


Controlling the heartbeat

  • chambers of the heart are stimulated to contract by electrical impulse

  • impulses originate from specialised groups of cells

    • atria: sinoatrial node (SAN)

    • between atria and ventricles: atrioventricular node (AVN)


  • heart labelled - SAN/AVN


Controlling heartbeat mechanism

  1. an action potential is initiated in the SAN


    • controlling heartbeat step 1

  2. wave of excitation spreads out from the SAN across the atria


    • controlling heartbeat step 2

  3. wave spreads across both atria causing them to contract and reaches AVN and slows conduction briefly

    • ensures atria have fully emptied into ventricles before ventricles contract


    • controlling heartbeat step 3

  4. AVN passes impulse to Bundle of His and down between the ventricles through the purkyne fibres


    • controlling heartbeat step 4

  5. purkyne fibres release electrical impulse so ventricles contract from the bottom up


    • controlling heartbeat step 5

  6. no electrical impulse, cardiac muscle relaxes - diastole

    • allows atria to fill with blood before next cardiac cycle


    • controlling heartbeat step 6


  • controlling heartbeat summary


Electrocardiogram interpretation

  • Normal (60-100bpm)


    • normal heart rhythm

  • Bradycardia (<60bpm)


    • bradycardia

  • Tachycardia (>100bpm)


    • tachycardia

  • Atrial fibrillation


    • atrial fibrillation

  • Ectopic heartbeat


    • ectopic heartbeat