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