HEARTBEATS!!

The pause is to allow the atria to fully empty into the ventricle.

Heartbeat on an ECG trace

P Interval (Ventricular Diastole)

• Atria and ventricles are relaxed
• blood is flowing into the atria from the veins.
• Atrial pressure increases above that of the ventricle, AV valves open allowing blood to flow into the ventricle
  

P Wave (Atrial Systole) P-Q

Signal transduction from SA to AV nodes.

• SA node fires
• Atria contract causing atrial systole
• which forces all blood into the ventricles
• emptying the atria.

Q Interval (End of Ventricular Diastole)

Depolarisation of interventricular (IV) septum

• AV valves remain open - all remaining blood squeezed into the ventricles.
• impulse from the SA node reaches the AV node
• which spreads the signal throughout the walls of the ventricles via bundles of His and Purkinje fibres
• R peak is the end of ventricular diastole and the start of systole.
  

R Interval (Ventricular Systole)

Ventricular contraction

• All blood is now within the ventricles
• so pressure is higher than in the atria - AV valves close
• ventricles start to contract although pressure is not yet high enough to open the SL (semilunar) valves

ST Segment (Ventricular Systole)

Ventricular contraction

• Pressure increases until it equals Aortic pressure,
• SL valves open
• blood is ejected into the Aorta (and pulmonary artery) as ventricles contract
• At this time the atria are in diastole and filling with blood returning from the veins.
  
• plateau in ventricular arterial pressure
  

T Wave (Ventricular Diastole)

T= moment of Ventricular repolarisation immediately before ventricular relaxation

• Ventricles relax
  
• ventricular pressure is once again less than the aortic pressure
• so SL valves close