The cardiac cycle is the sequence of events occurring in the heart during each beat, consisting of atrial and ventricular events with specific durations for systole and diastole. Key phases include isometric contraction, ejection period, rapid and slow filling, with heart sounds attributed to various valve closures and blood movements. Heart rate averages 72 beats per minute with each cycle lasting approximately 0.8 seconds.

1. CARDIAC CYCLE

2. Introduction
• Definition: Cardiac cycle is defined as the sequence
of coordinated events taking place in the heart
during each beat.
• Events: Systole (Contraction)
Diastole (Relaxation)

3. Duration
• Heart rate: 72/minute
• Cardiac cycle: 0.8 second
Events of cardiac cycle
1. Atrial events
2. Ventricular events

4. Atrial events
1. Atrial systole = 0.11 (0.1) sec
2. Atrial diastole = 0.69 (0.7) sec
Ventricular events
1. Ventricular systole = 0.27 (0.3) sec
2. Ventricular diastole = 0.53 (0.5) sec

5. Ventricular Systole
Time (second)
1. Isometric contraction = 0.05
2. Ejection period = 0.22
0.27
Ventricular Diastole
1. Protodiastole = 0.04
2. Isometric relaxation = 0.08
3. Rapid filling = 0.11
4. Slow filling = 0.19
5. Last rapid filling = 0.11
0.53

6. Atrial systole
Duration: 0.11 second
Known as last rapid filling phase or
presystole
Intra atrial pressure increases
Contraction of atrial musculature causes the
production of fourth heart sound

7. Atrial diastole
 Duration: 0.69 sec (0.7sec)
 Atrial filling takes place
 atrial diastole (first 0.3 sec) coincides with ventricular
systole > heart relaxes as a whole for 0.4 sec

9. Ventricular events
1. ISOMETRIC CONTRACTION PERIOD
 characterized by increase in tension, without any change in the
length of muscle fibers
 Also called isovolumetric contraction
 Closure of atrioventricular valves at the beginning of this
phase produces first heart sound.
 Significance: Pressure rise in ventricle, causes opening of
semilunar valves

10. 2. EJECTION PERIOD
• Duration: 0.22 second
• Blood is ejected out of both the ventricles due to the opening of semilunar
valves and isotonic contraction of ventricles
1. First Stage or Rapid Ejection Period
 Starts immediately after the opening of semilunar valves.
 Large amount of blood is rapidly ejected from both the ventricles.
 Duration: 0.13 second.
2. Second Stage or Slow Ejection Period
 Blood is ejected slowly with much less force.
 Duration: 0.09 second.

11. 1. PROTODIASTOLE
• Duration: 0.04 second
• first stage of ventricular diastole
Events:
 Due to the ejection of blood, the pressure in aorta
and pulmonary artery increases and pressure in
ventricles drops
 Closure of Semilunar valves produces second
heart sound

12. 2. ISOMETRIC RELAXATION PERIOD
• Duration: 0.08 second
• Characterized by decrease in tension without any
change in the length of muscle
Isometric relaxation
Reduces ventricular pressure
Opening of atrioventricular valves,
Filling of ventricles

13. 3. RAPID FILLING PHASE
• Duration: 0.11 second
• Sudden rush of blood from atria into ventricles
• 70% of ventricular filling takes place
• Significance: Production of third heart sound

14. 4. SLOW FILLING PHASE
• Duration: 0.19 second
• ventricular filling becomes slow
• 20% of ventricular filling takes place

15. 5. LAST RAPID FILLING PHASE
• Occurs because of atrial systole
• After slow filling period, the atria contract and push a
small amount of blood into ventricles (atrial kick)
• 10% of ventricular filling takes place
• End-diastolic Volume: amount of blood remaining in
each ventricle at the end of diastole. It is about 130 to
150 mL per ventricle.

16. INTRA-ATRIAL PRESSURE
Responsible for:
 Opening of the atrioventricular valves
 Ventricular filling
 Development of venous pulse

17. Heart sounds
HEART SOUNDS
1st
Heart Sound > Closure AV valves
2nd
Heart Sound > Closure of Semilunar valves
3rd
Heart Sound > Sudden rush blood from Atria to ventricles
4th
Heart Sound > Contraction of Atrial musculature

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