Question
Describe the events of the cardiac cycle. Explain systole, diastole, heart sounds, and the significance of different waves in an ECG.
(NEET 2023, similar pattern)
Solution — Step by Step
The cardiac cycle is one complete heartbeat — from the beginning of one heartbeat to the beginning of the next. At a normal heart rate of 72 beats/min, one cardiac cycle lasts about 0.8 seconds.
The cycle has three main phases: atrial systole, ventricular systole, and joint diastole (relaxation).
Atrial Systole (0.1 s): Both atria contract simultaneously, pushing blood into the ventricles through the open AV (atrioventricular) valves. The ventricles are relaxed and filling. The SAN (sinoatrial node) initiates this contraction.
Ventricular Systole (0.3 s): Both ventricles contract. This happens in two sub-phases:
- Isovolumetric contraction: Ventricles contract with all valves closed, building up pressure rapidly.
- Ejection phase: When ventricular pressure exceeds aortic/pulmonary pressure, the semilunar valves open and blood is ejected into the aorta and pulmonary artery.
Joint Diastole (0.4 s): Both atria and ventricles are relaxed. Venous blood flows into the atria and then passively into the ventricles through open AV valves. About 70% of ventricular filling happens during diastole without any atrial contraction.
Two main heart sounds are produced per cycle:
- First heart sound (lub / S1): Produced by the closure of the AV valves (tricuspid and mitral) at the beginning of ventricular systole. It is louder and longer.
- Second heart sound (dub / S2): Produced by the closure of the semilunar valves (aortic and pulmonary) at the beginning of ventricular diastole. It is shorter and sharper.
The pattern is: lub-dub, lub-dub — with a brief pause between each cycle.
An electrocardiogram (ECG) records the electrical activity of the heart:
- P wave: Represents atrial depolarisation (atrial contraction/systole). Generated when the impulse spreads from SAN across both atria.
- QRS complex: Represents ventricular depolarisation (ventricular contraction). The large amplitude reflects the thick ventricular muscle mass.
- T wave: Represents ventricular repolarisation (ventricular relaxation). Atrial repolarisation is hidden within the QRS complex.
- P-Q interval: Time for impulse to travel from SAN through AVN to ventricles. A prolonged P-Q interval indicates a conduction block.
Why This Works
The heart functions as a double pump — the right side pumps blood to the lungs (pulmonary circulation), and the left side pumps blood to the body (systemic circulation). The cardiac cycle ensures that both circuits operate in synchrony. The electrical conduction system (SAN → AVN → Bundle of His → Purkinje fibres) coordinates the timing so atria contract first, giving ventricles time to fill before they contract.
The ECG is clinically valuable because any disruption in the conduction pathway shows up as changes in wave shape, amplitude, or timing — allowing diagnosis of arrhythmias, heart blocks, and ischaemia.
NEET commonly asks to match ECG waves with cardiac events. Remember: P = atrial depolarisation, QRS = ventricular depolarisation, T = ventricular repolarisation. A question on cardiac output (CO = stroke volume x heart rate = ~5 L/min) is also frequently paired with the cardiac cycle.
Common Mistake
Students often write that the T wave represents “atrial repolarisation.” The T wave represents ventricular repolarisation. Atrial repolarisation does occur, but its electrical signal is masked by the much larger QRS complex and is not separately visible on a standard ECG.
Another common error: stating that the heart is at rest for 0.4 seconds out of 0.8 seconds. While joint diastole is 0.4 s, each atrium rests for 0.7 s and each ventricle rests for 0.5 s per cycle — because they are not contracting during the other phases either.