Cardiac Cycle
இதய சுழற்சி
The heart does not rest when you sleep; it merely optimizes its efficiency. A single cardiac cycle lasts exactly 0.8 seconds. If this timing deviates, your systemic circulation collapses. Do not confuse 'heart rate' with 'cardiac cycle duration'.
The idea
The cardiac cycle comprises sequential systole and diastole of atria and ventricles. Atrial systole lasts 0.1 seconds, while ventricular systole occupies 0.3 seconds. The remaining 0.4 seconds constitute joint diastole, where all four chambers are in a relaxed state. This rhythmic sequence ensures unidirectional blood flow through the heart.
Why it matters
To ensure the orderly filling and pumping of blood through the heart chambers.
NEET lens
NEET examiners frequently test the duration of each phase. Memorize the 0.1, 0.3, and 0.4-second split. Trap: Questions often ask for the duration of 'ventricular systole' specifically; do not confuse it with the total cycle time. Focus on the sequence of valve closures that produce the 'lub' and 'dub' sounds during these phases.
Clinical significance
Arrhythmias occur when the electrical conduction system disrupts this 0.8-second timing. If the joint diastole phase is shortened, ventricular filling is incomplete, leading to reduced stroke volume. This is the primary mechanism behind congestive heart failure symptoms.
Introduction
The cardiac cycle is the complete sequence of events in one heartbeat — atrial systole, ventricular systole, and diastole — occurring approximately 72 times per minute, each cycle lasting ~0.8 seconds.
Core concept
Duration: ~0.8 sec at 72 bpm. Phases: (1) Atrial systole (0.1 sec): SA node fires → atria contract → push blood into ventricles (AV valves open). (2) Ventricular systole (0.3 sec): ventricles contract → AV valves close (lub sound) → semilunar valves open → blood ejected into aorta/pulmonary artery. (3) Diastole/Joint diastole (0.4 sec): both chambers relax → semilunar valves close (dub sound) → AV valves open → ventricles fill passively. Heart sounds: 'Lub' = AV valves closing (start of ventricular systole). 'Dub' = semilunar valves closing (start of diastole).
Go deeper
Isovolumetric contraction: brief period at start of ventricular systole where ventricles contract but all valves closed → pressure builds without blood moving. Isovolumetric relaxation: brief period at start of diastole where ventricles relax but all valves still closed → pressure falls. These phases are important in understanding heart sounds and murmurs.
From Class 10
Each heartbeat is a carefully coordinated sequence of events — chambers filling, contracting, and relaxing in the right order. This complete sequence is the cardiac cycle.