Cardiac Cycle — Heart Physiology NEET Explained
What is the cardiac cycle?
The cardiac cycle is the sequence of mechanical and electrical events that occur during one heartbeat. It includes atrial contraction, ventricular contraction, and ventricular relaxation (atrial relaxation happens during ventricular systole). The entire cycle takes about 0.8 seconds at a resting heart rate of 75 bpm.
The cardiac cycle is the sequence of events that occur during one heartbeat. It has two main phases: systole (contraction) and diastole (relaxation). Blood flows through the heart's four chambers (right atrium, right ventricle, left atrium, left ventricle) in a precise sequence. Understanding the cardiac cycle — when each chamber contracts, when valves open/close, how pressure changes — is essential for NEET physiology.
Key NEET Facts
- •Cardiac cycle = systole (contraction) + diastole (relaxation), lasts ~0.8 seconds at rest (heart rate 75 bpm)
- •Atrial systole: both atria contract, pushing blood into ventricles; both atrioventricular (AV) valves open
- •Ventricular systole: both ventricles contract, pushing blood into arteries; AV valves close, semilunar valves open
- •Ventricular diastole: both ventricles relax, pressure drops, semilunar valves close, blood refills from atria
- •Heart sounds: S₁ (lub) = AV valve closure; S₂ (dub) = semilunar valve closure
- •Pressure: left ventricle reaches ~120 mmHg (systolic), drops to ~80 mmHg (diastolic)
Common Mistakes
- ✕Thinking the entire heart contracts at once — wrong. Atrial and ventricular contractions are coordinated but separate.
- ✕Confusing systole with diastole — systole is contraction (pressure ↑), diastole is relaxation (pressure ↓).
- ✕Forgetting valve directionality — AV valves (tricuspid, mitral) prevent backflow from ventricles to atria; semilunar valves (pulmonary, aortic) prevent backflow from arteries to ventricles.
NEET Frequency: 2-3 questions per year
Frequently Asked Questions
What is the difference between systole and diastole?›
Systole is the phase when the heart muscle contracts, forcing blood out of the chambers. Diastole is the phase when the heart muscle relaxes, allowing chambers to fill with blood. Systole = 'lub', diastole = 'dub'. Blood pressure is recorded as systolic/diastolic (e.g., 120/80 mmHg).
Why do heart valves open and close?›
Valves are one-way gates that ensure blood flows in the correct direction. AV valves (tricuspid, mitral) open during atrial systole to let blood into ventricles, then close during ventricular systole to prevent backflow. Semilunar valves (pulmonary, aortic) open during ventricular systole to let blood out, then close during diastole to prevent backflow from arteries.
What causes the 'lub-dub' heart sound?›
'Lub' (S₁) is caused by the closure of atrioventricular valves at the start of ventricular systole. 'Dub' (S₂) is caused by the closure of semilunar valves at the start of ventricular diastole. These vibrations are audible through a stethoscope.
How does blood pressure change during the cardiac cycle?›
Left ventricular pressure rises sharply during ventricular systole, reaching ~120 mmHg (systolic pressure). During ventricular diastole, pressure drops to ~80 mmHg (diastolic pressure). Aortic pressure follows a similar pattern. Atrial pressure changes are smaller (0-8 mmHg) because atria are thin-walled.
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