Cardiac Cycle — Core Principles

Core Principles

The cardiac cycle is the sequence of events in the heart from one heartbeat to the next, typically lasting 0.8 seconds at a resting heart rate of 75 bpm. It comprises two main phases: diastole (relaxation and filling) and systole (contraction and ejection).

The cycle begins with joint diastole, where all chambers relax and ventricles passively fill. This is followed by atrial systole, where atria contract to push the remaining blood into the ventricles, reaching the End-Diastolic Volume (EDV).

Next is ventricular systole, starting with isovolumetric contraction (all valves closed, pressure rising, S1 sound from AV valve closure), then ventricular ejection (semilunar valves open, blood pumped out, leaving End-Systolic Volume (ESV)).

Finally, ventricular diastole begins with isovolumetric relaxation (all valves closed, pressure falling, S2 sound from semilunar valve closure), followed by rapid and reduced ventricular filling, leading back to joint diastole.

Pressure gradients drive blood flow and valve actions, ensuring unidirectional circulation. The ECG correlates electrical events (P wave, QRS complex, T wave) with these mechanical phases.

Important Differences

vs Heartbeat

Aspect	This Topic	Heartbeat
Definition	The complete sequence of mechanical and electrical events occurring in the heart from the beginning of one contraction to the beginning of the next.	A single contraction and relaxation of the heart muscle, resulting in a pulse or 'beat'.
Scope	Encompasses all phases: atrial systole, ventricular systole (isovolumetric contraction, ejection), and diastole (isovolumetric relaxation, filling).	Refers primarily to the mechanical action of the heart, the 'thump' felt or heard.
Components	Involves pressure changes, volume changes, valve actions, heart sounds, and electrical events (ECG correlation).	The audible and palpable manifestation of the heart's pumping action.
Duration	Typically 0.8 seconds at rest (for 75 bpm).	The duration of one complete cardiac cycle.
Measurement	Analyzed through pressure-volume loops, ECG, phonocardiography.	Measured as heart rate (beats per minute) or felt as a pulse.

While often used interchangeably, the cardiac cycle is a more comprehensive term than a heartbeat. A heartbeat refers to the physical contraction and relaxation of the heart muscle, the 'lub-dub' sound and the pulse felt. The cardiac cycle, on the other hand, describes the entire, intricate sequence of electrical and mechanical events – including pressure and volume changes, valve movements, and blood flow – that occur from the start of one heartbeat to the start of the next. Essentially, one cardiac cycle constitutes one complete heartbeat, but the cycle details all the underlying physiological processes.