Anatomy and Physiology 2e · The Cardiovascular System: The Heart
Cardiac Cycle
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In 30 seconds
A single heartbeat — from the start of one beat to the start of the next — is the cardiac cycle: a precisely timed sequence of contraction (systole) and relaxation (diastole) in the atria and ventricles, driven entirely by pressure differences. Blood flows down pressure gradients; the valves simply ensure it flows one way. When a chamber's pressure rises above the chamber upstream, the valve between them closes; when its pressure falls below the chamber downstream, the valve opens. At a commonly taught resting rate of ~75 beats/min, one cycle lasts ~0.8 s: ~0.1 s of atrial systole, ~0.3 s of ventricular systole, ~0.4 s of relaxation. The heart spends more time filling than pumping — exactly what a muscle that never rests needs.
Why this matters
The cardiac cycle connects the chapter's threads: electrical events (ECG) trigger the mechanical events, which produce the pressures, sounds, and pulses you can observe. Clinically, the cycle explains the "lub-dub" Heart sounds S1 "lub" (AV closure) and S2 "dub" (semilunar closure) Full entry →, what a murmur means in general terms (turbulent flow, often from a valve that does not open or close properly), and why the wrist pulse matches ventricular ejection. Volume measurements from the cycle — stroke volume and ejection fraction — are daily clinical vocabulary, and heart failure is fundamentally inadequate output per cycle. Exam questions test sequence: what happens when, which valves are open or closed at each moment, and which sound marks which transition.
The college version
Core Concepts
Systole and diastole
Systole is contraction; diastole is relaxation. The atria and ventricles cycle out of sync: atrial systole occurs while the ventricles are still relaxed and filling, then ventricular systole follows as the atria relax. At ~75 bpm (commonly taught), ventricular systole lasts ~0.3 s and ventricular diastole ~0.5 s. Because diastole is longer, the ventricles spend most of their time filling — and the coronary arteries, squeezed shut during systole, fill during diastole.
The five phases of the cycle
1. Ventricular filling. The ventricles are relaxed, ventricular pressure is low, AV valves open, semilunar valves closed. Blood flows passively from atria to ventricles — rapidly at first, then slowly (diastasis). About 70–80% of filling happens with no atrial contraction at all.
2. Atrial systole ("atrial kick"). The atria contract, squeezing the last ~20–30% in. This topping-off matters most when ventricles are stiff or heart rate is fast, since passive filling time is short.
3. Isovolumetric "Same volume": all valves closed while pressure changes Full entry → ventricular contraction. Ventricular pressure rises above atrial pressure, slamming the AV valves shut (S1, "lub"), but stays below arterial pressure, so the semilunar valves remain closed. With all four valves shut, pressure rises steeply while volume stays constant ("isovolumetric").
4. Ventricular ejection. Once ventricular pressure exceeds pulmonary trunk and aortic pressure, the semilunar valves open and blood is ejected — rapidly, then more slowly. Ventricular pressure peaks here; this surge creates the arterial pulse.
5. Isovolumetric ventricular relaxation. The ventricles relax; ventricular pressure drops below arterial pressure and the semilunar valves snap shut (S2, "dub"). Pressure is still above atrial pressure, so the AV valves stay closed; volume holds constant while pressure falls. When ventricular pressure drops below atrial pressure, the AV valves reopen and filling resumes.
Pressure gradients and valve logic
One rule drives the whole cycle: blood flows from higher to lower pressure, and each valve opens only when downstream pressure is lower than upstream pressure. AV valves open when ventricular pressure falls below atrial pressure (filling) and close when it rises above (contraction). Semilunar valves open when ventricular pressure exceeds arterial pressure (ejection) and close when it falls below (relaxation). Know the pressures and you know the valve states.
Heart sounds
The two audible sounds are valve closures, not muscle contraction: S1 ("lub") = AV valves closing at the start of ventricular systole; S2 ("dub") = semilunar valves closing at the start of ventricular diastole. The pause between S1 and S2 is ventricular systole; the longer pause after S2 is diastole. Extra sounds can occur — S3 with rapid filling (sometimes heard in healthy young people), S4 with the atria contracting against a stiff ventricle; both are "extra" sounds whose significance depends on context. A murmur is turbulent blood flow, often from a narrowed (stenotic) or leaking (regurgitant) valve; murmurs are not always pathological.
Volumes that describe the pump
- End-diastolic volume (EDV) — blood in the ventricle after filling (commonly taught ~120 mL at rest).
- End-systolic volume (ESV) — blood left after ejection (commonly taught ~50 mL at rest).
- Stroke volume (SV) EDV − ESV: blood ejected per beat Full entry → = EDV − ESV — blood ejected per beat (commonly taught ~70 mL at rest).
- Ejection fraction (EF) SV ÷ EDV: fraction of the filled ventricle ejected Full entry → = SV ÷ EDV — fraction of the filled ventricle ejected (commonly taught ~55–65% at rest).
These are the practical outputs of the cycle: cardiac output is simply stroke volume × heart rate.
Common Confusions
| Do not confuse | With | Difference |
|---|---|---|
| "Systole" = ejection | Ventricular systole starts before ejection | Isovolumetric contraction precedes ejection |
| S1 = semilunar valves | S1 = AV valves ("lub") | S1 marks AV closure at start of systole; S2 marks semilunar closure |
| Atrial systole is the main filling mechanism | Filling is mostly passive (~70–80%) | Atrial kick adds only ~20–30% |
| Heart sounds = muscle contracting | Heart sounds = valve closures | Contracting muscle is nearly silent to a stethoscope |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Imagine a water balloon with two one-way doors. It stretches and fills from the faucet — mostly by itself, with a little squeeze at the end to top it off (the atrial kick). Then you squeeze: the faucet door slams shut ("lub"), and once the squeeze is strong enough, the far door bursts open and water shoots out. When you stop squeezing, the far door slams shut ("dub"), the balloon relaxes, and the faucet door reopens to refill. That whole squeeze-and-refill story is one heartbeat.
Worked example
Listening with a stethoscope. You hear "lub... dub... (pause)... lub... dub..." — about 75 cycles per minute. "Lub" (S1) occurs the instant ventricular pressure rises above atrial pressure: ventricular contraction has begun. The pause between "lub" and "dub" is ventricular systole: pressure climbs, the semilunar valves open, blood is ejected, and a pulse wave races down the arteries — hold a finger on the radial artery and you feel the pulse in this window. "Dub" (S2) occurs the instant ventricular pressure falls below arterial pressure: ventricular relaxation has begun, and aortic back-pressure slams the aortic valve shut. The long quiet after "dub" is diastole: AV valves reopen and the ventricles refill, mostly passively, while the coronary arteries get their blood supply.
Key takeaways
- One cycle ≈ 0.8 s at ~75 bpm (commonly taught): atrial systole ~0.1 s, ventricular systole ~0.3 s, relaxation ~0.4 s — more time filling than pumping.
- ~70–80% of ventricular filling is passive; atrial kick adds ~20–30%.
- Isovolumetric phases (contraction and relaxation): all four valves closed, volume constant, pressure changing.
- Valve logic: AV valves close when ventricular pressure > atrial pressure; semilunar valves open when ventricular pressure > arterial pressure.
- S1 "lub" = AV closure (start of systole); S2 "dub" = semilunar closure (start of diastole).
- Coronary arteries fill mainly during diastole.
- Murmur = turbulent flow, usually from a valve problem; S3/S4 are context-dependent extra sounds.
Check yourself
5 review questions from the chapter. Try each one, then open the answer.
At ~75 bpm, how long is one cardiac cycle, and how is the time divided?
Show answer
About 0.8 s: roughly 0.1 s of atrial systole, 0.3 s of ventricular systole, and 0.4 s of relaxation (commonly taught values at ~75 bpm).
How much of ventricular filling is passive, and what does atrial systole contribute?
Show answer
About 70–80% is passive, driven by the pressure difference between relaxed atria and ventricles; atrial systole adds ~20–30% as the atrial kick.
What defines an isovolumetric phase, and when do the two occur?
Show answer
An isovolumetric phase has all four valves closed, so chamber volume does not change while pressure does. The two occur at the start of ventricular systole (before ejection) and of ventricular diastole (before filling resumes).
Which valves close at S1 and S2, and what events do those sounds mark?
Show answer
S1 ("lub") = AV valves closing at the start of ventricular systole; S2 ("dub") = semilunar valves closing at the start of ventricular diastole.
If EDV is 120 mL and ESV is 50 mL, what are the stroke volume and ejection fraction?
Show answer
SV = 120 − 50 = 70 mL; EF = 70 ÷ 120 ≈ 58%, within the commonly taught ~55–65% resting range.
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- Systole / diastole
- Contraction / relaxation phases of a chamber
- Isovolumetric
- "Same volume": all valves closed while pressure changes
- EDV / ESV
- Blood in the ventricle after filling / after ejection
- Stroke volume (SV)
- EDV − ESV: blood ejected per beat
- Ejection fraction (EF)
- SV ÷ EDV: fraction of the filled ventricle ejected
- Heart sounds
- S1 "lub" (AV closure) and S2 "dub" (semilunar closure)
Sources & references
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