Cardiac Cycle Study Pack
Kibin's free study pack on Cardiac Cycle includes a 6-section study guide, 25 quiz questions, 30 flashcards, and 5 open-ended Explain review questions. Sign up free to track your progress toward mastery, plus upload your own notes and recordings to create personalized study packs organized by course.
Last updated May 27, 2026
Cardiac Cycle Study Guide
Trace the cardiac cycle from diastole through systole, covering the five key events, pressure-driven valve mechanics, S1 and S2 heart sounds, stroke volume calculation, and how to read a Wiggers diagram.
Key Takeaways
- •The cardiac cycle consists of two alternating phases — diastole (relaxation and filling) and systole (contraction and ejection) — that together constitute one complete heartbeat.
- •Pressure gradients drive blood movement: blood flows from areas of higher pressure to lower pressure, and valve opening or closing occurs whenever pressures across a valve equalize or reverse.
- •The cycle is divided into five distinct events: isovolumetric contraction, rapid ejection, isovolumetric relaxation, rapid ventricular filling, and reduced ventricular filling (diastasis).
- •The two heart sounds (S1 and S2) are produced by the closure of the atrioventricular valves and semilunar valves, respectively, not by the contraction of the myocardium itself.
- •Stroke volume — the amount of blood ejected per beat — equals end-diastolic volume minus end-systolic volume, and in a healthy adult at rest is approximately 70 mL.
- •The cardiac cycle takes roughly 0.8 seconds at a resting heart rate of 75 beats per minute, with diastole occupying about 0.5 seconds and systole about 0.3 seconds.
- •The wiggers diagram is the standard tool for visualizing simultaneous changes in ventricular pressure, aortic pressure, ventricular volume, and the electrocardiogram throughout a single cardiac cycle.
Foundations: Pressure, Volume, and Valve Mechanics
Every event in the cardiac cycle is governed by the relationship between pressure and volume inside the heart's chambers, and valves act as passive one-way gates that respond to those pressure differences automatically.
Pressure-Gradient Rule for Blood Flow
- •Blood always moves from a region of higher pressure to a region of lower pressure — no active pumping mechanism steers it through the valves.
- •When ventricular pressure exceeds atrial pressure, the atrioventricular (AV) valves — the mitral valve on the left and the tricuspid valve on the right — are forced shut.
- •When ventricular pressure exceeds pressure in the aorta or pulmonary trunk, the semilunar valves (aortic and pulmonic) are pushed open.
Isovolumetric Periods and Volume Changes
- •During isovolumetric contraction and isovolumetric relaxation, all four valves are closed simultaneously, so ventricular volume does not change even though pressure is changing rapidly.
- •Once a semilunar valve opens, ventricular volume decreases as blood is ejected; once an AV valve opens, ventricular volume increases as blood enters from the atria.
End-Diastolic and End-Systolic Volumes
- •End-diastolic volume (EDV) is the maximum blood volume in the ventricle just before contraction begins — approximately 120 mL in a resting adult.
- •End-systolic volume (ESV) is the residual blood remaining after ejection — approximately 50 mL at rest, meaning the heart does not empty completely with each beat.
Phases of Ventricular Systole
Ventricular systole covers the period of active myocardial contraction and encompasses two sequential events: isovolumetric contraction and the ejection phase.
Isovolumetric Contraction
- •Systole begins when the ventricles depolarize (corresponding to the QRS complex on the ECG), causing ventricular pressure to rise sharply.
- •All valves remain closed during this brief period because ventricular pressure has exceeded atrial pressure (closing the AV valves) but has not yet exceeded aortic or pulmonary trunk pressure (keeping the semilunar valves shut).
- •Ventricular volume stays constant — hence 'isovolumetric' — while tension in the myocardium builds.
Ventricular Ejection
- •Once left ventricular pressure surpasses approximately 80 mmHg (the diastolic aortic pressure), the aortic valve opens and rapid ejection begins.
- •During rapid ejection, roughly two-thirds of the stroke volume exits the ventricle within the first third of systole; flow then slows during the reduced ejection phase.
- •Ejection ends when ventricular pressure falls below aortic pressure, causing the aortic and pulmonic valves to snap shut — producing the second heart sound, S2.
Stroke Volume and Ejection Fraction
- •Stroke volume equals EDV minus ESV; at rest this is approximately 70 mL per beat.
- •Ejection fraction — stroke volume divided by EDV — is normally about 55–65% and is a clinically important index of ventricular function.
Phases of Ventricular Diastole
Diastole is the longer portion of the cardiac cycle and covers both the relaxation of the myocardium and the refilling of the ventricles with blood from the atria.
Isovolumetric Relaxation
- •Immediately after S2, the ventricles begin to repolarize (T wave on the ECG) and myocardial tension drops, causing ventricular pressure to fall rapidly.
- •Because ventricular pressure is still above atrial pressure, all valves remain closed and ventricular volume does not change — a second isovolumetric period.
Rapid Ventricular Filling
- •When ventricular pressure falls below atrial pressure, the AV valves open and blood that has been accumulating in the atria rushes passively into the ventricles.
- •Approximately 70–80% of ventricular filling occurs passively during this early rapid-filling phase, driven entirely by the pressure gradient — no atrial contraction is required.
- •The rush of blood into the ventricle can produce a third heart sound (S3), which is normal in children and young adults but may indicate ventricular dysfunction in older adults.
Diastasis and Atrial Systole
- •Diastasis is the mid-diastolic period of slow filling when the pressure gradient between atria and ventricles nearly equalizes and filling nearly stops.
- •Late diastole ends with atrial systole — the contraction of the atria, triggered by the P wave on the ECG — which contributes the final 20–30% of ventricular filling, called the 'atrial kick.'
- •A fourth heart sound (S4), when present, is associated with a stiff or non-compliant ventricle resisting the atrial kick and is always considered abnormal.
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Created by Kibin to help students review key concepts, prepare for exams, and study more effectively. This Study Pack was checked for accuracy and curriculum alignment using authoritative educational sources. See sources below.
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What is the approximate duration of one complete cardiac cycle at a resting heart rate of 75 beats per minute?
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Pressure-Gradient Rule for Blood Flow and Valve Operation
Explain how pressure gradients control blood flow and valve behavior throughout the cardiac cycle. How does this principle determine when each valve opens and closes, and why do valves not require active muscle control?
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