Cardiac Cycle Study Pack

Kibin's free study pack on Cardiac Cycle includes a 3-section study guide, 8 quiz questions, 10 flashcards, and 1 open-ended Explain review question. 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 22, 2026

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Cardiac Cycle Study Guide

Trace every phase of the cardiac cycle — from passive ventricular filling and atrial contraction through isovolumetric contraction, semilunar valve opening, and ejection — with clear coverage of valve mechanics, heart sounds S1 and S2, stroke volume, cardiac output, and the Wiggers diagram that ties pressure, volume, and ECG changes together.

Key Takeaways

  • •The cardiac cycle is the complete sequence of mechanical and electrical events that occurs with each heartbeat, divided into diastole (relaxation and filling) and systole (contraction and ejection).
  • •Ventricular filling during diastole depends on pressure gradients that draw blood from the atria; approximately 70–80% of ventricular filling is passive, with the final 20–30% delivered by atrial contraction.
  • •Ventricular systole generates enough pressure to open the semilunar valves — the aortic valve on the left and the pulmonary valve on the right — driving blood into the systemic and pulmonary circulations respectively.
  • •The four cardiac valves (mitral, tricuspid, aortic, pulmonary) open and close passively in response to pressure differentials, and their closure produces the characteristic heart sounds S1 and S2.
  • •Stroke volume — the volume of blood ejected per beat — combined with heart rate determines cardiac output, which in a healthy adult at rest is approximately 5 liters per minute.
  • •The isovolumetric contraction and isovolumetric relaxation phases are brief periods during which all four valves are closed simultaneously and ventricular volume remains constant while pressure changes dramatically.
  • •The Wiggers diagram organizes changes in aortic pressure, ventricular pressure, atrial pressure, ventricular volume, and the electrocardiogram on a shared time axis, making it the standard tool for analyzing cardiac cycle events.

Structural Basis of the Cardiac Cycle

Understanding the cardiac cycle requires a clear picture of the chambers, valves, and pressure relationships that drive blood flow through the heart.

Four-Chamber Architecture and Directional Flow

  • •The heart contains two functionally separate pumps operating in parallel: the right side receives deoxygenated blood from the systemic veins and pumps it to the lungs; the left side receives oxygenated blood from the pulmonary veins and pumps it to the body.
  • •The right ventricle pumps against pulmonary vascular resistance, which is much lower than systemic vascular resistance, so it generates pressures of roughly 25/8 mmHg compared to the left ventricle's 120/80 mmHg.
  • •Both ventricles contract and relax simultaneously, ensuring coordinated ejection into the pulmonary and systemic circuits.

Atrioventricular and Semilunar Valves

  • •The atrioventricular (AV) valves — the mitral valve (left) and the tricuspid valve (right) — sit between the atria and ventricles and prevent backflow during ventricular contraction.
  • •The semilunar valves — the aortic valve (left) and the pulmonary valve (right) — sit at the ventricular outflow tracts and prevent backflow from the great arteries into the ventricles during relaxation.
  • •All four valves are passive structures; they open and close solely in response to pressure gradients across them, with no direct neural control.

Pressure Gradients as the Driving Force

  • •Blood moves from regions of higher pressure to regions of lower pressure; every valve opening and every phase of the cardiac cycle is a direct consequence of changing pressure relationships.
  • •The atria, ventricles, aorta, and pulmonary trunk each have characteristic pressure ranges, and the cycle proceeds as each chamber's pressure rises above or falls below its neighboring chamber.

Phases of Ventricular Diastole: Relaxation and Filling

Ventricular diastole encompasses all events from the end of ejection through the completion of ventricular filling, and it occupies roughly two-thirds of a cardiac cycle at a resting heart rate of 75 beats per minute.

Isovolumetric Relaxation

  • •Immediately after the ventricles finish ejecting blood, ventricular pressure drops below aortic and pulmonary arterial pressures, causing the semilunar valves to snap shut — producing the second heart sound, S2.
  • •The AV valves have not yet opened because ventricular pressure still exceeds atrial pressure; both sets of valves are simultaneously closed, so ventricular volume remains constant while pressure falls rapidly.
  • •This phase is called isovolumetric relaxation because volume is fixed ('iso' = same) even as the myocardium actively relaxes through calcium re-uptake into the sarcoplasmic reticulum.

Passive Ventricular Filling

  • •Once ventricular pressure drops below atrial pressure, the AV valves open and blood flows down its pressure gradient from the atria into the ventricles without any muscular effort — this passive phase accounts for approximately 70–80% of total ventricular filling.
  • •Blood flows in a rapid early burst immediately after AV valve opening, then slows during a period called diastasis as the pressure gradient between atria and ventricles equilibrates.
  • •Ventricular compliance — the ability of the ventricular wall to stretch without generating excessive pressure — determines how much blood can fill passively; stiff ventricles (as in hypertensive heart disease) fill poorly.

Atrial Contraction and the 'Atrial Kick'

  • •Late in diastole, the sinoatrial (SA) node fires, depolarization spreads through the atria (generating the P wave on the ECG), and atrial contraction actively pushes the remaining 20–30% of filling volume into the ventricles.
  • •This final contribution is called the atrial kick, and it is particularly important during tachycardia, when diastole is shortened and passive filling is limited.
  • •The end-diastolic volume (EDV) — the total volume present in the ventricle just before contraction begins — typically reaches 110–130 mL in a healthy left ventricle at rest.

About this Study Pack

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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