Cardiac Cycle Phases Overview

Overview

This lecture explains the phases of the cardiac cycle, detailing chamber pressures, valve changes, and their relationship to EKG waves and heart sounds.

Definition and Timing of the Cardiac Cycle

• The cardiac cycle consists of all the mechanical events as blood flows through the heart, lasting about 0.8 seconds.
• The cycle involves changes in pressures, valve positions, and electrical activity (EKG).

Phases of the Cardiac Cycle

1. Mid to Late Ventricular Diastole (Ventricular Filling)

• Diastole means relaxation; ventricles are relaxed and filling with blood.
• Blood returns via systemic veins, coronary sinus, and pulmonary veins into atria.
• Atrial pressure is greater than ventricular pressure, causing AV (atrioventricular) valves to open.
• 70–80% of blood flows passively into ventricles; last 20% is pushed in by atrial contraction.
• Semilunar valves (aortic and pulmonary) remain closed since arterial pressure > ventricular pressure.
• The P wave on the EKG corresponds to atrial depolarization and contraction.

2. Isovolumetric Contraction (Early Ventricular Systole)

• Ventricles begin contracting, raising ventricular pressure.
• Ventricular pressure exceeds atrial pressure, thus AV valves snap shut (producing the "Lub" or S1 heart sound).
• Semilunar valves remain closed because arterial pressure is still greater than ventricular pressure.
• No blood is ejected yet; volume in ventricles does not change.
• The QRS complex on the EKG indicates ventricular depolarization.

3. Ventricular Ejection (Mid to Late Ventricular Systole)

• Ventricular pressure becomes greater than arterial (aorta/pulmonary trunk) pressure.
• Semilunar valves open, blood is ejected from ventricles to arteries.
• AV valves remain closed because ventricular pressure > atrial pressure.
• EKG still shows the QRS complex.

4. Isovolumetric Relaxation (Early Ventricular Diastole)

• Ventricles relax, ventricular pressure drops below arterial pressure.
• Semilunar valves snap shut ("Dub" or S2 heart sound), brief rise in aortic pressure called dicrotic notch.
• Both sets of valves (AV and semilunar) are closed for a brief period.
• Ventricular pressure is still greater than atrial pressure, keeping AV valves closed.
• EKG displays the T wave (ventricular repolarization).

Key Terms & Definitions

• Diastole — Phase when heart muscle relaxes and allows chambers to fill with blood.
• Systole — Phase when heart muscle contracts and pumps blood.
• AV (Atrioventricular) Valves — Tricuspid and mitral (bicuspid) valves between atria and ventricles.
• Semilunar Valves — Aortic and pulmonary valves between ventricles and arteries.
• EDV (End Diastolic Volume) — Blood volume in ventricles at the end of diastole.
• ESV (End Systolic Volume) — Blood remaining in ventricles after contraction.
• P Wave — EKG signal for atrial depolarization.
• QRS Complex — EKG signal for ventricular depolarization.
• T Wave — EKG signal for ventricular repolarization.
• S1 ("Lub") — First heart sound; AV valves closing.
• S2 ("Dub") — Second heart sound; semilunar valves closing.
• Dicrotic Notch — Brief increase in aortic pressure after semilunar valve closure.

Action Items / Next Steps

• Review the sequence and pressure changes of each cardiac cycle phase.
• Study EKG waves in relation to each cardiac phase.
• Prepare for upcoming material on cardiac output (EDV and ESV definitions will be relevant).