Cardiac Cycle Overview

Overview

This lecture explains the cardiac cycle step-by-step, focusing on the sequence of electrical and mechanical events in one heartbeat.

What is the Cardiac Cycle?

• The cardiac cycle is everything that happens from the start of one heartbeat to the start of the next.
• Blood returns to the heart via the atria, which then contract to push blood into the ventricles.
• Ventricles contract next, pushing blood out of the heart to the lungs or body.

Electrical Events of the Heart (ECG)

• The ECG (electrocardiogram) shows the heart’s electrical activity that triggers muscle contractions.
• The P wave signals atrial depolarization which initiates atrial contraction (atrial systole).
• The QRS complex signals ventricular depolarization, starting ventricular contraction (ventricular systole).
• The T wave represents ventricular repolarization, leading to ventricular relaxation.

Mechanical Events and Pressure Changes

• Atrial systole increases atrial pressure, pushing blood into the ventricles and increasing ventricular volume.
• Ventricular systole begins after the QRS complex, causing a sharp rise in ventricular pressure.
• Isovolumetric contraction: Both valves are closed, ventricular volume remains unchanged, but pressure rises.
• Semilunar valves open when ventricular pressure exceeds aortic pressure (~80 mmHg), starting the ejection phase.
• Ventricular volume decreases as blood is ejected into the body/aorta.

Relaxation and Passive Filling

• After the T wave, ventricles relax, pressure drops, and both valves are briefly closed (isovolumetric relaxation).
• When ventricular pressure falls below atrial pressure, atrioventricular valves open, and ventricles fill passively.
• The cycle repeats with the next P wave.

Heart Sounds (Phonocardiogram)

• "Lub" sound: Closure of atrioventricular valves at the start of ventricular contraction (after QRS complex).
• "Dub" sound: Closure of semilunar valves at the start of ventricular relaxation (after T wave).

Key Terms & Definitions

• Cardiac Cycle — sequence of events from one heartbeat to the next.
• Atrial Systole — contraction of the atria.
• Ventricular Systole — contraction of the ventricles.
• Depolarization — electrical activation causing contraction.
• Repolarization — electrical recovery causing relaxation.
• Isovolumetric Contraction — contraction with no change in ventricular volume as both valves are closed.
• Isovolumetric Relaxation — relaxation with no change in ventricular volume as both valves are closed.
• Semilunar Valves — valves between ventricles and arteries (aorta/pulmonary artery).
• Atrioventricular Valves — valves between atria and ventricles.

Action Items / Next Steps

• Review blood flow through the heart if you need more background.
• Pause and rewatch the video as needed to reinforce your understanding.
• Practice drawing and labeling the cardiac cycle graph.