Overview
This lecture explains the cardiac cycle, breaking down each step of a heartbeat, key electrical events (ECG), heart sounds, and valve actions.
The Cardiac Cycle Overview
- The cardiac cycle is the sequence of events from the start of one heartbeat to the start of the next.
- Blood returns to the heart via the atria: left atrium (from the lungs), right atrium (from the body).
- Atria contract, pushing blood into ventricles; ventricles contract, pushing blood out of the heart.
Electrical Events (ECG)
- The P wave on ECG shows atrial depolarization, triggering atrial contraction (atrial systole).
- The QRS complex represents ventricular depolarization, leading to ventricular contraction (ventricular systole).
- The T wave indicates ventricular repolarization, leading to ventricular relaxation (diastole).
Pressure and Volume Changes
- Atrial contraction (after P wave) increases atrial pressure and ventricular volume as blood moves into ventricles.
- Ventricular contraction (after QRS) causes sharp rise in ventricular pressure.
- Isovolumetric contraction: ventricles contract with all valves closed; volume is constant but pressure rises.
- When ventricular pressure exceeds aortic pressure (~80 mmHg), semilunar valves open and blood is ejected, reducing ventricular volume.
- Isovolumetric relaxation: ventricles relax with all valves closed; pressure drops, volume constant.
- When ventricular pressure drops below atrial pressure, atrioventricular valves open; ventricles fill passively.
Heart Sounds (Phonocardiogram)
- First heart sound ("lub") occurs when atrioventricular valves close after QRS complex.
- Second heart sound ("dub") occurs when semilunar valves close after T wave.
Key Terms & Definitions
- Cardiac Cycle — the complete sequence of events in one heartbeat.
- Systole — contraction phase of a heart chamber.
- Diastole — relaxation phase of a heart chamber.
- Depolarization — electrical activation of heart muscle leading to contraction.
- Repolarization — electrical resetting of heart muscle, leading to relaxation.
- Isovolumetric Contraction/Relaxation — phases where chamber volume remains unchanged as all valves are closed.
- Atrioventricular Valves — valves between atria and ventricles.
- Semilunar Valves — valves between ventricles and arteries (aorta, pulmonary artery).
Action Items / Next Steps
- Review the flow of blood through the heart for better understanding.
- Re-watch or pause the video as needed to clarify each phase of the cardiac cycle.