Overview
This lecture explains the cardiac cycle, detailing the sequence of events during each heartbeat, including the roles of systole, diastole, valve operations, and associated heart sounds.
Cardiac Cycle Phases
- The cardiac cycle consists of systole (ventricular contraction) and diastole (ventricular relaxation).
- Systole and diastole are subdivided into smaller phases involving changes in pressure and valve positions.
Electrical Initiation & Atrial Events
- The SA node initiates the cycle by causing atrial depolarization, seen as the P-wave on ECG.
- Atrial contraction follows, increasing atrial pressure and pushing blood into the ventricles.
- Most ventricular filling is passive, not due to atrial contraction.
Ventricular Systole
- As atrial contraction ends, atrial pressure drops; AV valves close as the pressure gradient reverses.
- Closing of AV valves creates the first heart sound (S1), marking the start of systole.
- QRS complex represents ventricular depolarization, leading to ventricular contraction.
- Isovolumetric contraction occurs when ventricles contract with all valves closed; no blood is ejected.
Ventricular Ejection & End of Systole
- When ventricular pressure exceeds aortic/pulmonary artery pressure, semilunar valves open and rapid blood ejection occurs.
- T-wave indicates ventricular repolarization as ejection force decreases.
- Closing of semilunar valves (as ventricular pressure drops below arterial) creates the second heart sound (S2) and ends systole.
Diastole & Ventricular Filling
- Isovolumetric relaxation phase begins diastole with both AV and semilunar valves closed; ventricular volume unchanged.
- Ventricular pressure drops rapidly; atria fill and atrial pressure rises.
- When ventricular pressure falls below atrial, AV valves open, allowing passive ventricular filling.
- Atrial contraction completes the filling phase, and the cycle repeats.
Key Terms & Definitions
- Systole — Ventricular contraction phase of the cardiac cycle.
- Diastole — Ventricular relaxation phase of the cardiac cycle.
- Isovolumetric Contraction — Ventricles contract with all valves closed; no volume change.
- Isovolumetric Relaxation — Ventricles relax with all valves closed; no volume change.
- AV Valves — Valves between atria and ventricles (tricuspid and mitral).
- Semilunar Valves — Valves between ventricles and arteries (aortic and pulmonic).
- S1 — First heart sound, caused by AV valve closure.
- S2 — Second heart sound, caused by semilunar valve closure.
- SA Node — Sinoatrial node, initiates the heartbeat.
- QRS Complex — ECG waveform representing ventricular depolarization.
- T-wave — ECG waveform representing ventricular repolarization.
Action Items / Next Steps
- Review ECG waveforms and their corresponding cardiac cycle events.
- Memorize the sequence of valve movements and associated heart sounds.
- Understand pressure changes and their influence on blood flow during the cycle.