Overview
This lecture explains the events of the cardiac cycle, detailing the sequence of contraction and relaxation phases of the heart and the roles of its valves.
Cardiac Cycle Basics
- The cardiac cycle is a repeating sequence of heart events from one heartbeat to the next.
- Cycle duration is calculated as 1 over heart rate; at 72 bpm, each cycle lasts ~0.8 seconds.
- Higher heart rates shorten the cycle; lower rates lengthen it.
- Each cycle includes both systole (contraction) and diastole (relaxation) of the atria and ventricles.
Phases of the Cardiac Cycle
- Atrial systole (0.1s): atria contract, finishing ventricular filling.
- Atrial diastole (0.7s): atria relax and fill with blood.
- Ventricular systole (0.3s): ventricles contract to eject blood.
- Ventricular diastole (0.5s): ventricles relax, fill with blood.
- Diastole is longer than systole in both chambers.
- Atrial and ventricular phases overlap; atrial systole precedes ventricular systole.
Flow of Blood and Valve Actions
- Blood enters RA (right atrium) from body via IVC/SVC (superior and inferior vena cava) and LA (left atrium) from lungs via pulmonary veins during atrial diastole.
- Passive flow fills ventricles when atrioventricular (AV) valves (tricuspid and mitral) are open.
- Atrial contraction pushes the last 10-20% of blood into the ventricles.
- AV valves close at the end of atrial systole; ventricles are now filled.
Ventricular Systole Detailed Phases
- Isovolumetric contraction: all valves closed, ventricles contract, pressure rises (no volume change).
- Rapid ejection: semilunar valves (pulmonary and aortic) open, blood is pumped out quickly.
- Reduced ejection: slower outflow as ventricular contraction wanes.
- Semilunar valves close at systole end.
Ventricular Diastole Detailed Phases
- Protodiastole: ventricular pressure drops, semilunar valves close.
- Isovolumetric relaxation: all valves closed, ventricles relax, pressure drops.
- Rapid filling: AV valves open, blood quickly enters ventricles.
- Slow filling (diastasis): slow ventricular filling until next atrial systole.
Valve Events and Cycle Summary
- Isovolumetric phases occur when all valves are closed.
- AV valves close at start of isovolumetric contraction; semilunar valves close at start of isovolumetric relaxation.
- Diastole includes isovolumetric relaxation, rapid, and slow filling phases.
- The Wiggers diagram visually represents all these sequential events in the cardiac cycle.
Key Terms & Definitions
- Systole — phase of muscle contraction (atria or ventricles).
- Diastole — phase of muscle relaxation (atria or ventricles).
- Atrioventricular (AV) valves — tricuspid (right) and mitral (left) valves between atria and ventricles.
- Semilunar valves — pulmonary (right) and aortic (left) valves at ventricular outlets.
- Isovolumetric contraction/relaxation — phases when all valves are closed, so chamber volume does not change.
- Diastasis — slow ventricular filling phase.
- Protodiastole — early part of ventricular diastole as pressure drops.
Action Items / Next Steps
- Review the Wiggers diagram to visualize phase relationships.
- Watch part two for details on volume changes during different phases.