Cardiac cycle simplified!
The cardiac cycle refers to the complete sequence of electrical and mechanical events that occur in the heart from the beginning of one heartbeat to the beginning of the next. In a healthy adult with a resting heart rate of 75 beats per minute, one complete cycle lasts approximately 0.8 seconds.
The cycle is driven by electrical impulses and relies on pressure gradients to move blood sequentially through the chambers and out into the body. It is broadly divided into two primary phases:
- Diastole: The relaxation phase, during which the heart muscle relaxes and the chambers fill with blood.
- Systole: The contraction phase, during which the heart muscle contracts and actively pumps blood out into the pulmonary and systemic circulation.
The Seven Phases of the Cardiac Cycle
To understand the mechanics of the heart, the cycle is typically broken down into seven specific stages, defined by changes in pressure, volume, and valve positions.
| Phase | Valve Status | Key Events |
| 1. Atrial Systole | AV Open, Semilunar Closed | The atria contract, pushing the final 20% of blood volume into the ventricles (often called the “atrial kick”). |
| 2. Isovolumetric Contraction | All Valves Closed | Ventricular contraction begins. Pressure rises rapidly within the ventricles, but because all valves are closed, the volume of blood remains unchanged. |
| 3. Rapid Ventricular Ejection | AV Closed, Semilunar Open | Ventricular pressure exceeds the pressure in the aorta and pulmonary artery, forcing the semilunar valves open. Blood is rapidly ejected. |
| 4. Reduced Ventricular Ejection | AV Closed, Semilunar Open | The ventricles begin to relax. Pressure starts to drop, and the rate of blood ejection slows down. |
| 5. Isovolumetric Relaxation | All Valves Closed | Ventricular pressure falls below arterial pressure, causing the semilunar valves to snap shut (producing the second heart sound). The ventricles continue to relax as closed chambers. |
| 6. Rapid Ventricular Filling | AV Open, Semilunar Closed | Ventricular pressure falls below atrial pressure. The atrioventricular (AV) valves open, and blood that accumulated in the atria rushes into the ventricles. |
| 7. Reduced Filling (Diastasis) | AV Open, Semilunar Closed | The ventricles continue to fill, but at a much slower rate as the pressure gradient between the atria and ventricles equalizes. |
(Note: AV valves = Mitral and Tricuspid; Semilunar valves = Aortic and Pulmonary)
Wiggers Diagram
The Wiggers Diagram is the gold standard for visualizing the simultaneous changes that occur during a single heartbeat. It aligns electrical activity (ECG), chamber pressures, ventricular volume, and heart sounds on a single timeline.
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About The Author
Johnson Francis
Former Professor of Cardiology, Calicut Govt. Medical Kozhikode, Kerala, India. Editor-in-Chief, BMH Medical Journal