The Cardiac Cycle 

The cardiac cycle refers to the sequence of electrical and mechanical events that occur during one heartbeat — ie;  from the beginning of one contraction (systole) through relaxation and filling (diastole) to the beginning of the next contraction (systole). It ensures the rhythmic pumping of blood through the heart and into the circulation. 
 
Phases of the Cardiac Cycle 

There are two main phases: 
Diastole – relaxation and filling of the heart. 
Systole – contraction and ejection of blood. 
 
A normal cardiac cycle lasts approximately 0.8 seconds (assuming a heart rate of 75 bpm), which indicates both relaxation and contraction phases.  The duration is inversely proportional to heart rate; at a faster heart rate, the cardiac cycle is shorter, while a slower heart rate corresponds to a longer cycle. 
 
With increased heart rate eg; during exercise, the cardiac cycle shortens.  The duration of diastole (relaxation) decreases more significantly than the duration of systole (contraction). 
 
Cycle Phases: The cycle includes atrial and ventricular diastole (relaxation) and systole (contraction).  At rest, diastole makes up about two thirds of the cycle, and systole about one third. 
  
Cycle Phases in detail (duration based on HR of 75bpm): 
 
1. Atrial Contraction (Systole) (0.1 s) 
Event: Atria contract, pushing the final ~30% of blood into ventricles. 
AV valves (mitral & tricuspid): Open 
Semilunar valves (aortic & pulmonary): Closed 
Pressures: 
Atrial pressure ↑ (4–6 mmHg) 
Ventricular pressure slightly ↑ (~5–10 mmHg) 
Aortic pressure ~80 mmHg (diastolic) 
 
2. Isovolumetric Ventricular Contraction (0.05 s) 
Event: Ventricles begin to contract; pressure rises rapidly. 
AV valves: Close (→ first heart sound, S₁) 
Semilunar valves: Still closed 
Volume: Constant (no blood flow yet) 
Pressures: 
LV pressure ↑ sharply (~10 → 80 mmHg) 
RV pressure ↑ (~5 → 15 mmHg) 
 
3. Ventricular Ejection (0.25 s) 
Event: Once ventricular pressure > aortic/pulmonary pressure, semilunar valves open and blood is ejected. 
AV valves: Closed 
Semilunar valves: Open 
Pressures: 
LV pressure peaks at ~120 mmHg 
Aortic pressure peaks at ~120 mmHg (=systolic blood pressure) 
RV pressure ~25 mmHg; pulmonary artery ~25 mmHg 
Aortic flow: Rapid ejection, then reduced ejection. 
 
4. Isovolumetric Relaxation (0.08 s) 
Event: Ventricles relax; pressure falls rapidly. 
Semilunar valves: Close (→ second heart sound, S₂) 
AV valves: Still closed 
Volume: Constant (no filling yet) 
Pressures: 
LV pressure drops (~100 → 5 mmHg) 
Aortic pressure shows dicrotic notch (from aortic valve closure rebound) 
 
5. Ventricular Filling (0.45 s) 
Occurs in two parts: 
Rapid filling phase: AV valves open, ventricles fill quickly. 
Diastasis (slow filling): Passive filling slows as pressures equalize. 
Atrial systole then tops up ventricles again. 
Pressures: 
LV pressure low (~5–10 mmHg) 
LA pressure slightly higher (~7–8 mmHg) 
Aortic pressure gradually falls (~80 mmHg) (=diastolic blood pressure) 
 

Heart Sounds 


Key Concepts: 
Isovolumetric phases: No blood flow, all valves closed. 
Pressure gradients determine valve opening and closing. 
Left heart pressures are much higher than right heart pressures (systemic vs pulmonary circulation). 
Cycle duration shortens with faster heart rates (mainly diastole shortens). 
 
Wigger’s Diagram – illustrates the varying pressures in the atria, ventricle and artery in one cardiac cycle