Cardiac Cycle – Phases of Contraction & Relaxation

Terms

• Systole = contraction phase.

• Diastole = relaxation phase.

• Cycle alternates between atrial and ventricular systole/diastole.

Phases

Phase 1 – Atrial Diastole & Ventricular End-Systole

• Atria are relaxing and filling with venous blood.

• Ventricles are just finishing systole (contracting/emptying).

• At this point: atria filling, ventricles nearly done squeezing.

Phase 2 – Atrial Systole & Ventricular Diastole

• Atria contract → push blood into ventricles.

• Ventricles are in diastole (relaxing, filling).

• Blood moves through the AV valves (tricuspid/mitral).

• Result: ventricles fill with blood.

Phase 3 – Atrial Diastole & Ventricular Systole

• Atria are relaxing and refilling.

• Ventricles are in systole (contracting).

• Ventricles contract upward and out through aorta and pulmonary artery.

• Ventricles empty while atria begin to refill.

• End of Phase 3 → returns to Phase 1 (cycle repeats).

Big Takeaways

1. Stroke Volume

   • Amount of blood ejected per beat.

   • Depends on venous return (blood returning to the heart).

   • Greater venous return → greater stroke volume.

2. All-or-None Law

   • Heart muscle contracts fully or not at all.

   • No partial contractions.

3. Not All Blood Leaves the Heart

   • A portion of blood remains in ventricles after systole.

   • Ejection Fraction (EF):

     • Ratio of blood ejected vs. blood remaining.

     • Normal healthy hearts: higher EF.

     • Stronger hearts = higher proportion of blood pumped out.

4. Coronary Circulation

   • Some blood remains in the heart for nourishment and oxygen supply.

Summary:

• The cardiac cycle has 3 repeating phases: atrial filling, atrial contraction/ventricular filling, and ventricular contraction/atrial refilling.

• Stroke volume depends on venous return.

• Heart contracts fully (all-or-none).

• Not all blood leaves the ventricles → ejection fraction measures efficiency.