The cardiac cycle
During each heartbeat, periods of heart-muscle contraction alternate with periods of relaxation. Systole refers to contraction and diastole to relaxation.
During atrial systole, the walls of the atria contract while the ventricles remain relaxed.
- Atrial contraction reduces the volume of the atria.
- The decrease in volume raises atrial pressure.
- Atrial pressure becomes greater than ventricular pressure.
- The atrioventricular (AV) valves are open.
- Blood is driven from the atria into the ventricles.
- As the ventricles receive blood, their volume increases and their pressure rises slightly.
| Feature | During atrial systole |
|---|---|
| Atria | Contracting; volume falls and pressure rises. |
| Ventricles | Relaxed and filling with blood. |
| AV valves | Open. |
| Semilunar valves | Closed. |
atrial pressure > ventricular pressure → AV valves open → blood enters ventricles
Exam Tip: Explain valve movement using pressure differences. During atrial systole, atrial pressure exceeds ventricular pressure, so the AV valves are open and blood passes into the ventricles.