Timing Parameters of the Cardiac Cycle
The atrial cycle is strictly divided into two main phases: systole (contraction and blood ejection) and diastole (relaxation and filling). An important physiological feature is that the chambers do not contract in absolute synchrony.
The right side of the heart begins its cycle slightly earlier. Contraction of the right atrium starts 10 ms earlier than the left. This asynchrony is also reflected in the overall duration of the phases.
| Parameter | Right Atrium | Left Atrium |
|---|---|---|
| Systole Duration | 110 ms | 100 ms |
| Diastole Duration | 690 ms | 700 ms |
Mechanism and Sequence of Systole
The period of atrial contraction is also referred to as presystole. During this phase, the atrial myocardium actively contracts, and force vectors are directed inward toward the chambers. A final volume of blood is actively pumped into the ventricles. Meanwhile, the atrioventricular (cuspid) valves remain open, while the aortic and pulmonary valves (semilunar) remain closed.
The contraction process is not instantaneous; it follows a precise sequence:
- Venous orifices. Systole originates here. Contraction of the areas surrounding the venous entry points constricts their orifices, effectively preventing the retrograde (backward) flow of blood from the atrial cavity back into the venous bed.
- Auricles (Atrial appendages). They contract immediately after the venous orifices, increasing overall ejection efficiency. Just 50 ms after the onset of systole, all blood is completely expelled from the auricles.
- Wall stretching. The blood expelled from the auricles causes additional stretching of the atrial walls. This engages the Frank-Starling law: initial stretching of cardiomyocytes predictably leads to an increase in their subsequent contraction force.
- Lateral walls. Due to the Starling effect, the cardiomyocytes of the lateral walls contract with maximal force.
- Direction of the wave. The overall wave of atrial volume reduction propagates smoothly and strictly toward the ventricles.
Physiological Significance of the Atria
The primary hemodynamic challenge at the venous inlet of the heart is very low venous pressure. On its own, this pressure is insufficient to adequately distend the massive, thick-walled ventricles to their necessary end-diastolic volume.
Without the atria, cardiac pumping efficiency would drop sharply, and venous congestion would inevitably occur.
The multi-chambered structure of the heart solves this problem through a stepwise increase in pressure. The atria represent a thin-walled compartment that fills effortlessly and without resistance with venous blood. Their key function is to pump an additional volume of blood into the ventricles immediately before ventricular systole, thereby ensuring fully adequate and complete ventricular filling.