Temporal parameters of the cardiac cycle
The atrial cycle is strictly divided into two main phases: systole (period of contraction and blood ejection) and diastole (period of relaxation and filling). An important physiological feature is that the chambers do not work in absolute synchrony.
The right side of the heart begins its work slightly earlier. Contraction of the right atrium starts 10 ms earlier than the left. This is also reflected in the total 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 called presystole. In this phase, the atrial myocardium actively contracts, and force vectors (conditional arrows) are directed inward into the chambers. An active portion of blood is pumped into the ventricles. At the same time, the atrioventricular (cuspid) valves remain open, while the aortic and pulmonary valves (semilunar) remain closed.
The contraction process is not instantaneous; it has a clear sequence:
- Venous orifices. Systole originates right here. Contraction of the areas where veins enter compresses their orifices, reliably preventing retrograde (backward) blood flow from the atrial cavity back into the venous bed.
- Auricles. They contract immediately after the venous orifices, increasing the 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 creates additional stretching of the atrial walls themselves. Here, Starling's law comes into play: primary stretching of cardiomyocytes naturally leads to an increase in the force of their subsequent contraction.
- Lateral walls. Thanks to the Starling effect, the cardiomyocytes of the lateral walls contract with maximum force.
- Direction of the wave. The overall wave of atrial volume reduction smoothly moves strictly toward the ventricles.
Physiological significance of the atria
The main hemodynamic problem at the heart inlet is very low venous pressure. By itself, it is unable to properly stretch the massive, thick-walled ventricles to the required end-diastolic volume.
If the atria did not exist, the heart's pumping efficiency would drop sharply, and venous blood stasis would inevitably occur.
The multi-chamber structure of the heart solves this problem through a stepwise pressure increase. The atria represent a thin-walled compartment that fills with venous blood without resistance and very easily. Their key function is to pump an additional amount of blood into the ventricles immediately before their systole, thereby ensuring absolutely adequate and complete filling.
Orifices — Auricles — Walls (OAW): remember this acronym to reproduce the contraction sequence in presystole.
Class and exam questions
Why is blood not regurgitated back into the veins during atrial systole?
Atrial contraction begins at the entry sites of the venae cavae. Their orifices are compressed first, which physically blocks retrograde blood flow.
How does Starling's law manifest during atrial function?
Blood expelled from the auricles in the first 50 ms additionally stretches the lateral atrial walls. According to Starling's law, this stretching significantly increases the force of their subsequent contraction.
Why are atria necessary if blood can flow directly into the ventricles?
Low venous pressure cannot stretch the thick walls of the ventricles. Thin-walled atria easily collect blood and actively pump an additional amount into the ventricles, preventing venous stasis.
What else to study on this topic
- Mechanisms of low venous pressure formation
- Function of atrioventricular valves during different phases of the cardiac cycle
- Hemodynamics in the orifices of the venae cavae
- Molecular basis of the Frank-Starling law in cardiomyocytes
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