What is the first stage of the cardiac cycle?

The first stage of the cardiac cycle is characterized by the contraction of both atria, which is the phase where the atrial muscles contract to push blood into the ventricles. This process is crucial as it helps to maximize the amount of blood that fills the ventricles before they contract in the next phase.

During this initial stage, the blood that has been returning to the heart from the body and the lungs fills the atria. Once the pressure in the atria exceeds that of the ventricles, the atria contract, and the atrioventricular (AV) valves open to allow blood to flow into the ventricles. This action ensures that the ventricles are filled with blood prior to their contraction, which occurs in the next phase of the cardiac cycle.

Understanding this stage is vital because it sets the foundation for efficient pumping of blood through the heart. The subsequent phases build on this initial filling to maintain effective circulation throughout the body.