Final answer:
The "atrial kick" is the surge of blood into the ventricles caused by the contraction of the atria, which contributes the final portion of blood to the ventricles before they contract.
Step-by-step explanation:
The surge of blood pushed into the ventricles as a result of atrial contraction is often referred to as the "atrial kick." This occurs during atrial systole, where the atria, after filling with blood, contract to add an additional push of blood into the ventricles. Initially, the ventricles receive about 70-80% of their capacity passively during diastole, and the atrial contraction contributes the remaining 20-30% of ventricular filling. This phase of the cardiac cycle ensures that the ventricles are filled to their optimal capacity before they contract themselves, facilitating efficient circulation.
Atrial systole is represented by the P wave on an electrocardiogram (ECG) and lasts about 100 milliseconds. Following atrial systole, the ventricles begin their own contraction phase, called ventricular systole, during which blood pressure quickly rises within them. This closes the tricuspid and mitral valves to prevent backflow into the atria, marking the isovolumic contraction phase of ventricular systole, before the blood is eventually ejected from the heart.