P Wave.
When you see a P wave, your heart is depolarized (SA node electrical impulse firing). Right and left atria both contract to compress blood into the ventricles throughout this phase when the mechanical component of the heart reacts to the electrical impulse that is produced. The "P-R" Interval is the period of time between the onset of the P wave, or atrial depolarization, and the onset of the QRS, or ventricular depolarization.
The atrial musculature begins to contract with atrial depolarization. The pressure inside the atrial chambers rises as the atria contract, pushing more blood over the open atrioventricular (AV) valves and causing a rapid flow of blood into the ventricles.
Here's another question with an answer similar to this about atrial depolarization: https://brainly.com/question/10577559
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