As a cardiac electrophysiologist, I specialize in the study of the electrical activity of the heart. The P wave on an electrocardiogram (ECG) is a crucial indicator of the heart's rhythm. Here's a detailed explanation: The P wave is caused by the depolarization of the atria, which is the initial phase of the heart's electrical conduction system. Depolarization is the process by which the heart muscle cells, or cardiomyocytes, lose their resting electrical charge and become ready to contract. This process begins in the sinoatrial (SA) node, which is the heart's natural pacemaker located in the right atrium. The electrical impulse generated by the SA node travels through both atria, causing the atrial muscle cells to depolarize. When the atria depolarize, they contract, pushing blood into the ventricles. The contraction of the atria is followed by the opening of the atrioventricular (AV) valves, which are the valves between the atria and ventricles. As the ventricles begin to expand, they create a suction effect that helps draw the blood from the atria into the ventricles. This is not solely due to gravity, as the reference suggests, but rather a combination of atrial contraction and the suction effect of the expanding ventricles. In summary, the P wave on an ECG is a direct representation of atrial depolarization, which precedes atrial contraction and the subsequent movement of blood into the ventricles. read more >>

The P Wave. The first wave (p wave) represents atrial depolarisation. When the valves between the atria and ventricles open, 70% of the blood in the atria falls through with the aid of gravity, but mainly due to suction caused by the ventricles as they expand.read more >>