Which signals indicate that the heart chambers are filling with blood?

The P wave represents the electrical activity associated with atrial depolarization, which occurs just before the atria contract and push blood into the ventricles. This contraction is part of the cardiac cycle that assists in filling the heart chambers. As the electrical signals travel through the atria, they cause muscle fibers to contract and effectively fill the ventricles with blood in preparation for the next heartbeat.

In contrast, the other choices represent different phases of the cardiac cycle. The QRS complex is primarily associated with ventricular depolarization, which corresponds to ventricular contraction and ejection of blood from the ventricles. The T wave signifies ventricular repolarization and reflects the recovery phase of the ventricles after contraction. The U wave, although less commonly discussed, is thought to represent the repolarization of the papillary muscles or some form of late ventricular repolarization.

Therefore, the P wave is the most relevant signal indicating the phase where the heart chambers are filling with blood, as it reflects atrial activity leading up to this important process.