What does the PR interval on an ECG indicate?

The PR interval on an ECG represents the time it takes for electrical impulses to travel from the atria to the ventricles. This interval begins at the onset of the P wave, which indicates atrial depolarization, and ends at the start of the QRS complex, which signifies ventricular depolarization.

The function of the PR interval is crucial as it reflects the conduction through the atrioventricular (AV) node and the bundle of His, highlighting the delay that allows the ventricles to fill with blood after atrial contraction. A normal PR interval indicates that electrical signals are being conducted properly, while changes in this interval can suggest various cardiac conditions, such as AV block.

In contrast to this, other options focus on aspects not related to the PR interval. The time for ventricles to contract pertains more to the QRS complex and the subsequent mechanical phase of heart filling. Atrial contraction is a component of the cardiac cycle but is specifically represented by the P wave, not the PR interval. Lastly, blood flow timing involves physiological processes but is not directly reflected on the ECG; rather, it is a consequence of the electrical events captured by the ECG tracing.