What does an electrocardiograph primarily trace?

An electrocardiograph (ECG or EKG) is a diagnostic tool that primarily traces the heart's electrical activity. It detects the various electrical signals generated by the heart as it beats, providing valuable information about the heart's rhythm and overall function.

The electrodes placed on the skin capture these electrical impulses, allowing the electrocardiograph to create a visual representation of the heart's electrical activity over time. This information is essential for diagnosing various heart conditions, assessing heart health, and monitoring heart function in clinical settings.

The other options, while related to heart function, do not accurately describe what an electrocardiograph measures. It does not directly measure the contraction and relaxation of the ventricles, nor does it provide insights into the opening and closing of heart valves or ultrasound waves.