Final answer:
The force exerted against arterial walls when the heart ventricles contract is measured as the systolic blood pressure reading, which is the higher number in a blood pressure measurement.
Step-by-step explanation:
The force with which blood is pushed against arterial walls upon the heart ventricles contracting is the systolic blood pressure reading. This systolic pressure is the higher number noted during a blood pressure measurement, for instance, 120 in a 120/80 mm Hg reading. It reflects the arterial pressure resulting from the ejection of blood during ventricular contraction, or systole. The other phase, diastole, occurs when the heart ventricles are relaxed, which is represented by the lower number, such as 80 mm Hg in the example.