Final answer:
Systolic pressure measures the pressure exerted by blood against the walls of blood vessels when the heart is contracting during systole, meaning when it's actively pumping blood into the arteries.
Step-by-step explanation:
The systolic pressure is the higher number recorded during blood pressure measurement and is an indicator of the pressure exerted by the blood against the blood vessel walls when the heart is contracting (during ventricular contraction or systole). It is typically around 120 mm Hg and represents the amount of pressure the blood exerts on vessels as the heart is beating, with blood being ejected from the heart into the arterial system. During systole, the artery walls stretch to accommodate the increase in pressure of the extra blood, maintaining optimal blood flow throughout the body. The correct answer is b. Blood against the blood vessel walls when the heart is contracting.