What is the explanation for using two times the diastolic pressure to calculate the MAP?

Question

Quizplus · Accepted Answer

The heart remains in systole for about one-third of the cardiac cycle duration; thus the arteries contain the systolic pressure one-third of the time and the diastolic pressure two-thirds of the time. Mean arterial pressure is roughly estimated using the following computation:
MAP = (Systolic pressure + 2[Diastolic pressure])/3

What Is the Explanation for Using Two Times the Diastolic