The minimum pressure in the artery is diastolic blood pressure, which is 80mm Hg. The maximum pressure in the artery is systolic blood pressure, which is 120mm Hg. When the artery expands, it expands from the pressure of 80mm Hg to a pressure of 120mm Hg. Thus, the pressure with which we feel the artery hitting our finger is 120 - 80 = 40mm Hg. It is also called pulse pressure.