What is an explanation of the numbers that record blood pressure?

Systolic/diastolic: Blood pressure is expressed as 2 numbers: systolic pressure over diastolic pressure. Systolic pressure, the first ("top") number, is the pressure in the arteries while the heart is actively pumping blood out & is higher. Diastolic pressure, the second ("bottom") number, is generated while the heart is filling, and is lower. Optimal blood pressure for healthy adults is <120 systolic, <80 diastolic.

Heart and vassels: The systolic blood pressure (the upper one) is recorded when the hearth contracts and pushes the blood out into the vascular space. The diastolic blood pressure (the lower one) is the reflection of the blood vessels tone mostly. The mechanism of the blood pressure control is obviously more complex.