What exactly is happening to my body when the systolic and diastolic pressures change?

Reduced perfusion: Hello sherry -- when the BP is low, the perfusion (blood flow and oxygen delivery) to the internal organs and tissues is reduced -- and when severe, it can cause ischemia and cell death.

Blood pressure: Blood pressure is produced by the interrelationships of the vascular resistance, the blood volume and the mechanical force of contraction of the heart pumping blood into the vascular tree. Changes in any of these parameters can change the blood pressure, so BP measures multiple components.