Let’s first take a look at what drives your blood pressure.
Unlike a river, which flows continuously downstream, blood flow is pulsatile, with ebb and flow driven by pumping heart muscle. This cycle repeats itself 60 or so times each minute, every time your heart beats. With each heartbeat occurring about once per second, the heart squeezes, or contracts, within a split-second. The rapid contraction forces about 90 cc of blood (approximately 1/3 cup) up, pushing the aortic valve open, and blood passes up into the aorta and rapidly distributes upward to the brain (via the carotid arteries on either side of the neck), the arms, and downward to the abdomen, pelvis, and legs. The force of heart contraction and its rapid distribution to the body is measured as systolic pressure, or the top number in blood pressure.
As the 90 cc or so of ejected blood distributes rapidly throughout the body, pressure in all the arteries drops over the ensuing half second, and the aortic valve closes. This resting phase of blood pressure is measured as diastolic pressure.
Thus, blood pressure is the driving force within the arteries of the body, literally the hydraulic pressure that drives blood flow to all organs.
Just understanding the basic phenomena behind blood pressure allows us to immediately identify some ways blood pressure can go haywire:
If the volume of blood contained in the entire circulation is increased (e.g., fluid retention resulting from excessive sodium in the diet or poor kidney function), then the volume of blood ejected from the heart increases and the entire network of arteries and veins in the body are filled towards greater distention, raising systolic pressure.
If the volume of blood contained in the entire circulation is decreased (e.g., dehydration, hemorrhage), then systolic pressure will drop. This is hypotension, the opposite of hypertension.
A factor that affects both systolic and diastolic pressure is the flexibility, or “give,” of the arteries of the body. Soft, flexible arteries expand with the increased blood volume of systole and easily relax with diastole. Rigid arteries can’t accommodate the increased volume of systole, nor relax with diastole; this is common in arteries lined with semi-rigid atherosclerotic plaque (often appropriately called “hardening of the arteries”) or with any condition that simply adds rigidity to the artery walls.
If the aortic valve is leaky, and blood flows backward back into the heart during diastole, then diastolic pressure drops to abnormal levels, e.g., 50 mmHg, indicating abnormally rapid distribution of blood.