What is blood pressure?

The blood in your veins is under constant pressure — not such a strange state for anything that’s supposed to get from Point A to Point B in decent time. For the cells that float along in the fire hose of your blood, Point A is the left ventricle of your heart, and Point B is its right atrium, with the path in between winding through any given point in your body, such as your right pinkie toe or your left earlobe.

The pressure your blood experiences is not measured in numbers of cups of coffee consumed or the days, minutes, or half-seconds between you and a deadline, but in millimeters of mercury, a perhaps archaic unit of measurement invented for the barometer. If you take all the air out of a long glass tube with one end stopped up, and place the open end into a dish of mercury, the weight of the atmosphere above you will press down upon the mercury in the dish, pushing that liquid metal upward, against gravity, into the tube until it can’t climb any higher. At sea level, it will have climbed about 760 millimeters. If the blood in your body at the precise moment your heart beats, instead of being inside you, was rather pushing down on the mercury of a barometer, it would nudge the mercury about one-seventh as far up the tube as the atmosphere does at sea level.

The pressure on your blood when your heart’s left ventricle ejects it like an octopus ejects ink ought not to be more than 120 millimeters of mercury. When your heart is resting and your blood is coasting on what remains of its momentum you’d be better off if it didn’t get higher than 80 millimeters of mercury.

That’s because your blood passes on the pressure it experiences to the fleshy tubes it flows through, pushing them outward to attain the space it needs. Though your blood vessels are reasonably springy, being designed to expand and contract — sometimes to accommodate and sometimes to influence shifting blood pressure — they can only stand so much. If the pressure of the blood rushing through them stretches them too far, they will weaken. Once weakened, a spot in the blood vessel wall is in danger of becoming a breach.

Even if it avoids breaching, a section of blood vessel under too much pressure may tear — not enough to leak much but enough to need mending. The mending it gets is scarring, and the formerly smooth inside of a blood vessel is smooth no more. The texture of scar tissue collects and hoards all manner of things that float by until the blood has less and less space to flow by. And eventually, the high pressure to get from Point A to Point B can restrict the blood’s only path for doing so.