What Do Blood Pressure Numbers Mean?
Blood pressure is one of those numbers that gets rattled off at every doctor’s appointment. Usually as something like “120 over 80.” The “over” doesn’t mean anything special. It’s just how the two numbers are said. They’re not a fraction or a calculation.
It’s the two numbers themselves that mean something, and each one measures a different part of the heartbeat.
The First Number
The first number is called systolic pressure. It’s usually the higher of the two because it measures the pressure when the heart contracts and pushes blood into the arteries.
So with a reading of 120/80, 120 is the systolic number. It reflects the pressure during the pumping part of the heartbeat.
Then the heart relaxes before the next beat, which is where the second number comes in.
The Second Number
The second number is called diastolic pressure. It measures the pressure in the arteries while the heart is relaxed between contractions.
The pressure doesn’t drop to zero between heartbeats. Blood is still moving through the circulatory system, and there’s still pressure in the arteries. That’s what the second number is measuring.
So in that same 120/80 reading, 80 is the diastolic number.
What the Numbers Are Actually Measuring
The numbers aren’t measuring how fast the heart is beating or how much blood it’s pumping. They’re measuring pressure, specifically the force of blood pushing against the walls of the arteries.
That pressure rises when the heart squeezes and pushes blood out, which is why the first number is usually higher. It falls as the heart relaxes between beats, but it doesn’t disappear. That’s why there’s still a second number to measure.
So the two numbers are really two points in the same cycle. One catches the pressure when the heart is actively pumping, and the other catches it while the heart is between beats.
In general, a higher number means there’s more pressure against the artery walls at that point in the heartbeat, while a lower number means there’s less. Blood pressure can be considered too high or too low, but what a particular reading means medically depends on more than simply seeing a bigger or smaller number.
If You Want to Know More
The basics are pretty simple once the two numbers make sense. But there’s a lot more to blood pressure than what those numbers mean.
The Cleveland Clinic has a more detailed explanation of blood pressure readings if you want to go deeper. And if you have questions about what your own blood pressure numbers mean, that’s a question for your doctor.