Understanding Systolic and Diastolic Blood Pressure

When people talk about "taking your blood pressure," it means they want to check two numbers that represent the force of the blood that your heart pumps through your body.

These two numbers are called the systolic blood pressure and the diastolic blood pressure. They are measured by using a cuff with an attached gauge so that the numbers can be read by a machine, or by a person hearing the pressure rise and fall through a stethoscope.

Both numbers are important but not always for the same reasons. This article looks at how blood pressure is taken, what levels are normal, and what some of the results can mean.

Systolic and Diastolic Blood Pressures
Verywell / JR Bee

Blood Pressure Overview

When the heart beats, blood pulses through the arteries to travel throughout the body. It is not the steady stream you might see from a garden hose or water faucet.

The pulse of the the blood flow and the pressure it exerts change from moment to moment. It's highest during the heartbeat (this is the systolic pressure) and lowest between beats (diastolic). A blood pressure reading includes both these measurements.

Doctors measure blood pressure in these numbers so that there is a standard way of describing the force of the pulsing blood. Both the systolic and diastolic pressures are important.

If the readings are too high, it may mean a person has high blood pressure. If the readings are too low, there may not be enough blood flowing to the brain and other critical organs. If there are changes in the difference between the two numbers, it's a clue that there may be a heart condition or other problem.

The Blood Pressure Reading

  • Your blood pressure reading is written in a format like this: 120/80.
  • It is spoken like this: “120 over 80.”
  • The systolic blood pressure reading is the higher number.
  • The diastolic blood pressure reading is the lower number.
  • The units are millimeters of mercury (mmHg).

What Is Systolic Blood Pressure?

During a heartbeat, the heart is pushing blood out into the arteries. Doctors call this "systole," and that's why it's called the systolic blood pressure. It's the pressure during a heartbeat and the highest pressure measured.

When the reading is 120 mmHg or a little below while a person is sitting quietly at rest, the systolic blood pressure is considered normal.

High Systolic Blood Pressure

The heart muscle pushes out blood with higher pressure when a person is exercising, under stress, or at similar times when the heart rate is increased. The systolic pressure goes up with it.

In these cases, the increased pressure is normal. However, when the pressure is high while a person is resting, that's considered high blood pressure.

That's why it is so important to take your blood pressure during periods of quiet rest to diagnose hypertension, or high blood pressure.

High systolic blood pressure is usually caused by narrowing of the arteries, which makes the heart have to work harder to push blood through.

Low Systolic Blood Pressure

If the systolic blood pressure is lower than normal, it's called hypotension. If this low blood pressure is serious enough, it can cause lightheadedness, dizziness, or fainting. If it lasts long enough and it isn't treated, it may cause organs like your kidneys to start shutting down.

Systolic hypotension can occur if the amount of blood in your body becomes too low. This can happen if you are severely dehydrated or you have major bleeding; there just isn't enough blood to push through the body.

It also may happen if the heart muscle is too weak to push blood normally, as in cases of cardiomyopathy (damaged heart muscle) or if the arteries suddenly widen too much, as in vasovagal syncope (a reflex that causes fainting).

Sometimes this low blood pressure happens when you change positions suddenly. You may feel dizzy when you stand up because gravity is pulling blood down toward your feet. This is a common condition called orthostatic hypotension.

Recap

Your systolic blood pressure is the higher and first number in a blood pressure reading. If it's too high while you are at rest, you may need to have it treated. But pressure that's too low may be a serious problem too, so be sure to discuss your situation with your doctor.

What Is Diastolic Blood Pressure?

The heart rests between beats so it can refill with blood. Doctors call this pause between beats "diastole." Your diastolic blood pressure is the measurement during this pause before the next heartbeat.

A normal diastolic blood pressure during quiet rest is 80 mmHg or a little below. If you have high blood pressure, the diastolic number is often higher even during quiet rest.

Low diastolic pressure may be seen with dehydration or with severe bleeding. It also may happen if the arteries relax and widen.

Improving Accuracy

Your systolic and diastolic pressures—the highest and lowest points of your heartbeat—change depending on your activity level, stress, fluid intake, and other factors. This means that it is better to limit how these other factors change your pressure when taking a blood pressure reading.

For the most accurate reading, it should be taken in a calm, warm space after you rest quietly for at least five minutes. You should be relaxed, with your arms at your sides, and the cuff should be placed on your arm at about the level of your heart. Your legs should be uncrossed, and your bladder should be empty—both of these can affect your reading.

Measuring blood pressure this way is a challenge in a busy doctor’s office, and makes it harder to check for high blood pressure. Many experts suggest recording blood pressures over an extended period of time, such as repeating measurements at home, before diagnosing high blood pressure.

Summary

Your blood pressure is a measurement of the pressures in your arteries while your heart is beating (systolic) and between beats (diastolic). These values are important in diagnosing and managing high blood pressure.

They're also important numbers to know for treating a range of conditions, including heart disease or severe blood loss. It's important to get an accurate reading under calm, quiet conditions.

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6 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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