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Systole vs. diastole: What’s the difference?

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The terms systole and diastole are used to measure your blood pressure. Systole refers to the time when the heart contracts, while diastole is when the heart relaxes.
Medically reviewed by Kristin Shaffer, PA-C
Written by Lily Ferguson
Updated on April 6, 2023

Blood pressure is an important measurement that lets you know how hard your heart needs to work to pump blood through the body. It measures two things: systolic pressure and diastolic pressure.

Systolic pressure is measured when the heart squeezes, and it’s always the larger, or top, number on the measurement. Diastolic pressure is measured when the heart is relaxed. It’s always the smaller, or bottom, number.

It’s important to keep track of your blood pressure, which is measured in millimeters of mercury (mm Hg). If it’s too high (hypertension) or too low (hypotension), you might experience negative effects on your health.

Knowing more about systolic pressure and diastolic pressure can help you take care of your health.

Systole and diastole: What’s the difference?

An older adult taking their blood pressure measurement
Siro Rodenas Cortes/Getty Images

The cardiac cycle is the term used to describe how the heart beats. This includes the pressure changes that happen in the heart so that it can pump blood around the body. It can be divided into two phases: systole and diastole.

Systole refers to the time when the heart muscle squeezes, moving blood through the veins and arteries. The blood pushes against the blood vessels, creating pressure. This is called systolic pressure.

Diastole refers to the time when the heart muscle is relaxed. During this time, the heart fills with blood, and blood pressure decreases as the measured pressure in the blood vessels lowers. This value is called diastolic pressure.

Both are important in measuring overall blood pressure. According to one 2019 study, high levels of either systolic pressure or diastolic pressure increase the likelihood of cardiovascular events or medical problems affecting the heart and blood vessels, such as heart disease or stroke, but systolic pressure has a greater influence overall.

Blood pressure

A blood pressure measurement is presented as two values. It’s always given with the systolic value first, or on top, followed by the diastolic number. An average blood pressure reading might be 120 systolic and 80 diastolic, represented as 120/80 mm Hg, or 120 over 80 mm Hg.

In 2017, the American College of Cardiology updated its guidelines on the different blood pressure categories. These are now as follows:

  • Normal: less than 120 systolic and less than 80 diastolic
  • Elevated: between 120 and 129 systolic and less than 80 diastolic
  • Stage 1 hypertension (high blood pressure): between 130 and 139 systolic or between 80 and 89 diastolic, or both
  • Stage 2 hypertension (very high blood pressure): over 140 systolic or over 90 diastolic, or both

Hypotension refers to significantly low blood pressure. Unlike hypertension, it is usually diagnosed by evaluating symptoms rather than through measurements. However, the National Heart, Lung, and Blood Institute defines hypotension as a reading below 90/60 mm Hg.

It’s also important to manage hypotension as it can lead to serious health problems if it’s persistent.

Low blood pressure: Causes, symptoms, and treatment

There are many potential causes of low blood pressure. These include:

For the most part, low blood pressure may not be dangerous unless it causes symptoms or is linked to an underlying cause. But if your blood pressure is lower than 90/60 mm Hg, it may be cause for concern.

Many symptoms can lead to a hypotension diagnosis. These include:

The usual approach to managing low blood pressure is to treat the underlying cause. But in severe cases of hypotension, a healthcare professional may prescribe specific medications to raise your blood pressure. Examples include:

If your blood pressure reading is lower than usual, you might want to seek advice from a healthcare professional. You may be able to prevent low blood pressure through lifestyle measures or medications and avoid serious complications.

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High blood pressure: Causes, symptoms, and treatment

According to the Centers for Disease Control and Prevention (CDC), 9 out of 10 adults in the United States will experience high blood pressure at some point. Several risk factors can increase your likelihood of having high blood pressure. These include certain lifestyle factors, such as:

  • high alcohol consumption
  • a diet too high in sodium and too low in potassium
  • smoking
  • a lack of exercise
  • high stress levels

Certain other factors can increase the risk of hypertension, such as:

  • Age: Older adults are more likely to experience hypertension.
  • Genetics: If you have a close family member with the condition, you may also experience it.

Research has also found that race might play a role in hypertension. According to 2019 statistics from the American Heart Association, Black people may experience high blood pressure at a higher rate than people of other races. However, this might be due to health effects of stress and barriers and inequities in healthcare.

Hypertension doesn’t usually cause any symptoms, so it’s important to monitor your blood pressure regularly, particularly if you’re more at risk of developing it.

You can treat hypertension by implementing lifestyle measures, such as:

  • exercising regularly
  • cutting out foods high in sodium and eating more fruits and vegetables
  • monitoring consumption of alcohol
  • quitting smoking if you smoke
  • drinking enough water
  • reducing stress

In addition to implementing these lifestyle measures, a healthcare professional may prescribe medication to help treat high blood pressure. Various medications are available for this, and some of the most common include:

This isn’t a complete list of all the medications available. You should be prescribed the right medication for your condition after discussing it with a healthcare professional.

Possible complications

One key characteristic of hypertension is that it generally doesn’t cause any symptoms, which can be very dangerous. It can cause a lot of damage to blood vessels without being noticed, which can lead to serious complications, such as:

So, regularly monitoring your blood pressure can be very important in preventing complications. You may want to speak with a healthcare professional to help you manage your blood pressure.


Systole and diastole are two phases the heart goes through when it’s pumping blood. Both of them are important in measuring blood pressure.

High blood pressure, or hypertension, happens when the heart works harder to pump blood. This leads to higher systole and diastole measurements.

It can pose a serious risk to your long-term health. That’s why it’s important to have your blood pressure checked regularly, either by a healthcare professional or by yourself at home.

If you have a blood pressure reading that is either too high or too low, you may want to speak with a healthcare professional about whether it’s cause for concern and discuss treatment options.

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