• Definitions
• Differences
• Healthy range
• High blood pressure
• Low blood pressure
• Summary

Diastole and systole refer to when the heart muscles relax and contract. The balance between diastolic and systolic pressure determines a person’s blood pressure. A blood pressure reading displays systolic and diastolic pressure.

The heart is a pump that supplies all tissues and organs of the body with oxygen-rich blood. The heartbeat is caused by the heart muscles relaxing and contracting.

During this cycle, the period of relaxation is called diastole and the period of contraction is called systole.

What are diastole and systole?

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Diastole and systole are both parts of the cycleTrusted Source the heart goes through to pump blood through the body.

Diastole

Diastole is defined by the following characteristics:

• when the heart muscle relaxes
• the chambers of the heart fill with blood
• a person’s blood pressure decreases
• the blood returns to the heart from the organs and tissues throughout the body

Systole

Systole is defined by the following characteristics:

• when the heart muscle contracts
• pushing the blood out of the heart and into the large blood vessels of the circulatory system
• the blood goes to all of the organs and tissues of the body
• a person’s blood pressure increases

Learn about the cardiovascular system.

Diastole vs. systole differences

Diastole and systole affect a person’s blood pressure differently, as follows:

• When the heart pushes blood around the body during systole, the pressure placed on the vessels increases. This is called systolic pressure.
• When the heart relaxes between beats and refills with blood, the blood pressure drops. This is called diastolic pressure.

How the heart works

The heart is a pump composed of four chambers. It is divided in the middle into a right and left side, and each side is divided further into two chambers — the upper and lower chambers.

The two upper chambers of the heart, called theTrusted Source atria, receive the blood entering the heart. The two lower chambers, called the ventricles, pump the blood out of the heart to the rest of the body.

To pump blood around the body, the heart contracts and then relaxes repeatedly in a cycle called the cardiac cycle. The cycle begins when the two atria contract, pushing blood into the ventricles. Then, the ventricles contract, forcing blood out of the heart.

The deoxygenated blood that returns from the body to the right side of the heart is then pumped through the lungs, where it picks up oxygen. The oxygenated blood then travels to the left side of the heart and is pumped to the rest of the body.

What number is more important?

According to the American Heart Association (AHA)Trusted Source, systolic blood pressure may be one of the best ways to predict any future cardiovascular events for people of all ages. However, the AHA also notes that in younger people diastolic blood pressure may still be important.

Diastolic blood pressure may provide further predictive information for those under the age of 50 years old.

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What is a healthy blood pressure?

When a person receives their blood pressure results, they will see two numbers that represent the diastole and systole measurements. These measurements are given as millimeters of mercury (mm Hg).

The first number is the systolic pressure and the second is the diastolic pressure.

According to the American College of Cardiology’s (ACC) updated guidelines, the current blood pressure categories are:

Category	BP readings
Normal blood pressure	under 120/80 mmHg
Elevated blood pressure	systolic pressure of 120 to 129 and a diastolic pressure of less than 80
Stage 1 hypertension	a systolic pressure of between 130 and 139 or a diastolic pressure of 80 to 89
Stage 2 hypertension	a systolic pressure of at least 140 or a diastolic pressure of at least 90
Hypertensive crisis	systolic pressure over 180 and diastolic pressure over 120

Blood pressure is always measured when the person is at rest and over several days. Its measurements are also called blood pressure readings.

Read more about blood pressure readings.

About high blood pressure

A note about sex and gender

Sex and gender exist on spectrums. This article will use the terms “male,” “female,” or both to refer to sex assigned at birth. Click here to learn more.

High blood pressure (hypertension) is when a person has abnormally high pressure against the walls of their blood vessels. This condition develops gradually over many years and may go unnoticed for a long time, as there are often no symptoms.

The following risk factorsTrusted Source may increase a person’s risk of high blood pressure:

• Age: Blood pressure typically increases with age.
• Gender: Males are more likely to have high blood pressure before the age of 55, but females are more likely to have the condition after the age of 55.
• Family history: Having a family member with high blood pressure increases the risk of a person developing high blood pressure in the future.
• Obesity: A person with overweight or obesity is more likely to develop high blood pressure. This is because a higher volume of blood circulates through more blood vessels to supply the cells with oxygen and nutrients. Because there is more blood circulating, there is a higher pressure on the vessel walls.
• Lifestyle habits: A lack of physical activity, smoking tobacco (including second-hand smoking), drinking a lot of alcohol, consuming too much salt (sodium) or too little potassium, and stress may increase the risk.
• Certain chronic conditions: Kidney disease, diabetes, and sleep apnea can increase the risk of high blood pressure.
• Pregnancy: In some cases, pregnancy can cause high blood pressure.

Complications

When left untreated, high blood pressure can cause complications and, eventually, serious health problems, such as:

• heart attack
• stroke
• heart failure
• peripheral artery disease
• aneurysm
• chronic kidney disease

About low blood pressure

Low blood pressure (hypotension) occurs when a person has abnormally low blood pressure against the walls of their blood vessels.

Risk factors that increase a person’s chance of developing the condition include:

• Age: Older people may be more likelyTrusted Source to experience symptoms of low blood pressure when standing up or after a meal.
• Certain medications: High blood pressure medicines, includingTrusted Source diuretics, can cause hypotension.
• Certain diseases: Conditions such as Parkinson’s, diabetes, and some heart conditions can increase the risk of low blood pressure.
• Other factors: Pregnancy, standing in the heat, or standing still for long periods of time can also cause low blood pressure.

A person with mild low blood pressure may experience fatigue, fainting, or dizziness.

More severe forms of low blood pressure can compromise oxygen-rich blood flow to the body’s major organs, including the brain. If this happens, a person may feel sleepy, confused, or light-headed. In serious cases, this can lead to heart or brain damage.

Cardiovascular health resources

Visit our dedicated hub for more research-backed information and in-depth resources on cardiovascular health.

Summary

Diastole and systole are two phases of the cardiac cycle. They occur as the heart beats, pumping blood through a system of blood vessels that carry blood to every part of the body. Systole occurs when the heart contracts to pump blood out, and diastole occurs when the heart relaxes and refills after contraction.

Anyone who believes that they may be experiencing high or low blood pressure or has concerns about blood pressure levels should consult their healthcare professional.

Even if a person is taking medication for blood pressure, they may still need to measure their blood pressure levels regularly, since the condition may not have any apparent symptoms.