Systole vs. diastole: The differences

Systole vs. diastole: The differences

The terms diastole and systole apply to the relaxing and contraction of heart muscles. A person’s blood pressure is determined by the difference between diastole and systèmeole.

The heart is a pump which supplies oxygen-rich blood to all body tissues and organs. The heart beat is caused by relaxing and contracting cardiac muscles.

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

In this article we’ll clarify how blood pressure is linked to diastole and systole. We also address what normal blood pressure is, along with high blood pressure (hypertension) and low blood pressure (hypotension) related risk factors and complications.

What are diastole and systole?

Diastole is when the heart muscle relaxes and systole is when the heart muscle contracts.
Diastole is when the heart muscle relaxes and systole is when the heart muscle contracts.

Diastole is defined by the following characteristics:

  • Diastole is when the heart muscle relaxes.
  • When the heart relaxes, the chambers of the heart fill with blood, and a person’s blood pressure decreases.

Systole is defined by the following characteristics:

  • Systole is when the heart muscle contracts.
  • When the heart contracts, it pushes the blood out of the heart and into the large blood vessels of the circulatory system. From here, the blood goes to all of the organs and tissues of the body.
  • During systole, a person’s blood pressure increases.


The heart is a four chamber composite pump. It is divided into a right and a left side in the centre, and each side is further divided into two chambers-the upper and lower chambers.

The heart’s two upper chambers, called the atria, collect the blood that goes into the heart. The two lower chambers are denominated ventricles. They pump the blood from the heart into the rest of the body.

The heart contracts to pump the blood throughout the body, and then relaxes again and again in a process called the cardiac cycle. The cycle begins when the two atrias contract, driving blood into the ventricles. Then, the ventricles contract, squeezing out the blood from the nucleus.

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

Diastole and systole differently affect a person’s blood pressure, 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.

What is a healthy blood pressure?

A doctor measuring blood pressure
Normal blood pressure will be under 120/80 mmHg.

Once a person receives the results of his blood pressure they can see two numbers reflecting the measures of the diastole and the systole. Such measurements are given as mercury millimeters (mm Hg).

The first is the systolic pressure and the other is the diastolic pressure.

According to the 2017 revised recommendations of the American College of Cardiology (ACC) the main definitions of blood pressure are:

  • Normal blood pressure: under 120/80 mmHg
  • Elevated blood pressure: a systolic pressure of between 120-129 and a diastolic pressure of under 80
  • Stage 1 hypertension: a systolic pressure of between 130-139 or a diastolic pressure of between 80 and 89 mmHg
  • Stage 2 hypertension: a systolic pressure of at least 140 or a diastolic pressure of at least 90 mmHg

Such revised recommendations would likely put 46 per cent of Americans in the high blood pressure group.

Blood pressure is often measured when, and over many days, the person is in rest. The blood pressure levels are also called the measurements.

High and low blood pressure

For several reasons, a person’s blood pressure may get too high or too low. If left untreated, both high and low blood pressure can lead to serious health consequences.

High blood pressure

Senior woman checking her blood pressure
Gender and age may increase a person’s risk of high blood pressure.

High blood pressure or hypertension occurs when an abnormally high pressure occurs exerted on the walls of the blood vessels. This disease slowly progresses for several years and can go unnoticed for a long time, as there are often no signs there.

The following risk factors raise an individual’s risk of hypertension:

  • Age. Blood pressure is usually higher with age.
  • Gender. Men are more likely to have high blood pressure before the age of 55, but women are more likely than men to have the condition after the age of 55.
  • Race. High blood pressure is more common in African Americans than Caucasian or Hispanic Americans.
  • 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 who is overweight or obese is more likely to develop high blood pressure. This is because a higher volume of blood circulates through 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 too much 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.

High blood pressure can cause complications when left untreated, and potentially serious health issues, such as:

  • Heart attack. A block in the flow of oxygen-rich blood to a portion of the heart, preventing that portion of the heart from getting oxygen.
  • Stroke. A stroke happens when there is a block in the flow of oxygen-rich blood to the High blood pressure can cause complications when left untreated, and potentially serious health issues, such as:, preventing that portion of the brain from getting oxygen.
  • Heart failure. Failure of the heart to pump enough blood to meet the body’s demands, caused by the increased pressure on the vessels.
  • Peripheral artery disease. This is the narrowing of blood vessels other than those that supply the heart or the brain, most commonly of the legs. Blood flow to that part of the body is affected.
  • Aneurysm. An aneurysm is the development of an abnormal bulge in a blood vessel wall, which may press on other organs, block blood flow, or eventually burst.
  • Chronic kidney disease. Kidney disease can be caused by narrowing of blood vessels in the kidneys, which prevents them from working properly.

Low blood pressure

High blood pressure or hypotension occurs if an individual has an abnormally low blood pressure over their blood vessel walls.

Risk factors which increase the chance of a person developing the condition include:

  • Age. People older than 65 are more likely to experience a drop in blood pressure while standing up, or after eating. Children and young people are more likely to experience a rapid drop in blood pressure accompanied by dizziness, blurred vision, and fainting, which is known as neurally mediated hypotension.
  • Certain medications. High blood pressure medicines, including diuretics, can cause hypotension.
  • Certain diseases. Conditions such as Parkinson’s, diabetes, and some heart conditions 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.

An individual with moderate low blood pressure can experience tiredness, fainting, or dizziness.

More extreme types of low blood pressure can compromise the supply of oxygen-rich blood to major organs of the body, including the brain. A person may feel tired, confused or light-headed if this happens. This can lead to heart or brain damage in severe cases.


Diastole and sistemale are two stages of the heart cycle. We occur as the heart beats, pumping blood through a blood vessel system which carries blood to every part of the body. Systole occurs as blood is pumped out from the heart, and diastole occurs when the heart relaxes following contraction.

A person who suspects having high or low blood pressure may contact their doctor to find out about the appropriate remedies, which could include changes in medicine or lifestyle.

Also if a person is taking medicine for high blood pressure, they should also periodically check their blood pressure levels, because there might be no noticeable signs of that condition.