Which is the pressure exerted on the arteries when the heart is at rest?

Diastolic pressure occurs near the beginning of the cardiac cycle. It is the minimum pressure in the arteries when the pumping chambers of the heart — ventricles — fill with blood. Near the end of the cardiac cycle, systolic pressure, or peak pressure, occurs when the ventricles contract.

As the heart beats, it pumps blood through a system of blood vessels, which carry blood to every part of the body. Blood pressure is the force that blood exerts on the walls of blood vessels. All or any of the events related to the flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next is called a cardiac cycle. Problems in the cardiac cycle can cause low or high blood pressure.

Blood pressure readings are measured in millimeters of mercury (mmHg) and are provided as a pair of numbers. For example, 110 over 70 (written as 110/70) systolic/diastolic.

The lower number is the diastolic blood pressure reading. It represents the minimum pressure in the arteries when the heart is at rest. The higher number is the systolic blood pressure reading. It represents the maximum pressure exerted when the heart contracts.

The following video from Khan Academy explains the two numbers in greater detail.

Measuring Systolic and Diastolic Blood Pressure

The instrument used to measure blood pressure is called a Sphygmomanometer. The blood pressure cuff is snugly wrapped around the upper arm, positioning it so that the lower edge of the cuff is 1 inch above the bend of the elbow. The head of the stethoscope is placed over a large artery then air is pumped into the cuff until circulation is cut off, then air is let out slowly.

Air is pumped into the cuff until circulation is cut off; when a stethoscope is placed over the cuff, there is silence. Then as the air is slowly let out of the cuff, blood begins to flow again and can be heard through the stethoscope. This is the point of greatest pressure (called Systolic), and is usually expressed as how high it forces a column of mercury to rise in a tube. At its highest normal pressure, the heart would send a column of mercury to a height of about 120 millimeters.

At some point, as more and more air is let out of the cuff, the pressure exerted by the cuff is so little that the sound of the blood pulsing against the artery walls subsides and there is silence again. This is the point of lowest pressure (called Diastolic), which normally raises the mercury to about 80 millimeters.

Normal Ranges for Diastolic and Systolic Pressure

In children, the diastolic measurement is about 65 mmHg. In adults it ranges from 60 – 80 mmHg. Systolic measurement in children ranges from 95 to 100 and in adults it ranges from 90 – 120 mmHg.

An adult is considered suffering from

• hypotension if the diastolic reading is < 60 mmHg and systolic reading is < 90 mmHg
• Prehypertension if the diastolic reading is 81 – 89 mmHg and systolic reading is 121 – 139 mmHg
• Stage 1 Hypertension if the diastolic reading is 90 – 99 mmHg and systolic reading is140 – 159 mmHg
• Stage 2 Hypertension if the diastolic reading is 100 mmHg and systolic reading is 160 mmHg

Clinical Significance and Cardiovascular Risk

In the past, more attention was paid to diastolic pressure but it is now recognized that both high systolic pressure and high pulse pressure (the numerical difference between systolic and diastolic pressures) are risk factors. In some cases, it appears that a decrease in excessive diastolic pressure can actually increase risk, probably due to the increased difference between systolic and diastolic pressures.

Cardiovascular risk in those middle-aged and older is often more accurately predicted by using systolic blood pressure measurements than diastolic blood pressure measurements. Diastolic blood pressure can then be used to better understand the risks identified by systolic blood pressure.[1]

In a video titled What is the Clinical Importance of Systolic and Diastolic Blood Pressure, Dr. Len Saputo cites a research study published in the journal The Lancet examining how systolic and diastolic blood pressure in 30-year olds could predict risk of cardiovascular disease later in life. He explains that the difference between the two types of blood pressure is probably more important than either number alone.

Age Factor

Diastolic readings are particularly important in monitoring the blood pressure in younger individuals. Systolic blood pressure is known to rise with age as a result of hardening of the arteries.

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Your blood pressure reading is composed of two numbers (e.g., 120/80). Systolic blood pressure, or the top number, is the amount of force put on the arteries as the heart beats. Diastolic blood pressure (the bottom number) is the amount of force on the arteries when the heart is resting.

This article delves into the differences between systolic and diastolic blood pressure, why both numbers are important, and what your blood pressure readings mean.

When the heart beats, blood pulses through the arteries to travel throughout the body. However, it is not a steady stream like you might see from a garden hose.

The pulse of the blood flow and the pressure it exerts change from moment to moment. It's highest during the heartbeat (systolic pressure) and lowest between heartbeats (diastolic pressure).

Providers measure blood pressure using these numbers because it is a standard way of describing the force of the pulsing blood.

Your systolic and diastolic blood pressures are both important. If the readings are too high, you could have high blood pressure. If the readings are too low, there may not be enough blood flowing to your brain and other organs.

Furthermore, if there are changes in the difference between the two numbers, it's a clue that there could be a heart condition or other health problem.

During a heartbeat, the heart pushes blood out into the arteries. Systolic pressure is the measure of this force. This phase, known as systole, is the point at which blood pressure is the highest.

Systolic blood pressure is considered normal when the reading is below 120 mmHg (millimeters of mercury) while a person is sitting quietly at rest.

The heart muscle pushes out blood with higher pressure when a person is exercising, under stress, or at any time 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 is not normal and is considered high blood pressure.

Since your blood pressure can go up when you're active, it's important to take your blood pressure during periods of quiet rest to accurately diagnose high blood pressure (hypertension).

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

There is such a thing as too-low systolic pressure, however. When the reading is significantly below normal, it's called hypotension. This can cause lightheadedness, dizziness, or fainting. If low blood pressure is not treated, it may cause organs like the kidneys to start shutting down.

Systolic hypotension can happen if the amount of blood in your body becomes too low. For example, low blood pressure can happen when you are severely dehydrated or when you have major bleeding. In these cases, there just isn't enough blood to push through the body.

Low blood pressure can also happen if the heart muscle is too weak to push blood normally—for example if the heart muscle is damaged (cardiomyopathy) or if the arteries suddenly widen too much (as in vasovagal syncope, a reflex that causes fainting).

Sometimes, 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.

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

Normal diastolic blood pressure during quiet rest is below 80 mmHg. 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.

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.

You need to do your best to limit how these other factors might change your pressure when you're taking a blood pressure reading.

For the most accurate reading, check your blood pressure when you are in a calm, warm space after you have been able to 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, as both of these factors can affect your reading.

Measuring blood pressure this way is a challenge in a busy provider's office. Your provider might suggest you take your blood pressure at home.

Many experts suggest recording blood pressures that are taken over an extended time (which can include repeating the measurements at home) before diagnosing high blood pressure.

You should also know that your blood pressure will be different throughout the day. It tends to be highest in the morning and lower at night.

Your provider may want you to take your blood pressure multiple days a day—usually once in the morning and once in the afternoon or evening. Try to avoid taking it early in the morning right when you wake up or right after dinner.

Instead, try to take your morning reading before your breakfast (especially if you drink coffee or take medication) and your evening reading when you're getting ready for bed (and again, before you take any medications).

Once you decide what time you'll check your blood pressure, it's important that you're consistent. You'll get the most accurate results and comparisons if you take your blood pressure at about the same time every day.

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

Systolic and diastolic blood pressure are also important to know for treating a range of conditions, including heart disease or severe blood loss. It's important to get an accurate blood pressure reading under calm, quiet conditions.

Frequently Asked Questions

• Which is more important: Systolic blood pressure or diastolic blood pressure?

  Both systolic and diastolic blood pressure readings give important information about your health. However, systolic pressure is the number your doctor pays the greatest attention to if it is high.

• What is normal blood pressure by age?

  A blood pressure below 120/80 mmHg is ideal for most healthy adults. Children and teens may have slightly lower normal blood pressure. For example, a blood pressure of 100/65 could be considered normal for a toddler and a healthy, active teen could have a blood pressure of around 115/70. Older adults may also have higher or lower blood pressures that are considered normal. For example, a person who is in their 60s may have a blood pressure of around 130/60.

• What is considered dangerously high blood pressure?

  Dangerously high blood pressure, also known as a hypertensive crisis, is when systolic blood pressure (the top number) is 180 or higher or diastolic blood pressure (the bottom number) is 120 or more.
  

• What is the best time to take your blood pressure?

  The specific time you take your blood pressure is not as important as being consistent about it, day after day. That said, it is best to avoid taking a reading first thing in the morning, after you have a meal, or right after you take medication.