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.
Why Your Blood Pressure Matters
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.
Systolic Blood Pressure
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.
High Systolic Blood Pressure
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.
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.
Low Systolic Blood Pressure
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.
Diastolic Blood Pressure
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.
Improving Blood Pressure Reading 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.
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.
What Time of Day Should I Check My Blood Pressure?
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.