Systolic and Diastolic Blood Pressure

measuring blood pressure
Measuring Blood Pressure. BSIP/UIG/Getty Images

Systolic and Diastolic Blood Pressure

When your blood pressure is measured, you get two different numbers. These numbers represent the systolic blood pressure and the diastolic blood pressure. Both numbers reflect the pressure exerted by the blood flowing within your arteries.

You will see your blood pressure reading written like this: 120/80. The systolic blood pressure reading is the top number, and the diastolic blood pressure reading is on the bottom.

Both values represent the pressure in millimeters of mercury, or mmHg.

Both the systolic and diastolic numbers are important. If the readings are too high, hypertension may be present. If too low, there may be insufficient blood flow to critical organs, such as the brain.

What Is Systolic Blood Pressure?

The pressure exerted by flowing blood on the arteries is not constant, but is cyclical, according to what the heart is doing at a given moment.

When the heart is actively beating (an event called “systole”), blood is being ejected from the heart and into the arteries. Because blood is being actively pushed into the arteries during systole, the pressure within the arteries increases. The peak blood pressure during cardiac contraction is called the systolic blood pressure.

A “normal” systolic blood pressure during quiet rest is 120 mmHG or below. The systolic blood pressure increases during exertion, during periods of stress, or during any other event that causes the heart to beat more strongly.

In hypertension, the systolic blood pressure is often increased even during quiet rest.

Systolic hypotension (lower blood pressure than normal) can be seen with severe dehydration, with major bleeding episodes, or when the heart muscle is too weak to eject the blood normally (a condition known as cardiomyopathy).

What Is Diastolic Blood Pressure?

The diastolic blood pressure is the pressure the blood exerts within the arteries when the blood is not being actively ejected by the heart. After the heart is finished contracting, the ventricles relax momentarily so that they can be refilled with blood, in preparation for the next contraction. This period of ventricular relaxation is called “diastole,” and the blood pressure during diastole is called the diastolic blood pressure.

A “normal” diastolic blood pressure during quiet rest is 80 mmHG or below. In hypertension, the diastolic blood pressure may be increased during quiet rest. Diastolic hypotension may be seen with dehydration or with bleeding episodes.

Importance of Measuring Blood Pressure During Quiet Rest

Blood pressure is a very dynamic thing. The level of your blood pressure depends on the activity of your heart and the elasticity of your arteries, and changes from moment to moment according to your state of activity, your state of stress, your state of hydration, and several other factors.

This means that, in order to diagnose hypertension accurately, it is important to control for as many “external” factors as possible. The standard recommended by experts requires the blood pressure to be taken in a calm, warm environment after you have been resting quietly for at least five minutes. Better yet, experts say, is to record the blood pressure periodically during ambulatory monitoring.


Siu AL, U.S. Preventive Services Task Force. Screening for High Blood Pressure in Adults: U.S. Preventive Services Task Force Recommendation Statement. Ann Intern Med 2015; 163:778.

Daskalopoulou SS, Rabi DM, Zarnke KB, et al. The 2015 Canadian Hypertension Education Program recommendations for blood pressure measurement, diagnosis, assessment of risk, prevention, and treatment of hypertension. Can J Cardiol 2015; 31:549.

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