Understanding Systolic and Diastolic Blood Pressure

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The measurement of a person’s blood pressure is recorded as two different numbers—the systolic blood pressure and the diastolic blood pressure. These two numbers reflect different aspects of the pressure being exerted by your blood as it pulses through your arteries.

When your heart pumps blood into your arteries, it pushes the blood along under a head of pressure. Doctors measure your blood pressure as a way of quantifying the force being exerted by this moving blood against the walls of your arteries.

Because the heart beats, the blood flow through the arteries is not steady (as with a fire hose), but pulsatile, and the flow of blood, and the pressure it exerts, fluctuate from moment to moment.

The Blood Pressure Reading

  • Your blood pressure reading is written as 120/80.
  • It is spoken like this: “120 over 80.”
  • The systolic blood pressure reading is the higher number.
  • The diastolic blood pressure reading is the lower number.
  • The units are millimeters of mercury (mmHg).

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

Systolic and Diastolic Blood Pressures
Verywell / JR Bee

What Is Systolic Blood Pressure?

The pressure exerted by your blood flowing through your arteries is not constant but is dynamic, and constantly reflects what the heart is doing at a given moment.

When the heart is actively beating (an event called “systole”), it is ejecting blood out into the arteries. This dynamic ejection of blood into the arteries causes the pressure within the arteries to rise. The peak blood pressure reached during active cardiac contraction is called the systolic blood pressure.

A “normal” systolic blood pressure when a person is sitting quietly is 120 mmHg or below.

High Systolic Blood Pressure

When a person is exercising, during periods of emotional stress, or at any other time when the heart is stimulated to beat more strongly than at rest, the force of cardiac contraction increases—and the systolic pressure goes up.

The increase in systolic blood pressure that occurs during these conditions of cardiac stress is entirely normal. This explains why it is so important to measure the blood pressure during periods of quiet rest before diagnosing hypertension.

Low Systolic Blood Pressure

If the systolic blood pressure is lower than normal, systolic hypotension is said to be present. If systolic hypotension is severe enough, it can cause lightheadedness, dizziness, syncope, or (if it lasts long enough), organ failure.

Systolic hypotension can occur if the blood volume becomes too low (as with severe dehydration or a major bleeding episode), if the heart muscle becomes too weak to eject the blood normally (a condition known as cardiomyopathy), or if the blood vessels become too dilated (as in vasovagal syncope).

A common condition that produces systolic hypotension is orthostatic hypotension.

What Is Diastolic Blood Pressure?

The diastolic blood pressure is the pressure the blood exerts within the arteries in between heartbeats, that is, when the heart is not actively ejecting blood into the arteries.

After the heart is finished contracting, the cardiac 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 is often increased during quiet rest.

Diastolic hypotension (when the diastolic blood pressure is low) may be seen with dehydration or with bleeding episodes, or if the arteries become abnormally dilated.

Quiet Rest for Accuracy

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. As we have seen, the blood pressure is actively changing from moment to moment as the heart cycles between systole and diastole.

In addition, your systolic and diastolic blood pressure (the highest and the lowest blood pressure reached during any given cardiac cycle) can change substantially from minute to minute depending on your state of activity, your state of stress, your state of hydration, and several other factors.

What this means is 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.

Measuring blood pressure this way is a challenge in today’s typical, harried doctor’s office, making the accurate diagnosis of hypertension much more of a challenge than it should be. This is why most experts today recommend recording the blood pressure over an extended period of time, with ambulatory monitoring, before making the diagnosis of hypertension.

A Word From Verywell

Systolic and diastolic blood pressures represent the pressures within the blood vessels during different parts of the cardiac cycle. Accurately measuring both of these values is important in diagnosing and managing hypertension.

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Additional Reading
  • 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.
  • 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.