What is happening during the systolic phase of blood pressure?

Blood pressure is the force of the blood pushing against the artery walls. The force is generated with each heartbeat as blood is pumped from the heart into the blood vessels. The size and elasticity of the artery walls also affect blood pressure. Each time the heart beats (contracts and relaxes), pressure is created inside the arteries.

The pressure is greatest when blood is pumped out of the heart into the arteries. When the heart relaxes between beats (blood is not moving out of the heart), the pressure falls in the arteries.

Two numbers are recorded when measuring blood pressure.

• The top number, or systolic pressure, refers to the pressure inside the artery when the heart contracts and pumps blood through the body.

• The bottom number, or diastolic pressure, refers to the pressure inside the artery when the heart is at rest and is filling with blood.

Both the systolic and diastolic pressures are recorded as "mm Hg" (millimeters of mercury). This recording represents how high the mercury column in the blood pressure cuff is raised by the pressure of the blood.

Blood pressure is measured with a blood pressure cuff and stethoscope by a nurse or other healthcare provider. You can also take your own blood pressure with an electronic blood pressure monitor. These are available at most pharmacies.

The National Heart, Lung, and Blood Institute (NHLBI) of the National Institutes of Health (NIH) has determined 2 levels of high blood pressure for adults:

• Stage 1

  • 140 mm Hg to 159 mm Hg systolic pressure—higher number

and

• 90 mm Hg to 99 mm Hg diastolic pressure—lower number

• Stage 2

  • 160 mm Hg or higher systolic pressure

and

• 100 mm Hg or higher diastolic pressure

The NHLBI defines prehypertension as:

• 120 mm Hg to 139 mm Hg systolic pressure

and

• 80 mm Hg to 89 mm Hg diastolic pressure

The NHLBI guidelines define normal blood pressure as follows:

• Less than 120 mm Hg systolic pressure

and

• Less than 80 mm Hg diastolic pressure

Use these numbers as a guide only. A single elevated blood pressure measurement is not necessarily an indication of a problem. Your healthcare provider will want to see multiple blood pressure measurements over several days or weeks before making a diagnosis of high blood pressure and starting treatment. If you normally run a lower-than-usual blood pressure, you may be diagnosed with high blood pressure with blood pressure measurements lower than 140/90.

What are the risk factors for high blood pressure?

Nearly one-third of all Americans have high blood pressure, but it is particularly prevalent in:

• People who have diabetes, gout, or kidney disease

• African Americans (particularly those who live in the southeastern U.S.)

• People in their early to middle adult years; men in this age group have higher blood pressure more often than women in this age group

• People in their middle to later adult years; women in this age group have higher blood pressure more often than men in this age group (more women have high blood pressure after menopause than men of the same age)

• Middle-aged and elderly people; more than half of all Americans age 60 and older have high blood pressure

• People with a family history of high blood pressure

• People consuming a high salt diet

• Overweight people

• Heavy drinkers of alcohol

• Women who are taking oral contraceptives

• People with depression

How does blood pressure increase?

The following conditions contribute to high blood pressure:

• Being overweight

• Excessive sodium intake

• A lack of exercise and physical activity

How is high blood pressure controlled?

These steps can help you control your blood pressure:

• Take prescribed medicine exactly as directed by your healthcare provider

• Choose foods that are low in sodium (salt)

• Choose foods low in calories and fat

• Choose foods high in fiber

• Maintain a healthy weight, or losing weight if overweight

• Limit serving sizes

• Increase physical activity

• Reduce or omit alcoholic beverages

Sometimes daily medicine is needed to control high blood pressure. If you have high blood pressure, have your blood pressure checked routinely and see your healthcare provider to monitor the condition.

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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