Difference Between Systolic and Diastolic Blood Pressure


The diastolic phase of the heart's pumping cycle begins at its lowest pressure. When the ventricles (the heart's pumping chambers) are completely filled with blood, that's the lowest pressure in the arteries. Maximum pressure, known as systolic pressure, is reached when the ventricles contract at the conclusion of the cardiac cycle.

Blood is pumped throughout the body by the heart with each pulse. That which the blood does to the artery walls is measured as blood pressure. The cardiac cycle refers to the series of events that occur between the onset of one heartbeat and the onset of the next and may or may not be directly tied to blood flow or blood pressure. Blood pressure issues, both low and high, may be traced back to problems in the cardiac cycle.

Blood Pressure Reading

Individuals' systolic and diastolic blood pressure readings are given as a pair of values in millimeters of mercury (mmHg). To illustrate, systolic/diastolic blood pressure is expressed as 110 over 70 (or 110/70).

The diastolic blood pressure measurement is the lowest of the two numbers. What this number indicates is the lowest possible pressure in the arteries while the heart is at rest. The systolic blood pressure value is the highest of the two numbers. It measures the greatest force generated by a cardiac contraction.

Measuring Systolic and Diastolic Blood Pressure

The Sphygmomanometer is the device used to take blood pressure readings. The upper arm is wrapped tightly with the blood pressure cuff, with the bottom border of the cuff resting an inch above the elbow's crease. Placing the stethoscope's head over an important artery, pumping air into the cuff until circulation is stopped, and then gently releasing the air.

When a stethoscope is put over the cuff, sounds of the heart and lungs are muffled because air has been pumped into it to the point where circulation is completely shut off. The return of normal blood flow may be seen by the stethoscope when the air is gradually released from the cuff. The systolic reading is the highest of the two and is measured by the height to which the pressure in a tube causes a column of mercury to rise. If you were to put a column of mercury against the heart at its maximum normal pressure, it would rise around 120 millimeters.

As more air is released from the cuff, the pressure imposed by the cuff decreases until the blood no longer pulsates against the artery walls, and quiet reigns once again. Diastole refers to the lowest pressure when the mercury rises to its highest natural level (about 80 mm Hg).

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 to be suffering from −

  • Hypotension if the diastolic reading is < 60 mmHg and the systolic reading is < 90 mmHg.

  • Prehypertension if the diastolic reading is 81 – 89 mmHg and the systolic reading is 121 – 139 mmHg.

  • Stage 1 Hypertension if the diastolic reading is 90 – 99 mmHg and systolic reading is 140 – 159 mmHg.

  • Stage 2 Hypertension if the diastolic reading is 100 mmHg and the systolic reading is 160 mmHg.

Clinical Significance and Cardiovascular Risk

High systolic and high pulse pressure (the numerical difference between systolic and diastolic pressures) are increasingly acknowledged as risk factors; in the past, more focus was placed on diastolic pressure. Reducing diastolic blood pressure below normal can sometimes raise systolic blood pressure, which in turn raises the risk.

Systolic blood pressure readings are frequently more reliable predictors of cardiovascular risk in persons aged middle age and older than diastolic blood pressure measurements. The diastolic reading can then be utilized to provide further insight into the dangers shown by the systolic reading.

Dr. Len Saputo, in a video titled "What is the Clinical Importance of Systolic and Diastolic Blood Pressure," refers to a study published in The Lancet that looked at the relationship between systolic and diastolic blood pressure and the risk of cardiovascular disease in adults aged 30 and older. It is the disparity between the two measurements, he says, rather than either one individually, that is likely to be most significant.

Differences between Systolic and Diastolic Blood Pressure

The following table highlights the major differences between Systolic and Diastolic blood pressure −

Characteristics

Systolic Blood Pressure

Diastolic Blood Pressure

Definition

It measures the amount of pressure that blood exerts on arteries and vessels while the heart is beating.

It is the pressure that is exerted on the walls of the various arteries around the body in between heartbeats when the heart is relaxed.

Normal range

90 – 120 mmHg (adults) −95 mmHg (infants) − 100 mmHg (6 to 9 years)

60 – 80 mmHg (adults) − 65 mmHg (infants) − 65 mmHg (6 to 9 years)

Age factor

As a person's age increases, so does the importance of their systolic blood pressure measurement.

Diastolic readings are particularly important in monitoring blood pressure in younger individuals.

Blood Pressure

Systolic represents the maximum pressure exerted on the arteries.

Diastolic represents the minimum pressure in the arteries.

Ventricles of the heart

Left ventricles contract

Fill with blood

Blood Vessels

Contracted

Relaxed

Blood Pressure reading

The higher number is systolic pressure.

The lower number is the diastolic pressure.

Conclusion

Diastolic readings are particularly important in monitoring the blood pressure in younger individuals, while Systolic blood pressure is known to rise with age due to the hardening of the arteries.

Updated on: 31-Jan-2023

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