Blood pressure is the force of blood pushing against the inner walls of your arteries as the heart circulates it through the body. This measurement is a fundamental indicator of your general health. The pressure is generated by the heart as it pumps blood into the arteries, the vessels that carry oxygenated blood from the heart to all other parts of your body.
Systolic Pressure: The Force of the Squeeze
Systolic pressure measures the force in your arteries at the moment your heart contracts or “squeezes.” This contraction phase of the heartbeat is known as systole. During systole, the heart’s muscular lower chambers, the ventricles, push blood out into the large arteries, causing the pressure inside these vessels to reach its highest point. For this reason, systolic pressure is the peak pressure exerted on your artery walls.
This measurement is the first, or top, number in a blood pressure reading. For instance, in a reading of 120/80, the 120 represents the systolic pressure. To visualize this, think of a garden hose. The systolic pressure is comparable to the maximum pressure that builds inside the hose when you squeeze the nozzle to spray water.
The force is necessary to ensure blood overcomes resistance within the vascular system to reach every tissue and organ. Healthy arteries are flexible and stretch to accommodate this surge of blood. If arteries become stiff due to age or other health conditions, the heart must work harder to pump blood, which can lead to a higher systolic reading.
Diastolic Pressure: The Pressure Between Beats
Following the heart’s contraction is a period of relaxation known as diastole. Diastolic pressure captures the force of blood against your artery walls when the heart is in this resting phase, refilling with blood for the next beat. This measurement represents the lowest pressure within the arteries during the cardiac cycle. It reflects the baseline pressure constantly present in the vascular system.
In a blood pressure reading, the diastolic pressure is the second, or bottom, number. Using the example of 120/80, the 80 signifies the diastolic pressure. To return to the garden hose analogy, the diastolic pressure is similar to the residual pressure in the hose when the nozzle is not being squeezed. It is the constant pressure keeping the hose expanded between sprays.
This resting pressure indicates how much resistance your blood faces when the heart is not actively pushing it. If the smaller arteries, known as arterioles, are narrowed, they create more resistance, which can cause the diastolic pressure to rise. This number gives a clear picture of the condition of your blood vessels when they are not under the peak stress of a heartbeat.
How to Read Your Blood Pressure Results
Blood pressure is measured in units called millimeters of mercury, abbreviated as mm Hg. This unit of measurement originated with early pressure gauges that used mercury columns. Both the systolic and diastolic numbers are used together to evaluate your cardiovascular health.
The American Heart Association provides standard categories to interpret these readings. A normal blood pressure reading is one where the systolic number is below 120 mm Hg and the diastolic number is below 80 mm Hg. Readings in this range indicate your cardiovascular system is functioning under a healthy amount of pressure.
When readings move outside the normal range, they fall into different classifications:
- Elevated blood pressure: A systolic reading between 120 and 129 mm Hg with a diastolic pressure still below 80 mm Hg.
- Hypertension Stage 1: Systolic pressure consistently ranges from 130 to 139 mm Hg or diastolic pressure is between 80 and 89 mm Hg.
- Hypertension Stage 2: A more serious condition involving systolic readings of 140 mm Hg or higher or diastolic readings of 90 mm Hg or higher.
- Hypertensive crisis: A medical emergency where systolic pressure exceeds 180 mm Hg and/or diastolic pressure is over 120 mm Hg.