When your heart beats, it pumps blood round your body to give it the energy and oxygen it needs. As the blood moves, it pushes against the sides of the blood vessels. The strength of this pushing is your blood pressure. If your blood pressure is too high, it puts extra strain on your arteries (and your heart) and this may lead to heart attacks and strokes.

Blood pressure chart

Use the blood pressure chart below to see what your blood pressure means. The blood pressure chart is suitable for adults of any age. (The level for high blood pressure does not change with age.)  Blood pressure readings have two numbers, for example 140/90mmHg.
The top number is your systolic blood pressure. (The highest pressure when your heart beats and pushes the blood round your body.) The bottom one is your diastolic blood pressure. (The lowest pressure when your heart relaxes between beats.)

The blood pressure chart below shows ranges of high, low and healthy blood pressure readings.

Blood pressure chart for adults

Using this blood pressure chart: To work out what your blood pressure readings mean, just find your top number (systolic) on the left side of the blood pressure chart and read across, and your bottom number (diastolic) on the bottom of the blood pressure chart. Where the two meet is your blood pressure.

Function

Our circulation is similar to a highly sophisticated form of plumbing – blood has ‘flow’ and arteries are ‘pipes.’ A basic law of physics gives rise to our blood flow, and this law also applies in a garden hose pipe. Blood flows through our body because of a difference in pressure.

Our blood pressure is highest at the start of its journey from our heart – when it enters the aorta – and it is lowest at the end of its journey along progressively smaller branches of arteries. That pressure difference is what causes blood to flow around our bodies. Arteries affect blood pressure in a similar way to the physical properties of a garden hose pipe affecting water pressure. Constricting the pipe increases pressure at the point of constriction. Without the elastic nature of the artery walls, for example, the pressure of the blood would fall away more quickly as it is pumped from the heart.

While the heart creates the maximum pressure, the properties of the arteries are just as important to maintaining it and allowing blood to flow throughout the body. The condition of the arteries affects blood pressure and flow, and narrowing of the arteries can eventually block the supply altogether, leading to dangerous conditions including stroke and heart attack.

Measurement

When the pressure from the arm cuff stops the pulse briefly, it gives the top figure of arterial blood pressure that we are familiar with from medical dramas – for example, “140 over 90”

The device used to measure blood pressure is a sphygmomanometer, it consists of a rubber armband – the cuff that is inflated by hand or machine pump. Once the cuff is inflated enough to stop the pulse, a reading is taken, either electronically or on an analogue dial. The reading is expressed in terms of the pressure it takes to move mercury round a tube against gravity. This is the reason for pressure being measured using the unit millimeters of mercury, abbreviated to mm Hg.

Readings

A stethoscope identifies the precise point when the pulse sound returns and the pressure of the cuff is slowly released. Using the stethoscope enables the person measuring the blood pressure to listen out for two specific points. Blood pressure readings consist of two figures – the systolic pressure first and the diastolic pressure second. The reading is given as, for example, 140 over 90 mm Hg. The systolic pressure is the higher figure caused by the heart’s contraction, while the diastolic number is the lower pressure in the arteries, during the brief ‘resting’ period between heartbeats.

Ranges

The National Institutes of Health cite normal blood pressure to be below 120 mm Hg systolic and 80 mm Hg diastolic.

However, blood pressure changes naturally, a fact that cardiologists explored while writing about blood-pressure variability in Nature in March 2013:

“Blood pressure is characterized by marked short-term fluctuations occurring within a 24-hour period (beat-to-beat, minute-to-minute, hour-to-hour, and day-to-night changes) and also by long-term fluctuations occurring over more prolonged periods of time (days, weeks, months, seasons, and even years).”

The guidelines state that for blood pressures above a figure of 115/75 mm Hg, every rise of 20/10 mm Hg doubles the risk of cardiovascular disease.  The overall guidelines for high blood pressure received an update in November 2017. They allow for earlier intervention. Since 2017, the American Heart Association (AHA) has advised that people with high blood pressure should receive treatment at 130/80 mm Hg rather than 140/90 mm Hg. They also removed the “prehypertension” category between 120-139/80-89 mm Hg. A blood pressure reading of 140/90 mm Hg now qualifies as stage II hypertension and not stage I, as it used to be.

This category now forms two separate ranges:

·         elevated blood pressure, from 120-129/less than 80 mm Hg

·         stage I hypertension, from 130-139/80-89 mm Hg

In these new guidelines, the AHA also advises that doctors should only prescribe medication in cases of a previous heart attack or stroke, or in the presence of risk factors for these conditions, such as age, a diabetes mellitus diagnosis, or chronic kidney disease.  Treatment at the earlier stages should instead come mainly through lifestyle changes.