How blood pressure is measured

Perhaps the best place to start is to explain exactly what blood pressure is. In simple terms, your heart is a muscular pump that pushes blood into the large blood vessels, allowing the blood to deliver oxygen and other nutrients to all your organs and tissues.1

As the blood is forced into the vessels (like arteries), it puts pressure on their walls.1

Blood pressure measurements are made up of two values:

Systolic blood pressure (also known as the ‘top’ number)

This is the pressure when the heart muscle is contracting (squeezing) and pumping oxygen-rich blood into the blood vessels.

Diastolic blood pressure (also known as the ‘bottom’ number)

This is the pressure on the blood vessels when the heart muscle relaxes. The diastolic pressure is always lower than the systolic pressure.

Measuring blood pressure (BP)

Perhaps you’ve seen a blood pressure reading written down as 140mmHg. Hg is the chemical symbol for mercury, and the 140mm component means the pressure would push a up column of liquid mercury 140 mm or 14 cm.

Old style blood pressure monitor

You may have even seen this in action if your doctor used a mercury sphygmomanometer. (See image)

These days you’re more likely to see a more modern digital blood pressure device, but the principle is the same:

A cuff is wrapped around your upper arm and inflated until no blood flows through the large artery in your arm (called the brachial artery). Then the air is slowly released from the cuff. As soon as the blood starts to flow, that means the pressure in the cuff is the same as the systolic blood pressure. If you listened with a stethoscope, you’d hear the sound of the blood pulsing through the arteries.

As the cuff continues to deflate, the pulsing starts to disappear. When it stops, that represents the diastolic pressure.

Digital devices, like the one shown here, ‘listen’ for vibrations in the arterial wall. As the cuff is automatically deflated, vibrations are detected when the blood begins to flow and capturers the reading (Systolic pressure).

Omron blood pressure monitor

When the cuff pressure falls below your diastolic pressure, blood flows smoothly through the artery without any vibration being set up in the wall.

Good quality, certified home blood pressure monitors are accurate, very easy to use, and above all, quite affordable. Measuring at home is a great way to monitor your cardiovascular health. It provides a more accurate picture of what’s happening with your blood pressure because it’s not just a single snapshot – it looks at patterns over a longer period of time.4

It allows your doctor to see if your medications are working or if the dosage needs to be adjusted – or even a switch to a different medicine if required.

Finally, it avoids white coat hypertension. White coat hypertension is a phrase that is used to explain the situation where your blood pressure is elevated when you’re in the doctor’s rooms but is normal at other times. It affects around 20% of the population, so it’s fairly common.5

For a great digital blood pressure device, check out the Omron range here.

Always read & follow the instructions for use & health warnings. For people with high blood pressure. Consult your doctor to evaluate the readings. Check your device periodically for accuracy.


  1. https://www.ncbi.nlm.nih.gov/books/NBK279251/
  2. https://theconversation.com/health-check-what-do-my-blood-pressure-numbers-mean-29212
  3. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1121444/
  4. https://www.ama-assn.org/delivering-care/hypertension/what-you-need-know-about-self-measured-blood-pressure-monitoring
  5. https://www.health.harvard.edu/blog/could-white-coat-hypertension-harm-your-heart-2019112918384



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