Systolic and Diastolic Blood Pressure

What do the two numbers mean in a blood pressure reading?

Thanks to my new paid subscribers, you are awesome. Also, thanks to those who joined my talk on Thursday—I hope you enjoyed it.

Now to something I wondered about for years before actually finding out what it was:

The heart is a really, really amazing thing. Prints of this

When a doctor or nurse takes your blood pressure, you’ll usually be told two numbers — for example, 120 over 80. But what do these numbers actually mean?

A blood pressure reading measures how strongly your blood pushes against your artery walls as your heart beats. About once every second, your heart contracts and relaxes — your heartbeat. Each contraction pushes blood to your lungs to exchange carbon dioxide for oxygen and sends oxygen-rich blood around your body.

Inside your heart, a set of clever valves ensures blood flows in just one direction — out to your body during each contraction, then back in to refill the heart as it relaxes.

Systolic Blood Pressure

When your heart contracts, it forces blood out at its peak pressure — this is called systolic blood pressure. It’s the higher of the two numbers in a blood pressure reading.

For example, in a reading of 120/80 mmHg, the 120 is your systolic pressure. It shows how hard your heart is working to pump blood through your arteries during the systole phase.

Diastolic Blood Pressure

When your heart relaxes between beats, the pressure in your arteries drops — this is called diastolic blood pressure. It’s the lower number in your reading and represents the baseline pressure as blood continues to flow even while the heart rests. This occurs during the diastole phase, when the heart refills ready for the next contraction.

An Example Blood Pressure Reading: 120/80 mmHg

So, in a normal blood pressure range like 120/80:

• 120 – Systolic blood pressure (heart contracting)

• 80 – Diastolic blood pressure (heart relaxing)

Together, these two numbers give a clear picture of how your heart and blood vessels are functioning — whether your blood pressure is normal, low, or high.

The pressure readings, e.g. 120 mmHg, are values above atmospheric air pressure, which at sea level is usually around 760 mmHg. So the absolute pressure in this case would be 880 mmHg (120 + 760). (added thanks to the comment below)

The unit mmHg means millimetres of mercury. It corresponds to early pressure gauges that measured pressure by the height of mercury in a glass column.

In case it’s not obvious, this is not medical advice, and I am not a doctor—I just wanted to know what those two numbers really meant when someone took my blood pressure. Many blood pressure readings are considered ‘normal’. If you have concerns, please consult a qualified medical professional!

Hoping you have a week that doesn’t raise your blood pressure too much,

Jono

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Also see:

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• The half-life of caffeine

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• Proprioception

• Crossmodal perception

• Anatomy of a wave