Blood Pressure: What is it?

4 min readMar 11, 2021


Blood pressure is, in short, the force exerted by the blood when it pushes against your blood vessel walls because of your heart beating.

However, if you’ve ever seen a blood pressure reading it looks like 110/70 mmHg with the two numbers changing wildly from person to person. So, what are they?

The first number (110 in this example) is known as systolic, and it represents the force exerted by your blood against your blood vessel walls precisely at the moment when your heart is at its maximum contraction capability, right before it starts relaxing to continue its cycle. The second number (70) is known as diastolic, and it represents the same force, but when the heart is at its most relaxed state, just before it starts contracting again.

Right: Systole, at the high of the pumping process. Left: Diastole, at the peak of the filling process.

How is it measured?

Blood pressure is most commonly measured employing the auscultatory method by using a stethoscope and a sphygmomanometer, which is also called a blood pressure monitor and it is composed of an inflatable cuff and mercury or aneroid manometer.

Left: Sphygmomanometer with a mercury manometer. Right: Sphygmomanometer with an aneroid manometer.

Mercury sphygmomanometers are considered the gold standard in clinical studies because of their accuracy and for not requiring calibration like their aneroid counterparts. This is why mmHg or millimeter of mercury is still used today to measure blood pressure (1mmHg = 133.3224 Pa).

Another mechanism employed is the oscillometric method which also uses an inflatable cuff, but instead of a stethoscope and a manometer, it uses an electronic pressure sensor to observe cuff pressure oscillations and interpret them as blood pressure readings. This is the method employed by most electronic blood pressure monitors out there and it is not as accurate as mercury sphygmomanometers.

Other methods like palpation, pulse wave velocity and some AI approaches have been proposed, but they all tend to be less accurate in general.

What is considered high and low blood pressure?

Interestingly enough this is not a simple question to answer, since it depends on which region of the world you live in. This is mainly because virtually every country sets its own guidelines on how to classify blood pressure readings.

For instance, in the US you have the guidelines from the American College of Cardiology and the American Heart Association known as the ACA/AHA. Also in the US, you have the Joint National Committee guidelines or JNC and from Europe, it is the European Society of Cardiology and the European Society of Hypertension guidelines known as ESC/ESH. These are only to name a few, all around the world there are health guidelines specific to different regions.

Hypertension (High Blood Pressure)

Despite what I said earlier, the differences between these guidelines are small, so it is possible to answer the question of when your blood pressure is high by just looking at the AHA guidelines.

Blood Pressure Categories

As you can see a blood pressure is considered normal only when the systolic is less than 120 mmHg and the diastolic is less than 80 mmHg. Meaning that if you have 130/80 mmHg of consistent blood pressure it is considered as hypertension stage 1 which puts nearly half of American adults (108 million) into the hypertense category.

Hypotension (Low Blood Pressure)

Within certain limits, the lower your blood pressure reading is, the better. In fact, only when your measurements go below 90/60 mmHg consistently and you are presenting symptoms (like the ones we’ll discuss below) is when you should get worried about low blood pressure.

Consequences of high blood pressure

Hypertension is known as the “Silent Killer” because most people don’t present any symptoms or warning signs at first. Therefore it can remain elevated for years and even decades before the first symptoms start to appear like headaches, nosebleeds, irregular heart rhythms, vision changes, buzzing in the ears, fatigue, nausea, vomiting, chest pain, confusion, and muscle tremors. This is why most doctors recommend measuring your blood pressure regularly.

By the point any of these symptoms show up, the continuous high blood pressure may have caused severe damage to your circulatory system and it can cause complications like:

  • Heart attacks.
  • Heart failure.
  • Irregular heartbeats, which can lead to sudden death.
  • Strokes.
  • Chest pains (also known as angina).

Symptoms of low blood pressure

Hypotension on the other hand can also be dangerous, but most doctors will only consider it as such when noticeable symptoms are present. These are dizziness or lightheadedness, nausea, fainting, lack of concentration, fatigue, and blurred vision.

As you can see there are similarities with the symptoms of high blood pressure, so make sure to always measure it before making a diagnosis.


There is a lot more to know about blood pressure and I plan on writing more articles like this one touching on specific issues in more depth. If there is a subject you are interested in let me know in the comments.

Also, check out my app to help you manage your blood pressure. It’s free on the Play Store.




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