Why Is Blood Pressure Measured In mmHg?

When you go to your medical professional and get your blood pressure checked, you’ll get a readout measured in mmHg. Knowing what this means can be confusing for the average person. However, there is a good reason why blood pressure is measured in mmHg.

So, why is blood pressure measured in mmHg? This article discusses what blood pressure is, how it is measured, why it is measured in mmHg, and other factors you need to know.

What Is Blood Pressure?

First, you should know what blood pressure is. Blood pressure is a measurement of how hard your blood is pushing against the walls of your arteries. It can be an indication of your overall health and cardiovascular-related conditions.

Your blood pressure may vary throughout the day and depends on your stress level, activity, and posture. Blood pressure may also depend on your overall health and level of cardiovascular fitness. Various illnesses and diseases can also lower or elevate blood pressure. A healthy adult’s normal resting blood pressure is 120/80 mmHg.

How Is Blood Pressure Measured?

When you go to the doctor, they will wrap a cuff around your upper arm and pump air into it. This device is known as a sphygmomanometer, and it uses mercury to assist in determining blood pressure. We will later discuss why mercury is used.

Blood pressure is measured by recording two values. This includes systolic blood pressure, the first number, and diastolic blood pressure, the second number.

Systolic blood pressure indicates how much pressure your blood exerts against the artery’s walls every time your heart beats.

Diastolic blood pressure indicates how much pressure is exerted against your artery walls when your heart is resting in between beats.

What Is mmHg?

Before we move on, you should know that mmHg stands for millimeters of mercury. Blood pressure monitors use mercury to determine your blood pressure. The medical definition of mmHg is a unit of pressure equal to the pressure exerted by a column of mercury 1 millimeter high at 0° Celsius under the acceleration of gravity and nearly equivalent to 1 torr, or about 133.3 pascals.

Why Is Blood Pressure Measured In mmHg

Why Is Blood Pressure Measured in mmHg?

To understand why blood pressure is measured in mmHg, you need to know more about the device used to measure blood pressure, the sphygmomanometer. However, to keep things simple, we’re just going to refer to it as a blood pressure meter or a blood pressure monitor.

A blood pressure monitor consists of different components, including an inflatable cuff, a pump and valve, and a mercury-filled manometer.

These devices are seen as the gold standard, as they are generally the most accurate. They also require minimal setup and are very simple to use. There has been a recent ban on mercury devices in US offices and hospitals, which has led to a shift in the type of device used to measure blood pressure.

When you get your blood pressure read, the inflatable cuff will be wrapped around your arm. Next, the practitioner will inflate the cuff by squeezing the small air pump.

You’re going to feel the cuff get much tighter. The practitioner will keep inflating the cuff until the mercury gauge reaches a certain level, generally around 30 points above the systolic pressure that you are expected to have.

They will then open the air valve to slowly release the air and listen for the sound of your pulse through a stethoscope. They will note the gauge’s pressure reading when they hear the first pulse. This is your systolic blood pressure, and it’s always the higher of the two readings.

The level of mercury in the gauge will continue to fall until the doctor can no longer hear the pulsing sound through the stethoscope. They will take the reading at this time, your diastolic blood pressure.

However, explaining exactly why mercury is used in these devices is slightly more difficult. It has to do with the way mercury reacts to changes in air pressure in a sealed tube, such as inside the pressure gauge in a blood pressure monitor.

When the pressure in the cuff of the blood pressure monitor increases, it forces the mercury to travel up the gauge. In other words, the increased air pressure causes the mercury to expand up the tube, providing an accurate blood pressure reading.

Blood Pressure Categories

According to the American Heart Association, there are five categories of blood pressure ranges. Let’s take a quick look at each one.

  • Normal: Normal blood pressure in a healthy adult should be around 120/80 mm Hg. This is generally an indication of a person in good health.
  • Elevated: If you have elevated blood pressure, your systolic blood pressure is between 120 and 129, and your diastolic blood pressure is under 80 mm Hg. Therefore, you will likely develop high blood pressure if you have elevated blood pressure.
  • Hypertension Stage 1: Hypertension stage one is characterized as having systolic blood pressure between 130 and 139 and diastolic blood pressure between 80 and 89 mmHg. At this point, you will likely be prescribed lifestyle changes and medication to decrease your risk of cardiovascular disease and stroke.
  • Hypertension Stage 2: If your blood pressure rate is around 140/90 mmHg or higher, this is very high blood pressure. Here you will undoubtedly be prescribed lifestyle changes and blood pressure medications.
  • Hypertensive Crisis: If you are in hypertensive crisis, your blood pressure readings exceed 180/120 mmHg very suddenly. If this very high reading persists over 5 minutes, you will require immediate medical attention.

Final Thoughts

Blood pressure is measured in mmHg because blood pressure monitors, sphygmomanometers, use mercury in their operation, with mm Hg being a measure of millimeters of mercury.