Why do we measure blood pressure in mercury?
We measure blood pressure in millimeters of mercury (mmHg) because it is a convenient and accurate method of measuring the pressure exerted by blood on the walls of the arteries. The use of mercury has historical significance and it continues to be the standard unit for blood pressure measurement.
To understand why mercury is used in measuring blood pressure, we need to know a little bit about the physics behind blood pressure measurement. Blood pressure is measured using a device called a sphygmomanometer, which consists of an inflatable cuff attached to a meter or gauge. The cuff is wrapped around the upper arm and inflated to temporarily stop the blood flow in the brachial artery. Then, the air is slowly released, and a stethoscope is used to listen to the sounds of blood flow in the artery. The pressure at which the sounds are first heard (systolic pressure) and the pressure at which the sounds disappear (diastolic pressure) are recorded.
Mercury is used in the sphygmomanometer system because of its properties. It is a dense, heavy liquid metal that allows for precise measurement of pressure. When the cuff is inflated, it compresses the brachial artery, stopping the blood flow. As the air is released, the pressure in the cuff decreases until it matches the pressure of the blood flowing through the artery. This pressure is then read on the gauge, which is usually calibrated in millimeters of mercury.
The use of mercury as the standard unit of measurement is rooted in historical practices. In the early 20th century, mercury-filled manometers were commonly used to measure blood pressure. The height of the mercury column was an accurate indicator of the blood pressure exerted on the walls of the arteries. Over time, mercury became the reference standard for blood pressure measurement, and even with the advent of electronic pressure sensors, the use of mercury millimeters of mercury (mmHg) remains the conventional unit for blood pressure measurement. It provides a consistent and universally recognized scale for healthcare professionals to interpret blood pressure readings accurately.
It is important to note that due to environmental concerns and the potential hazards associated with the handling and disposal of mercury, alternative methods using electronic devices have been developed and are increasingly being used in medical settings. However, the scale is still referred to as millimeters of mercury (mmHg) for consistency and compatibility with historical data.