Answer
A sphygmomanometer is a medical device used to measure blood pressure (BP). It operates based on the principle of auscultatory or oscillometric methods. The most common type of sphygmomanometer is the mercury column sphygmomanometer. Here's how it works and why blood pressure is expressed in millimeters of mercury (mm Hg):
**Principle of Operation:**
A sphygmomanometer consists of an inflatable cuff that is wrapped around the arm, a pressure gauge (manometer), and a stethoscope. The process involves the following steps:
1. The cuff is inflated to a pressure higher than the expected systolic blood pressure. This temporarily occludes the brachial artery, cutting off blood flow.
2. The pressure in the cuff is gradually released. As the pressure decreases, blood flow begins to return through the artery. This is indicated by the onset of Korotkoff sounds, which are the sounds of turbulent blood flow in the artery as it starts to open during each heartbeat.
3. The pressure level at which the first Korotkoff sound is heard corresponds to the systolic blood pressure.
4. As the pressure in the cuff continues to decrease, the Korotkoff sounds become clearer and louder.
5. The pressure level at which the Korotkoff sounds stop or become faint corresponds to the diastolic blood pressure.
**Measurement in Millimeters of Mercury (mm Hg):**
Blood pressure is expressed in millimeters of mercury (mm Hg) because the original mercury column sphygmomanometer used mercury as the pressure-measuring medium. Here's why mm Hg is used:
1. **Mercury as a Column Height:** In a mercury column sphygmomanometer, the pressure of the cuff compressing the artery is balanced against the weight of a column of mercury in the manometer tube. The height of the mercury column indicates the pressure being exerted on the cuff.
2. **Direct Pressure Measurement:** The pressure of the mercury column directly reflects the pressure applied to the cuff and, consequently, the blood pressure within the artery.
3. **Standardization and Precision:** Mercury is a dense liquid, allowing for precise and accurate measurements of pressure. The mm Hg unit provides a standardized and reproducible way to measure blood pressure.
4. **Historical Usage:** The use of mercury-based sphygmomanometers has been a common practice for decades, and the mm Hg unit has been widely adopted in medical practice and research.
While modern digital sphygmomanometers have largely replaced mercury-based ones due to safety and environmental concerns, the tradition of expressing blood pressure in mm Hg continues because of its historical significance and the established conventions in medical terminology.
Work Step by Step
A sphygmomanometer is a medical device used to measure blood pressure (BP). It operates based on the principle of auscultatory or oscillometric methods. The most common type of sphygmomanometer is the mercury column sphygmomanometer. Here's how it works and why blood pressure is expressed in millimeters of mercury (mm Hg):
**Principle of Operation:**
A sphygmomanometer consists of an inflatable cuff that is wrapped around the arm, a pressure gauge (manometer), and a stethoscope. The process involves the following steps:
1. The cuff is inflated to a pressure higher than the expected systolic blood pressure. This temporarily occludes the brachial artery, cutting off blood flow.
2. The pressure in the cuff is gradually released. As the pressure decreases, blood flow begins to return through the artery. This is indicated by the onset of Korotkoff sounds, which are the sounds of turbulent blood flow in the artery as it starts to open during each heartbeat.
3. The pressure level at which the first Korotkoff sound is heard corresponds to the systolic blood pressure.
4. As the pressure in the cuff continues to decrease, the Korotkoff sounds become clearer and louder.
5. The pressure level at which the Korotkoff sounds stop or become faint corresponds to the diastolic blood pressure.
**Measurement in Millimeters of Mercury (mm Hg):**
Blood pressure is expressed in millimeters of mercury (mm Hg) because the original mercury column sphygmomanometer used mercury as the pressure-measuring medium. Here's why mm Hg is used:
1. **Mercury as a Column Height:** In a mercury column sphygmomanometer, the pressure of the cuff compressing the artery is balanced against the weight of a column of mercury in the manometer tube. The height of the mercury column indicates the pressure being exerted on the cuff.
2. **Direct Pressure Measurement:** The pressure of the mercury column directly reflects the pressure applied to the cuff and, consequently, the blood pressure within the artery.
3. **Standardization and Precision:** Mercury is a dense liquid, allowing for precise and accurate measurements of pressure. The mm Hg unit provides a standardized and reproducible way to measure blood pressure.
4. **Historical Usage:** The use of mercury-based sphygmomanometers has been a common practice for decades, and the mm Hg unit has been widely adopted in medical practice and research.
While modern digital sphygmomanometers have largely replaced mercury-based ones due to safety and environmental concerns, the tradition of expressing blood pressure in mm Hg continues because of its historical significance and the established conventions in medical terminology.
