When taking a subject’s blood pressure you are actually measuring the difference between total pressure and which other pressure?
A. gauge pressure
B. blood pressure
C. atmospheric pressure
D. standard atmospheric pressure
The Correct Answer and Explanation is:
Correct Answer: A. Gauge pressure
Explanation:
When measuring a subject’s blood pressure using a sphygmomanometer (blood pressure cuff), you are essentially measuring gauge pressure, not absolute pressure. Understanding this concept requires a distinction between absolute pressure, gauge pressure, and atmospheric pressure.
1. Absolute Pressure vs. Gauge Pressure:
- Absolute pressure is the total pressure exerted, including atmospheric pressure.
- Gauge pressure, on the other hand, is the pressure relative to atmospheric pressure. It is calculated as: Gauge Pressure=Absolute Pressure−Atmospheric Pressure\text{Gauge Pressure} = \text{Absolute Pressure} – \text{Atmospheric Pressure}Gauge Pressure=Absolute Pressure−Atmospheric Pressure Most pressure-measuring instruments, including blood pressure cuffs, use atmospheric pressure as a reference point, so they measure gauge pressure.
2. How Blood Pressure is Measured:
When a clinician uses a blood pressure cuff:
- The cuff inflates and stops the flow of blood in the artery.
- As the cuff deflates, blood starts to flow again, and the clinician listens with a stethoscope to detect Korotkoff sounds.
- The systolic pressure (top number) is recorded when blood just begins to flow (maximum arterial pressure).
- The diastolic pressure (bottom number) is recorded when the sound disappears (minimum arterial pressure).
These values are recorded in millimeters of mercury (mmHg), and they reflect the pressure above atmospheric pressure, which is why it’s called gauge pressure.
3. Why Other Options Are Incorrect:
- B. Blood pressure is the measurement itself, not the pressure being subtracted.
- C. Atmospheric pressure is the baseline, not the quantity being measured.
- D. Standard atmospheric pressure (typically 760 mmHg at sea level) is a reference, not what’s being compared directly in this context.
Thus, when taking a subject’s blood pressure, you’re measuring gauge pressure — the difference between total pressure in the arteries and the surrounding atmospheric pressure.