Blood Pressure Measurement with a Sphygmomanometer
PhysicsChapter 6: Fluid Mechanics
Blood Pressure Measurement with a Sphygmomanometer
Explanation level
Overview
A sphygmomanometer measures arterial blood pressure by using an inflatable cuff and listening to blood-flow sounds as cuff pressure is slowly released.
Learning Outcomes
Cuff pressure can temporarily compress the artery.
Above systolic pressure, flow stops and no sound is heard.
At systolic pressure, intermittent turbulent flow begins and the first sound is heard.
At diastolic pressure, the artery remains open and flow becomes smooth, so the sound disappears.
Typical textbook values are about 120 torr systolic and 75–80 torr diastolic.
Intermediate Level
Equipment
Inflatable cuff or air bag.
Rubber bulb and release valve.
Pressure gauge.
Stethoscope placed over the artery.
Measurement Sequence
Cuff condition
Blood flow
Sound
Reading
Pressure above systolic
artery collapsed; no flow
none
not yet recorded
Pressure equals systolic
blood spurts through narrowed artery; turbulent
first sound appears
systolic pressure
Pressure between systolic and diastolic
intermittent or disturbed flow
sounds continue
—
Pressure reaches diastolic
artery remains open; laminar flow resumes
sound disappears
diastolic pressure
Fluid-Mechanics Explanation
External cuff pressure changes the cross-section of the artery. When the vessel is partly compressed, blood passes rapidly and turbulently through the narrow region, producing audible pressure disturbances. Once compression is removed, smoother laminar flow produces no gurgling sound.
Textbook Reference Values
Key equation
Psystolic≈120\ torr
Key equation
Pdiastolic≈75–80\ torr
These are illustrative textbook values and vary with age and individual conditions.
Systolic versus Diastolic
Term
Meaning in the measurement
Systolic
higher arterial pressure; identified by the first audible sound
Diastolic
lower arterial pressure; identified when the sounds disappear
Common Mistakes
Use the scientific terms precisely.
Do not confuse related but different physical quantities.
State the conditions and observations in the correct order.
Short-Question Preparation
Q. What is a sphygmomanometer?
A. An instrument used to measure arterial blood pressure.
Q. How is systolic pressure identified?
A. It is the cuff-pressure reading when the first blood-flow sound is heard during slow deflation.
Q. How is diastolic pressure identified?
A. It is the reading when the sounds disappear as smooth flow returns.
Q. Why are sounds heard in the middle stage?
A. Blood passes turbulently through a partly compressed narrow artery.
Sticky Notes
Cuff pressure can temporarily compress the artery.
Above systolic pressure, flow stops and no sound is heard.
At systolic pressure, intermittent turbulent flow begins and the first sound is heard.
At diastolic pressure, the artery remains open and flow becomes smooth, so the sound disappears.
Typical textbook values are about 120 torr systolic and 75–80 torr diastolic.
60-Second Revision
A sphygmomanometer measures arterial blood pressure by using an inflatable cuff and listening to blood-flow sounds as cuff pressure is slowly released.
Summary
A sphygmomanometer measures arterial blood pressure by using an inflatable cuff and listening to blood-flow sounds as cuff pressure is slowly released.
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Exam preparation
Exam Focus
Important points
Cuff pressure can temporarily compress the artery.
Above systolic pressure, flow stops and no sound is heard.
At systolic pressure, intermittent turbulent flow begins and the first sound is heard.
At diastolic pressure, the artery remains open and flow becomes smooth, so the sound disappears.
Typical textbook values are about 120 torr systolic and 75–80 torr diastolic.
Common mistakes
Use the exact definitions and sequence.
Quick revision
Revision Zone
Review the key facts, formulas and checks before you move on.
Blood Pressure Measurement with a Sphygmomanometer
Sticky Notes · Quick Revision
1.
Key Point 1
Cuff pressure can temporarily compress the artery.
2.
Key Point 2
Above systolic pressure, flow stops and no sound is heard.
3.
Key Point 3
At systolic pressure, intermittent turbulent flow begins and the first sound is heard.
4.
Key Point 4
At diastolic pressure, the artery remains open and flow becomes smooth, so the sound disappears.
