The blood pressure test
BMJ Group Medical Reference
Doctors usually use a device called a sphygmomanometer to take your blood pressure. This measures pressure as millimetres of mercury (mm Hg). Most doctors now use digital sphygmomanometers, which don't actually contain mercury. But the measurement is still given in this way.
This is how your doctor will take your blood pressure. 
Your doctor wraps a cuff around your arm above your elbow and pumps air into it. The cuff inflates and stops the blood flow in the main blood vessel in your arm.
Then, your doctor slowly releases the pressure in the cuff.
If they are using a digital machine, your doctor will check the reading on the side of the machine. It will give both the systolic pressure and the diastolic pressure. Digital machines are more common, but they are not suitable for everyone. People with an irregular pulse may need to be tested with an old-style machine.
If they are using an old-style machine, your doctor will look at a gauge on the sphygmomanometer while they use a stethoscope to listen for the sound of the blood rushing back through.
The first measurement is taken when your doctor hears the first thumping sound. This is your blood pressure during a heartbeat. It's called the systolic pressure.
As the pressure in the cuff drops further, and your blood can flow past more easily, the thumping sound stops. At this point your doctor reads off the second measurement. This is your blood pressure as your heart is relaxing between beats. It's called the diastolic pressure.
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