Pressures are usually measured with an instrument called a
sphygmomanometer. A cuff containing a rubber balloon is placed around the
upper arm and inflated to a pressure above the systolic so that no blood can
get through. Pressure in the cuff is then slowly released while an observer
listens over the artery at the elbow below the cuff. As the blood starts to
flow again at the height of the pressure wave it creates a thumping noise and
the pressure in the cuff at this time is equal to the systolic pressure. As the
pressure in the cuff falls further the sound suddenly becomes muffled and
then disappears as the flow is no longer obstructed. Some doctors take the
point of muffling as the diastolic pressure, but the majority now use the
point at which the sounds disappear.