Like water flowing in a smooth river, blood flowing in blood vessels is usually silent, but when blood or water passes through a narrow tube to form a swirl, it can make sound. The sphygmomanometer for measuring human blood pressure is designed based on this principle.
At present, the indirect method is used to measure blood pressure in clinical practice, and the sphygmomanometer used consists of a balloon, a cuff and a pressure gauge. Two sleeved rubber bags connect the balloon and the pressure gauge respectively, forming a closed piping system.
There are two types of pressure gauges: mercury column type and spring type.
When measuring blood pressure, the cuff wrapped around the upper arm is inflated with a balloon and the pressure is applied to the brachial artery through the soft tissue. When the applied pressure is higher than the systolic pressure, the balloon will slowly deflate outward and the pressure inside the cuff will decrease accordingly.
When the pressure inside the cuff is equal to or slightly lower than the systolic blood pressure, blood bursts out of the blocked blood vessel, creating a vortex that can be heard beating with a stethoscope. At this time, the pressure value displayed by the pressure gauge corresponds to the systolic pressure. Continue to deflate slowly to gradually reduce the pressure inside the cuff.
When the pressure inside the cuff is lower than systolic but higher than diastolic, a sound can be heard every time the heart contracts. When the cuff pressure drops to equal or slightly lower than the diastolic pressure, blood flow resumes, and the sound accompanying the heartbeat suddenly diminishes or disappears. At this time, the pressure value displayed by the pressure gauge corresponds to the diastolic pressure.