jitendragorashia:
Many blood pressure sensor have an IR source and detector. By passing the IR through the finger, the detector will be interrupted as a blood surge passes by. The surges are counted and displayed on a per minute rate.
Elecmec

Elecmec is right about the pulse measurement. The blood pressure is also fairly simple. Inside the cuff is a microphone that is sensitive enough to actually "hear" the blood rushing through the arteries. The algorithm that is in the microprocessor that is controlling the instrument works like this: cuff expands until it can hear no sound. That means that the artery is completely closed off. Then the program tells the cuff to slowly start to release air. Once the microphone picks up the first sound, it knows that blood is beginning to start to flow again. This pressure point is the “systolic pressure” and is recorded. The cuff continues to release air until there once again is no sound. No sound means that the artery is completely open. This pressure point is the “diastolic” pressure. It is recorded and both readings are then displayed. Sounds easy enough but the hard part is developing an algorithm that can recognize and not display erroneous readings caused by a badly placed cuff and not over or under inflate causing much discomfort to the patient! Go to webmd.com to find out what your doc does with the data.