Blood pressure is measured with a sphygmomanometer (figure). The oldest kind of sphygnomanometer consists of a mercury manometer on one side attached to a closed bag-the cuff. The cuff is wrapped around the upper arm at the level of the heart and is then pumped up with air. The manometer measures the gauge pressure of the air in the cuff. 

At first, the pressure in the cuff is higher than the systolic pressure-the maximum pressure in the bronchial artery that occurs when the heart contracts. The cuff pressure squeezes the artery closed and no blood flows into the forearm. A valve on the cuff is then opened to allow air to escape slowly. When the cuff pressure decreases to just below the systolic pressure, a little squirt of blood flows past the constriction in the artery with each heartbeat. The sound of turbulent blood flow past the constriction can be heard through the stethoscope. 

As air continues to escape from the cuff, the soundof blood flowing through the constriction in the artery continues to be heard. When the pressure in the cuff reaches the diastolic pressure in the artery – the minimum pressure that occurs when the heart muscle is relaxed – there is no longer a constriction in the artery, so the pulsing sounds cease. The gauge pressure for a healthy heart are nominally around 120 mmHg (systolic) and 80 mmHg (diastolic). Take atmospheric pressure as 760 mmHg. specific gravity of mercury as 13.6 (approx.)