In a blood pressure reading, the diastolic pressure (DIA) is the bottom number and represents the pressure in the arteries when the heart muscle relaxes between beats. When a doctor or nurse measures blood pressure, they use a cuff that can be inflated with air, a pressure meter (manometer) for measuring air pressure in the cuff, and a stethoscope to listen to the blood moving through the artery. They inflate the cuff to a pressure higher than the systolic blood pressure, and it will tighten around the arm. As the cuff deflates, the first sound they hear through the stethoscope is the systolic blood pressure. The point where this noise goes away marks the diastolic blood pressure. The unit of measurement used to measure blood pressure is millimeters of mercury (mmHg) .