Blood pressure is highest when the heart contracts and drives the blood into the arteries. This is called the systolic pressure. When the heart muscle is relaxing, no blood is being pushed from it and blood is leaving the arteries through the arterioles.
Blood pressure is measured by a sphygmomanometer, which is constructed on the following principle.
A rubber bag enclosed in a cloth bag is wrapped around the bare upper arm of the subject. This rubber bag has two openings, each of which is connect with a rubber tube. One of the tubes runs to a rubber bulb which acts as a pump, and the other is connected with a mercury manometer which indicates the pressure on a millimeter scale. A stethoscope is placed over the artery in the bend of the elbow, while by means of the bulb the rubber bag or cuff around the arm is inflated with air. When the air pressure in the cuff slightly exceeds the blood pressure in the artery, the flow of blood is closed off.