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.