The heart is not a continuous pump. It beats and then relaxes. Each
heart-beat produces a pressure wave. The peak or maximum of this wave is
I called the systolic pressure and the trough or lowest point between beats is
the diastolic pressure. The height of the systolic pressure is partly due to the
amount of blood expelled when the heart contracts and partly to the
resistance to flow of blood produced by the blood vessels. In younger people
the larger blood vessels are fairly elastic: the ‘give’ which this produces
tends to damp down the pressure wave. 