Definition
The heart action is divided into two phases, an expulsion phase, in which blood is pumped from the chambers into the blood vessels, and a filling phase, in which the pumped-out heart fills with blood again. The heart works like a suction-pressure pump, so to speak. The expulsion phase is known as systole, the filling phase as diastole.
Introduction
What do these phases of the heart action have to do with blood pressure? There is a certain pressure in the vessels, the diastolic blood pressure, which is caused by the blood in the vessels during the filling phase. This pressure depends on the amount of blood the heart pumps at any given time and the diameter of the blood vessels.
The diastolic blood pressure should be around 80mmHg (i.e. millimeters of mercury). In the expulsion phase, however, the heart must generate a higher pressure than the diastolic pressure in order to pump the blood into the vessels. This is because the blood always flows from the higher to the lower pressure.
During systole, the heart generates a pressure of about 120 mmHg, which is pumped into the vessels and from there through the body’s circulation. During the filling phase of the heart, the blood pressure drops back to the diastolic “low point”. This means that the blood pressure is made up of two values, one systolic and one diastolic: 120/80.
These two values can be abnormally high or low. In most cases of arterial hypertension, the high blood pressure we are familiar with, both values are elevated. However, the systolic or diastolic may also be too high in isolation. Both values depend on the blood volume on the one hand and on vascular resistance on the other hand, because the smaller the diameter, the higher the pressure. Thus there is volume hypertension (too much volume), which can be distinguished from resistance hypertension (too small vessel diameter).