Definition
Blood pressure (vessel pressure) is the pressure of the blood that prevails in blood vessels. It is defined as the force per unit area exerted between blood and the vessel walls of arteries, capillaries or veins. The term blood pressure usually refers to the pressure in the large arteries.
The unit of measurement of blood pressure is mmHg (millimetres of mercury), this is also the legal unit of measurement for blood pressure in the EU and may only be used for this purpose. In medical practice, blood pressure is usually understood as arterial blood pressure and is measured at the arm arteries at heart level by applying a blood pressure cuff (see: Measuring blood pressure). This measurement gives two values, the systolic and the diastolic value. The systolic value arises during the ejection phase of the heart and is indicated by the upper value, the diastolic (the lower) value describes the pressure that prevails permanently in the arterial vascular system. The blood pressure values should be about 130/80 mmHg for the arm artery.
Classification of blood pressure
The following list clarifies the classification of the measured blood pressure values and shows that above a value of 140/90 one speaks of too high blood pressure, the so-called arterial hypertension. – Optimal:<120/<80
- <120/<80
- Normal:120-129/80-84
- 120-129/80-84
- High standard:130-139/85-89
- 130-139/85-89
- High blood pressure grade 1:140-159/90-99
- 140-159/90-99
- High blood pressure grade 2:160-179/100-109
- 160-179/100-109
- High blood pressure grade 3:>179/>110
- >179/>110
- <120/<80
- 120-129/80-84
- 130-139/85-89
- 140-159/90-99
- 160-179/100-109
- >179/>110
General information
The pressure conditions in the different areas of the blood circulation are different. When “blood pressure” is spoken of without a more detailed definition, it usually refers to the arterial pressure in large vessels at heart level. This pressure is usually measured in one of the large arm arteries (brachial artery).
Here, the blood pressure readings are positive pressures compared to the atmosphere. Nevertheless, they are not given in the SI unit Pascal (Pa), but in the traditional unit mm Hg. This has a historical background, because in the past blood pressure was measured with a mercury manometer.
The blood pressure is then indicated in a pair of numbers consisting of a systolic and a diastolic value. The systolic value is the maximum value, which is determined by the heart ejection rate, among other things. The diastolic value is the minimum value in the heart filling phase.
For this reason, it is dependent on the elasticity and filling state of the large vessels, among other things. For example, one speaks then of a blood pressure of “110 to 70”. Depending on the position of the body, the values can vary. For example, due to gravity, the blood pressure in the lower half of the body is higher when standing than when lying down, but above the hydrostatic level it is lower when standing than when lying down. As a rule, the mean blood pressure values correspond to the values when lying down.
All articles in this series:
