Blood pressure

Last revised by Daniel J Bell on 9 Jul 2019

The blood pressure (BP) is defined as the force exerted by the circulating blood on the walls of the blood vessels. Fundamentally the blood pressure depends upon the interaction of:

  • blood volume
  • cardiac contractility
  • compliance of the arterial walls

Blood pressure is traditionally measured in millimeters of mercury (mmHg), although mercury-containing sphygmomanometers have all but disappeared from most modern hospitals. However, it is increasingly common to see values also expressed in the derived SI unit, kilopascals (kPa). 1 mmHg equates to 0.133 kPa 2.

Systolic blood pressure is the maximal pressure in the main circulation during systole as the heart contracts. The diastolic blood pressure is the minimal pressure reached during diastole, just before the next systole. Pulse pressure is the difference between the systolic and diastolic pressures.

In general when the term blood pressure is used without further qualification, it is referring to the pressure within the systemic arterial circulation. However blood pressure can also be used to refer to the pressure in other circulations, such as the pulmonary or portal venous systems.

Normal values of blood pressure are given as a range as - like most measured biological values - there is a spectrum of normality, which varies with both age and gender.

The systemic arterial blood pressure is usually expressed as two figures separated by a forward slash, e.g. 110/70. The first figure is the systolic pressure, the second the diastolic pressure. 

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