Pressure exerted by blood on the blood vessels during flow.
Blood pressure is measured during two distinct phases: The first phase, the systolic pressure, is measured during the contraction of the heart and it is mainly affected by the strength of the contraction. The reading is called systolic blood pressure value.
The second phase is the diastolic pressure, during the relaxation phase of the heart and it reflects mainly the resistance of the vessels to flow, or in other words the elasticity of the vessel, for which the measurement is taken. The reading is called diastolic blood pressure value.
Blood pressure is expressed in units of mm Hg (height of mercury column in millimeters).
The instrument used for this measurement is called sphygmomanometer and blood pressure is usually measured in the brachial artery (in the arm).
A cuff is put on the arm and inflated until the pressure exerted on the artery is higher than the systolic value. Thus, in fact, blood flow in the artery is interrupted.
In the next stage, the pressure in the cuff is slowly released and at the same time pulse sounds are auscultated by means of a stethoscope.
When pulse can be heard again (blood flow is restituted), the artery pressure is equal to the cuff pressure. This point is the systolic value. The point in which pulse sounds disappear again is the diastolic pressure value.
Blood pressure tends to be affected by age, gender, race, body weight, nutrition, genetics and more.
Blood pressure values may also differ at different day times and depend on exercise, season of the year and other factors.
If blood pressure values on repeat measurements exceed 140/90, the condition is called Hypertension, and follow-up and treatment are mandatory.