The heart works like a pump that keeps the blood in our body flowing in constant circulation. The arteries distribute oxygen and nutrients to the whole body. The circulatory system needs a certain amount of pressure - blood pressure - to distribute oxygen and nutrients and to remove toxic substances and carbon dioxide. Blood pressure is determined in part by the force used by the heart to push blood into the vessels and also by the elasticity of the walls of the blood vessels.

The heart usually contracts and expands about 60–80 times each minute on average. This expansion and contraction pumps the blood into the arteries to supply the organs of the body with oxygen and nutrients. The arteries branch again and again, down to hair-like blood vessels (capillaries). The circulatory system needs a certain amount of pressure to reach the capillaries.

Pressure is greatest at the moment of the heart beat, that is, when the heart contracts. This pressure is called systolic blood pressure. Between two heart beats, when the heart muscle relaxes, the blood pressure is at its lowest (diastolic blood pressure).

Blood pressure is given in mmHg. The systolic value is shown first, followed by the diastolic value, for example: 120/80 mmHg. 1 mmHg is the pressure exerted by one millimetre (mm) of a mercury column (Hg). This means 1 mmHg corresponds to 0.00133 bar.