What Is Blood Pressure, And What Do The Numbers Mean?

What exactly is blood pressure?

Your blood flows from your heart into your body through blood vessels called arteries, to organs, muscles etc., and back to the heart through blood vessels called veins. Blood pressure is the amount of pressure that blood exerts on the walls of blood vessels.

Blood pressure varies throughout the body, depending on factors like distance from heart and diameter of the blood vessel. The greater distance blood has traveled away from the heart, the lower the pressure. By the time that blood returns to the heart the pressure is much lower. For ease of measurement and to standardize measurements, the pressure in large (major) arteries is measured.

Blood pressure is measured in mm Hg, which is millimeters (mm) of mercury (Hg). Think of this pressure measurement like an old style mercury thermometer, as the pressure increases the top of a mercury column moves up beside a scale (ruler), which is marked in millimeters.

Why are there two numbers for blood pressure?

When your heart beats it pushes blood into your arteries. This leads to a temporary increase in the pressure in your arteries as the blood is pushed through. This is the higher number, or systolic pressure. Between heart beats the pressure drops, and this is the lower number or diastolic pressure. For example for a reading of 120/80; the pressure of 120 mm Hg (systolic) is the maximum (peak) pressure as the heart pumps into the large arteries, and the pressure of 80 mm Hg (diastolic) is when the heart is between beats and so no blood is being pushed into the large arteries.

- If either or both numbers are high, this is considered to be high blood pressure. Both numbers do not need to be elevated for a diagnosis of high blood pressure.

What do the actual numbers represent?

Blood pressure is classed as Low, Normal, Prehypertension, Stage 1 Hypertension or Stage 2 Hypertension. The table below gives the current classifications.

Your blood pressure should be taken when under standard conditions, with no recent exercise, caffeine intake, smoking etc. See how to measure blood pressure and a list of factors that may cause short or long term changes to blood pressure.

Note: If your diastolic and systolic readings are in different classes, your overall class is the higher of the two classes.

 * In those with diabetes or kidney disease the prehypertensive range is smaller, with pressure of 130/80 and above being considered high blood pressure.

 ** If blood pressure is always low, then low blood pressure does not increase any risk factor.

Sudden drops in blood pressure should be investigated.