Hypertension (High Blood Pressure)

What is it?

Hypertension is persistently high pressure in the arteries that can, over time, cause damage to organs such as the kidneys, brain, eyes, and heart. Arterial blood pressure, the amount of force blood exerts on the walls of the arteries, depends on the force and rate that the heart contracts as it pumps oxygenated blood from the left ventricle (compartment) of the heart into the arteries and the resistance to that flow. The amount of resistance depends on the elasticity and diameter of the smaller blood vessels and how much blood is flowing through them.

Blood pressure is dynamic; it rises and falls depending on a person’s level of activity, the time of day, and physical and emotional stresses. In healthy people it is regulated by hormones produced by the adrenal gland that affect the amount of sodium, potassium, and fluids removed by the kidneys (which therefore affects the volume of blood) and is altered by the nervous system decreasing and increasing the rate that the heart beats and the diameter of small blood vessels. When one or more of the regulating processes is not able to respond appropriately, then the pressure of the blood may become persistently high.

Two pressures are recorded when blood pressure is measured. Systolic pressure is the peak force on the blood vessel walls when the heart is contracting and diastolic pressure is the pressure present when the heart is relaxing between beats. Both pressures are measured in millimeters of mercury (mm Hg) and are expressed as systolic over diastolic pressure. For instance, a blood pressure of 120/80 mm Hg corresponds to a systolic pressure of 120 and a diastolic pressure of 80 and would be spoken of as 120 over 80. Blood pressures in adults are generally classified as follows:

• Normal blood pressure - a systolic of less than 120 and a diastolic of less than 80 mm Hg 
• Pre-hypertension (a state of blood pressure that is elevated above normal and that may eventually become hypertension) - a systolic pressure of 120-139 and/or a diastolic of 80-89 mm Hg
• Hypertension, Stage 1 - a systolic of 140-159 and/or a diastolic of 90-99 mm Hg
• Hypertension, Stage 2 - a systolic over 160 and/or a diastolic over 100 mm Hg

Usually diastolic pressure mirrors systolic pressure, but as people get older the diastolic changes less than systolic pressure, and hypertension due to high systolic pressure (called isolated systolic hypertension) becomes more common. In general, the higher the blood pressure and the longer the period of high pressure, the greater the likelihood of damage.

Blood pressure in children is assessed differently to adults and is compared with the 95th percentile of children of the same age, height and sex.