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. It is largely regulated by the autonomic nervous system changing the rate that the heart beats and the diameter of small blood vessels. Hormones like adrenaline produced by the adrenal glands and angiotensin II produced by the kidneys also constrict small blood vessels. Another hormone produced by the adrenal glands, aldosterone, alters blood pressure by affecting the amount of sodium, potassium and fluids excreted by the kidneys. When one or more of these regulating processes is not able to respond appropriately, blood pressure 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 millimetres 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.
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 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 (or 90 mm Hg in the elderly).
- Mild hypertension - a systolic pressure of 120-139 and/or a diastolic of 80-89 mm Hg. This usually responds to lifestyle changes like reducing salt intake and taking more exercise.
- Moderate hypertension- a systolic of 140-179 and/or a diastolic of 90-109 mm Hg. A number of measurements are likely to be made to confirm these values which usually require drug treatment in addition to lifestyle changes.
- Severe hypertension - a systolic over 180 and/or a diastolic over 110 mm Hg. This will almost certainly require referral to a specialist for urgent investigation and treatment.
Blood pressure in children is assessed differently than in adults, and is compared with the 95th percentile values in children of the same age, height and sex.