What is it? Hypertension, also known as high blood pressure, is a persistent elevation in blood pressure that taxes the heart and can, over time, cause damage to organs such as the kidneys, brain, eyes, and heart. Blood pressure (BP) is the amount of force blood exerts on the walls of the arteries and veins. BP depends on the force and rate of the contraction of the heart 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 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, time of day, and physical and emotional stresses. In healthy people, it is largely controlled by the autonomic nervous system but is also regulated by hormones, including:
- Angiotensin II — produced by the kidneys, it causes increased resistance in blood vessels.
- Aldosterone — produced by the adrenal glands in response to angiotensin II, it affects the amount of sodium, potassium, and fluids excreted by the kidneys.
- Catecholamines — such as epinephrine, also called adrenaline, produced by the adrenal glands in response to stress and increases heart rate and resistance in blood vessels.
When one or more of the regulating factors is not able to respond appropriately to the demands of the body, then the pressure of the blood may become persistently increased.
Two pressures are measured when blood pressure is evaluated. They are systolic pressure – the peak amount of force exerted on the blood vessel walls when the heart is contracting – and diastolic pressure – the amount of pressure present when the heart is relaxing between beats. Both 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 reported as 120 over 80. The National Heart, Lung and Blood Institute (NHLBI) web site has a table that lists the categories of blood pressure levels in adults.
Usually, diastolic pressures will mirror systolic pressures, but as people age, the diastolic pressure tends to level out and hypertension that involves primarily the systolic pressure (called isolated systolic hypertension) becomes more common. In general, the greater the blood pressure for extended periods of time, the greater the potential for damage.
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