What is hypertension?
Hypertension, also known as high blood pressure, is a persistent elevation in blood pressure that taxes the heart and can, over time, cause serious damage to the heart as well as other organs such as the kidneys, brain, and eyes. Blood pressure (BP) is the amount of force that blood exerts on the walls of the arteries. BP depends on the strength and rate of the heart's contraction as it pumps blood into the arteries and on 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 rises and falls during the day depending on a person's level of activity and physical and emotional stresses. Largely controlled by the autonomic nervous system, BP is also affected by several different hormones. These include angiotensin II (produced by the kidneys) that causes increased resistance in blood vessels, aldosterone (produced by adrenal glands) that affects the amount of sodium, potassium, and fluids excreted by the kidneys, and catecholamines (like adrenaline) that are produced by the adrenal glands in response to stress and increase heart rate and resistance in blood vessels. When one or more of these hormones is out of balance, high blood pressure or hypertension may result.
Measuring BP takes into account two pressures: systolic and diastolic. Systolic pressure refers to the force exerted on the blood vessel walls when the heart is contracting and diastolic pressure reflects the force present when the heart relaxes between beats. Both are measured in millimeters of mercury (mm Hg) and BP is expressed as systolic over diastolic pressure. For instance, a blood pressure of 120/80 mm Hg or 120 over 80 corresponds to a systolic pressure of 120 and a diastolic pressure of 80. This is considered the highest acceptable number for normal blood pressure. A single measurement of blood pressure is not diagnostic. Typically, multiple readings are taken on different days and, if measurements are consistently high, a diagnosis of high blood pressure is made.
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.
The classifications of blood pressure based on recommendations of the National Heart Lung and Blood Institute (NHLBI) Seventh Report of the Joint National Committee on Prevention, Detection, Evaluation, and Treatment of High Blood Pressure (JNC 7) are summarized in the table below. These categories apply to most adults with no known illness at the time of testing.
Blood Pressure Classifications
|Category||Systolic mm Hg (top number-force when heart is pumping)||Diastolic mm Hg (bottom number-force when heart is at rest)|
|Normal||Less than 120||and||Less than 80|
|Hypertension Stage I||140-159||or||90-99|
|Hypertension Stage II||Equal to or greater than 160||or||Equal to or greater than 100|