In a bicycle tire, air pressing against the inner wall of the tire keeps it inflated. Depending on how much air is in the tire, it is either full of air and ready to roll, or too low and looking "flat." In the body, blood pressure works the same way. Blood pressure is the force of blood pressing against the inner walls of blood vessels. Just like air in a bike tire, the force of the blood inside a blood vessel determines the blood pressure.
To measure a person's blood pressure, doctors take several readings using a blood pressure cuff (also called a sphygmomanometer). Blood pressure is expressed as 2 numbers and is often read as one number "over" another (e.g., "120 over 70"). These 2 numbers represent the pressure inside the blood vessels when the heart contracts (systolic - top number) and when the heart is relaxed (diastolic - bottom number).
Blood pressure is measured in millimeters of mercury (mm Hg). A person's blood pressure varies throughout the day. It changes depending on the time of day and the person's activity and may increase in stressful situations.
When blood exerts too much pressure on the inner walls of the blood vessels (the arteries and veins), this is known as high blood pressure or hypertension. Even though the word hypertension contains "tension," this is not strictly a disease of anxious people, although high stress levels over a prolonged period of time can influence the level of blood pressure. A person who has a systolic pressure equal to or above 140 mm Hg or a diastolic pressure equal to or above 90 mm Hg (i.e., 140/90 mm Hg) as measured by a doctor (or higher than 135/85 mm Hg when measure at home) on a number of consecutive readings is said to have high blood pressure. Normal blood pressure is between 120 mmHg and 129 mm Hg for systolic blood pressure and between 80 mmHg and 84 mm Hg for diastolic blood pressure. If you have diabetes or kidney disease, high blood pressure is anything at or above 130/80 mm Hg.
About 20% of Canadians who have hypertension are not aware of their condition. Many people with high blood pressure discover their condition during a routine yearly examination with their doctor.
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