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 a person's activity and may increase in stressful situations.
What is meant by high blood pressure?
When blood exerts too much pressure on the inner walls of the blood vessels (the arteries and veins), this is known as hypertension. Even though the word 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 on a number of consecutive readings is said to have high blood pressure. Optimal blood pressure is less than 120 mm Hg for systolic blood pressure and less than 80 mm Hg for diastolic blood pressure.
About 42% 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.
Causes and classes
In the 95% of cases where the exact cause of high blood pressure is unknown, it is called essential hypertension or primary hypertension. Lifestyle, environment, and family history of the condition all play a role in these cases.
In the other 5% of cases, the cause can be linked to a medical condition. This is called secondary hypertension. Reviewing medical and family histories often helps determine the underlying medical problem that led to high blood pressure. Some of the medical conditions that may increase a person's blood pressure include:
- a previous heart attack
- a previous stroke
- hormonal disorders
- kidney disease
If you have secondary hypertension, the underlying medical condition will be treated first.
Classes
Doctors divide hypertension into classes based on the blood pressure reading:
- pre-hypertension: systolic blood pressure from 120 to 139, diastolic from 80 to 89
- stage 1: systolic 140 to 159, diastolic 90 to 99
- stage 2: systolic ≥ 160, diastolic ≤ 100
A diagnosis may be made if either of the numbers, systolic or diastolic, is out of the normal range.
Written and reviewed by the MediResource Clinical Team
