Blood pressure (BP) is the pressure exerted by blood upon the walls of blood vessels. During each heartbeat, blood pressure varies between a maximum (systolic) and a minimum (diastolic) pressure. The BP in the circulation is principally due to the pumping action of the heart.
The rate of mean blood flow depends on the resistance to flow presented by the blood vessels. The pressure decreases as the circulating blood moves away from the heart through arteries, capillaries and veins due to viscous loss of energy. Gravity affects the pressure via hydrostatic forces (e.g. during standing). Valves in veins, breathing, and pumping from contraction of skeletal muscles also influence blood pressure in veins.
BP measurement refers to systemic arterial pressure measured at a person’s upper arm. It is the measure of the pressure in the brachial artery. It is expressed as systolic pressure over diastolic, and is measured in millimetres of mercury (mmHg), for example 120/80.
BP often varies from person to person. Normal values fluctuate through a day, with highest readings in the afternoons and lowest readings at night.
Various factors, such as age and gender influence a person’s average blood pressure and variations. In children, the normal ranges are lower than those for adults. As adults age, systolic pressure tends to rise and diastolic tends to fall. In elderly people, BP tends to be above the normal adult range, largely because of reduced flexibility of the arteries. Also, an individual’s blood pressure varies with exercise, emotional reactions, sleep, digestion and time of day.
Differences between left and right arm BP measurements tend to be random and average to nearly zero if enough measurements are taken.
The table below indicates the classification of blood pressure for adults according to the American Heart Association. It assumes the values are a result of averaging blood pressure readings measured at two or more visits to the doctor.
Disorders related to control of BP include:
- High blood pressure
- Low blood pressure
|Average blood pressure in (mmHg):|
|1 year||6–9 years||adults|
|95/65||100/65||110/65 – 140/90|
The table below indicates the classification of BP for adults according to the American Heart Association. It assumes the values are a result of averaging BP readings measured at two or more visits to the doctor.
|Category||Systolic (mmHg)||Diastolic (mmHg)|
|Hypotension||< 90||< 60|
|Stage 1 Hypertension||140–159||90–99|
|Stage 2 Hypertension||160–179||100–109|
|Hypertensive Crisis||≥ 180||≥ 110|