Blood pressure (BP) is the pressure exerted by circulating blood upon the walls of blood vessels, and is one of the principal vital signs. During each heartbeat, BP varies between a maximum (systolic) and a minimum (diastolic) pressure. The mean BP, due to pumping by the heart and resistance to flow in blood vessels, decreases as the circulating blood moves away from the heart through arteries. Blood pressure drops most rapidly along the small arteries and arterioles, and continues to decrease as the blood moves through the capillaries and back to the heart through veins. Gravity, valves in veins, and pumping from contraction of skeletal muscles, are some other influences on BP at various places in the body.
The term blood pressure usually refers to the pressure measured at a person's upper arm. It is measured on the inside of an elbow at the brachial artery, which is the upper arm's major blood vessel that carries blood away from the heart. A person's BP is usually expressed in terms of the systolic pressure over diastolic pressure (mmHg), for example 120/80.
|Category||systolic, mmHg||diastolic, mmHg|
The following US classification of blood pressure applies to adults aged 18 and older. It is based on the average of seated BP readings that were properly measured during 2 or more office visits. In the UK, hypertension is considered when a patient's reading is above 140/90 mmHg. According to the American Heart Association the following are the blood pressure categories:
While average values for arterial pressure could be computed for any given population, there is often a large variation from person to person; arterial pressure also varies in individuals from moment to moment. Additionally, the average of any given population may have a questionable correlation with its general health, thus the relevance of such average values is equally questionable. However, in a study of 100 subjects with no known history of hypertension, an average blood pressure of 112/64 mmHg was found, which are the normal values.
Various factors influence a person's average BP and variations. Factors such as age and gender influence average values. In children, the normal ranges are lower than for adults and depend on height. As adults age, systolic pressure tends to rise and diastolic tends to fall. In the elderly, BP tends to be above the normal adult range, largely because of reduced flexibility of the arteries. Also, an individual's BP 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. However, in a small percentage of cases there is a consistently present difference greater than 10 mmHg which may need further investigation, e.g. for obstructive arterial disease.
The risk of cardiovascular disease increases progressively above 115/75 mmHg. In the past, hypertension was only diagnosed if secondary signs of high arterial pressure were present, along with a prolonged high systolic pressure reading over several visits. Regarding hypotension, in practice blood pressure is considered too low only if noticeable symptoms are present.
Clinical trials demonstrate that people who maintain arterial pressures at the low end of these pressure ranges have much better long term cardiovascular health. The principal medical debate concerns the aggressiveness and relative value of methods used to lower pressures into this range for those who do not maintain such pressure on their own. Elevations, more commonly seen in older people, though often considered normal, are associated with increased morbidity and mortality.
Average blood pressure in (mmHg):
|1 year||6 - 9 years||adults|
|95/65||100/65||110/65 - 140/90|
There are many physical factors that influence arterial pressure. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs or alcohol, stress, obesity, and so-forth. 
Some physical factors are:
In practice, each individual's autonomic nervous system responds to and regulates all these interacting factors so that, although the above issues are important, the actual arterial pressure response of a given individual varies widely because of both split-second and slow-moving responses of the nervous system and end organs. These responses are very effective in changing the variables and resulting BP from moment to moment.
Moreover, blood pressure is the result of cardiac output increased by peripheral resistance: blood pressure = cardiac output X peripheral resistance. As a result, an abnormal change in blood pressure is often an indication of a problem affecting the heart's output, the blood vessels' resistance, or both. Thus, knowing the patient's blood pressure is critical to assess any pathology related to output and resistance.