Blood pressure (BP) is the pressure of blood on the walls of blood vessels. During each heartbeat, BP varies between a maximum (systolic) and a minimum (diastolic) pressure. BP decreases as the circulating blood moves away from the heart through arteries. It 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 personâ€™s upper arm. It is measured on the inside of an elbow at the brachial artery, which is the upper armâ€™s major blood vessel that carries blood away from the heart. BP is usually measured by systolic and diastolic pressures, for example 120/80 millimetres of mercury (mmHG), which is 16/11 kPa, for comparison.
BP is usually measured with a sphygmomanometer, which historically used the height of a column of mercury to measure pressure. BP values are reported in millimetres of mercury (mmHg), though some BP devices now do not use mercury.
For each heartbeat, BP varies between systolic and diastolic pressures. Systolic is peak pressure in the arteries, near the end of the cardiac cycle when ventricles are contracting. Diastolic pressure is minimum pressure in the arteries, near the beginning of the cardiac cycle when the ventricles are filled with blood. Normal measured values for a resting, healthy adult human is about 120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg, and spoken as â€œone-twenty over eightyâ€).
Systolic and diastolic arterial BPs are not static but undergo natural variations from one heartbeat to another and throughout the day (in a circadian rhythm). They also change in response to stress, nutritional factors, drugs, disease, exercise, and momentarily from standing up. Sometimes the variations are large.Hypertension refers to arterial pressure being abnormally high, as opposed to hypotension, when it is abnormally low. Along with body temperature, respiratory rate, and pulse rate, BP is one of the four main vital signs routinely monitored by medical professionals and healthcare providers.
Arterial pressures are usually measured non-invasively, without penetrating skin or artery. Measuring pressure invasively, by penetrating the arterial wall to take the measurement, is much less common and usually restricted to a hospital setting.
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 is in the normal range.
Various factors influence a personâ€™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 individualâ€™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. In the UK, patientsâ€™ readings are considered normal up to 140/90 mmHg.
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.
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.