Blood pressure
Blood pressure is the force exerted by circulating blood against the walls of blood vessels. Most of this pressure results from the heart pumping blood through the circulatory system. When used without qualification, the term refers to the pressure in a brachial artery where it is most commonly measured. Blood pressure is usually expressed as systolic pressure over diastolic pressure in millimetres of mercury or kilopascals. Systolic pressure represents the maximum pressure during one heartbeat while diastolic pressure is the minimum between two heartbeats. The difference between these values is known as pulse pressure and the average pressure during a cardiac cycle is mean arterial pressure. This measurement serves as one of the vital signs alongside respiratory rate and body temperature that healthcare professionals use to evaluate patient health.
Traditionally a health-care worker measured blood pressure non-invasively by auscultation listening through a stethoscope for sounds in an arm's artery. The artery was squeezed closer to the heart by an aneroid gauge or a mercury-tube sphygmomanometer. Auscultation remains generally considered the gold standard of accuracy for non-invasive readings in clinic settings. However semi-automated methods have become common largely due to concerns about potential mercury toxicity. Early automated alternatives were often seriously inaccurate but modern devices validated to international standards achieve an average difference of 5 mmHg or less between standardized reading methods. Most semi-automated methods measure blood pressure using oscillometry which measures small oscillations of intra-cuff pressure accompanying heartbeat-induced changes in pulse volume. Fully automated oscillometric measurement has been available since 1981 and this principle has recently been used to measure blood pressure with a smartphone.
In the short term blood pressure is regulated by baroreceptors which act via the brain to influence the nervous and endocrine systems. Baroreceptors in high pressure receptor zones detect changes in arterial pressure and send signals ultimately to the medulla of the brain stem specifically to the rostral ventrolateral medulla. These receptors adjust mean arterial pressure by altering both the force and speed of the heart's contractions as well as systemic vascular resistance. The most important arterial baroreceptors are located in the left and right carotid sinuses and in the aortic arch. The renin-angiotensin system allows the kidney to compensate for loss in blood volume or drops in arterial pressure by activating an endogenous vasoconstrictor known as angiotensin II. This system targets pharmacologically by ACE inhibitors and angiotensin II receptor antagonists while aldosterone release stimulates sodium retention
and potassium excretion by the kidneys.
In pregnancy it is the fetal heart and not the mother's heart that builds up the fetal blood pressure to drive blood through the fetal circulation. The blood pressure in the fetal aorta is approximately 30 mmHg at 20 weeks of gestation and increases to approximately 45 mmHg at 40 weeks of gestation. The average blood pressure for full-term infants ranges from 65, 95 mmHg systolic and 30, 60 mmHg diastolic. In children normal ranges for blood pressure are lower than for adults and depend on height. Reference blood pressure values have been developed for children in different countries based on the distribution of blood pressure in children of these countries. In adults in most societies systolic blood pressure tends to rise from early adulthood onward up to at least age 70 while diastolic pressure tends to
begin to rise at the same time but start to fall earlier in mid-life approximately age 55.
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Common questions
What is blood pressure and how is it measured?
Blood pressure is the force exerted by circulating blood against the walls of blood vessels. It is usually expressed as systolic pressure over diastolic pressure in millimetres of mercury or kilopascals.
How do healthcare workers measure blood pressure accurately?
Healthcare workers traditionally measure blood pressure non-invasively by auscultation listening through a stethoscope for sounds in an arm's artery. Auscultation remains generally considered the gold standard of accuracy for non-invasive readings in clinic settings.
When was fully automated oscillometric measurement available since 1981?
Fully automated oscillometric measurement has been available since 1981 and this principle has recently been used to measure blood pressure with a smartphone. Modern devices validated to international standards achieve an average difference of 5 mmHg or less between standardized reading methods.
Where are the most important arterial baroreceptors located?
The most important arterial baroreceptors are located in the left and right carotid sinuses and in the aortic arch. These receptors adjust mean arterial pressure by altering both the force and speed of the heart's contractions as well as systemic vascular resistance.
What is the fetal blood pressure at 20 weeks of gestation?
The blood pressure in the fetal aorta is approximately 30 mmHg at 20 weeks of gestation and increases to approximately 45 mmHg at 40 weeks of gestation. The average blood pressure for full-term infants ranges from 65, 95 mmHg systolic and 30, 60 mmHg diastolic.