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  What is Blood Pressure?  

Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure is understood to mean arterial blood pressure, i.e., the pressure in the large arteries, such as the brachial artery (in the arm). The pressure of the blood in other vessels is lower than the arterial pressure. Blood pressure values are universally stated in millimetres of mercury (mm Hg), and are always given relative to atmospheric pressure—the absolute pressure of the blood in an artery with mean arterial pressure stated as 100 mm, on a day with atmospheric pressure of 760 mm, is 860 mm.

The mean arterial pressure and pulse pressure are other important quantities.

Typical values for a resting, healthy adult are approximately 120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg), with large individual variations. These measures of blood pressure are not static, but undergo natural variations from one heartbeat to another or throughout the day (in a circadian rhythm); they also change in response to stress, nutritional factors, drugs, or disease.

 

 

  Measurement  

Invasive measurement

Arterial blood pressure (BP) is most accurately measured invasively by placing a cannula into a blood vessel and connecting it to an electronic pressure transducer. This invasive technique is regularly employed in intensive care, anesthesiology, and for research purposes, but it is, rarely, associated with complications such as thrombosis, infection, and bleeding.

Non-invasive measurement

 

Non-invasive measurement
The non-invasive auscultatory (from the Latin for listening) and oscillometric measurements are simpler and quicker, require less expertise in fitting, have no complications, and are less unpleasant and painful for the patient, at the cost of somewhat lower accuracy and small systematic differences in numerical results. These methods actually measure the pressure of an inflated cuff at the points where it just occludes blood flow, and where it just permits unrestricted flow. These are the methods more commonly used for routine examinations and monitoring.

 

 

  Normal values of blood pressure  
Normal ranges for blood pressure in adult humans are:
Systolic between 90 and 135 mmHg (or 90 and 135 Torr, 12 to 18 kPa)
Diastolic between 50 and 90 mmHg (or 50 and 90 Torr, 7 to 12 kPa)

 

In children the observed normal ranges are lower; in the elderly, they are often higher, largely because of reduced flexibility of the arteries. Clinical trials demonstrate that people who maintain blood pressures at the low end of these pressure ranges have much better long term cardiovascular health and are considered optimal. The principal medical debate is the aggressiveness and relative value of methods used to lower pressures into this range for those who don't maintain such pressure on their own. Elevations, more commonly seen in older people, though often considered normal, are associated with increased morbidity and mortality. The clear trend from double blind clinical trials (for the better strategies and agents) has increasingly been that lower BP is found to result in less disease and better long-term outcomes.

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