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.
The peak pressure in the arteries during the cardiac cycle is the systolic pressure, and the lowest pressure (at the resting phase of the cardiac cycle) is the diastolic pressure. Typical values for the arterial blood pressure of a resting, healthy adult are approximately 120 mmHg systolic and 80 mmHg diastolic (written as 120/80 mmHg), with large individual variations.
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Blood pressure is most accurately measured invasively by placing a cannula into a blood vessel and connecting it to a electronic pressure transducer. This invasive technique is regularly employed in intensive care medicine, anesthesiology, and for research purposes, but it is associated with complications such as thrombosis, infection, and bleeding. Therefore, the less accurate techniques of manual or oscillometric measurement predominate in routine examinations.
Most often, arterial blood pressure is measured manually using a sphygmomanometer. This is an inflatable cuff placed around the upper arm, at roughly the same vertical height as the heart in a sitting person, attached to a manometer. The cuff is inflated until the artery is completely occluded. Listening with a stethoscope to the brachial artery at the elbow, the examiner slowly releases the pressure in the cuff. When blood flow barely begins again in the artery, a "whooshing" or pounding sound (first Korotkoff sound) is heard. The pressure is noted at which this sound began. This is the systolic blood pressure. The cuff pressure is further released until no sound can be heard (fifth Korotkoff sound). This is the diastolic blood pressure.
Oscillometric methods are used in long-term measurement as well as in clinical practice. Oscillometric measurement (also termed NIPB = Non-Invasive Blood Pressure) is incorporated in many bedside patient monitors. It relies on a cuff similar to that of a sphygmomanometer, which is connected to an electric pump and a pressure transducer. The cuff is placed on the upper arm and is automatically inflated. When pressure is gradually released, the small oscillations in cuff pressure that are caused by the cyclic expansion of the brachial artery are recorded and used to calculate systolic and diastolic pressures.
Values are usually given in millimetres of mercury (mmHg). Normal ranges for blood pressure in adult humans are:
- Systolic between 90 and 135 mmHg (12 to 18 kPa)
- Diastolic between 50 and 90 mmHg (7 to 12 kPa)
In children the observed normal ranges are lower, in the elderly, they are more often higher. Clinical trials demonstrate that people who maintain blood pressures in 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 ends up being demonstrated to result in less disease/better outcomes long term.
The mean blood pressure in the arteries supplying the body is a result of the heart pumping blood from the veins back into the arteries.
The mean blood pressure value is determined by the volume of blood the heart is pumping per minute, termed cardiac output, versus the resistance of the 20,000 to 30,000 arterioles, termed total peripheral resistance, through which the blood must flow to reach the capillaries and then veins.
The up and down fluctuation of the arterial blood pressure results from the pulsatile nature of the cardiac output. The pulse pressure is determined by the interaction of the stroke volume versus the volume and elasticity of the major arteries.
The larger arteries, including all large enough to see without magnification, are low resistance (assuming no advanced atherosclerotic changes) and conduit vessels with high flow rates but producing very little pressure drop. For instance, about 5 mmHg mean pressure decrease in the blood flow traveling from the heart all the way to the toes is typical, assuming the individual is supine (horizontal with respect to gravity).
If the blood pressure exceeds these normal values, one speaks of arterial hypertension. Blood pressure that is too low is known as hypotension. The former is rarely an acute problem, while the latter may be a sign of severe disease and requires more urgent medical attention.
Any level of blood pressure puts mechanical stress on the arterial walls. The higher the pressure, the more stress that is present and the more atheroma tend to progress. Veins, when used as surgical bypasses, typically develop atheroma much more rapidly than arteries in the same individual.
When blood pressure and blood flow is very low, the perfusion of the brain may be critically decreased (i.e. the blood supply is not sufficient), causing lightheadedness, weakness and fainting. Sometimes the blood pressure drops significantly when a patient stands up from sitting. This is known as orthostatic hypotension. Other causes of low blood pressure include:
- Toxins including toxic doses of blood pressure medicine
- Hormonal abnormalities, such as Addison's disease
Venous pressure is the blood pressure in a vein. It is much less than arterial blood pressure. e.g. typically about 5 mmHg in the right atrium, 8 mmHg in the left atrium. Measurement of pressures in the venous system and the pulmonary vessels plays an important role in intensive care medicine but requires invasive techniques.