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Blood pressure or arterial blood pressure is the pressure (force per unit area) exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure refers to the pressure in the large arteries, such as the brachial artery (in the arm), e.g. 110/70 mmHg. The pressure of the blood in other blood vessels differs from the arterial pressure.
The reference pressure level , termed zero, is considered to be at heart level, i.e. towards the middle of the chest and level with the right atrium.
Actually the barometric pressure around the body, about 760 mmHg, is called zero. Blood pressure is read as the number of mmHg above atmospheric pressure, which is used as the zero reference.
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.
Arterial blood pressure is usually measured in millimeters of mercury (mm Hg) 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 is heard. The pressure is noted at which this sound began. This is the systolic blood pressure. The cuff pressure is further released until the sound can no longer be heard. This is the diastolic blood 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.
Normal ranges for blood pressure in adult humans are:
In children the observed normal ranges are lower, in the elderly, they are more often higher. Clinical trails 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 not healthy. 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, termed Systolic (the top number) and the Diastolic (the bottom number) result 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) 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.
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.
At branch points in blood vessels, shear stress and flow turbulence is greater, apparently involved in altering the behavior of the endothelial lining of the arteries. Increased turbulence over the endothelium, along with other factors (nutritional, inflammatory, toxic) contribute to the development of the atheroma of atherosclerosis. It has long been observed that atheroma are nearly always located on the inside of bends and at the outside of arteries past branch points while the outside, higher speed flow side of the artery is more commonly free of fatty streaks, much less more advanced atheroma.
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. This is known as orthostatic hypotension. Other causes of low blood pressure include:
Shock is a complex condition which leads to critically decreased blood perfusion. Low blood pressure is a sign of shock and can also contribute to further decreasing perfusion.
If there is a significant difference in the pressure from one arm to the other, that may indicate a narrowing (e.g. due to thrombosis or embolism) of an artery.
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