Blood pressure
From Wikipedia, the free encyclopedia.
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.
Blood pressure is not static, but undergoes natural variations from one heartbeat to another or throughout the day (in a circadian rhythm); it also changes in response to stress, nutritional factors, drugs, or disease.
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Measurement
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 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 (Riva Rocci) 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 NIBP = 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:
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.
Incidentally, the absolute BP is obtained by adding the atmospheric pressure (760 mmHg at sea level) to the values obtained by the sphygmomanometer. If the BP had not been greater than the atmospheric pressure, the blood vessels would collapse and the blood would have never flowed through the vessels!
Physiology
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).
Pathophysiology
Effects of high blood pressure
See main page Hypertension
Blood pressure exceeding normal values is called arterial hypertension. It itself is only rarely an acute problem; see hypertensive crisis. But because of its long-term indirect effects (and also as an indicator of other problems) it is a serious worry to physicians diagnosing it.
Any level of blood pressure puts mechanical stress on the arterial walls. This leads to unhealthy tissue growth (atheroma) that develops within the walls of arteries. The higher the pressure, the more stress that is present and the more atheroma tend to progress.
Persistent hypertension is one of the risk factors for strokes, heart attacks, heart failure and arterial aneurysm, and is a leading cause of chronic renal failure.
Effects of low blood pressure
See main page hypotension
Blood pressure that is too low is known as hypotension.
Low blood pressure may be a sign of severe disease and requires more urgent medical attention.
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.
Factors influencing blood pressure
The physics of the circulatory system, as of any fluid system, are very complex. That said, there are many physical factors that influence blood pressure. Each of these may in turn be influenced by physiological factors, such as diet, exercise, disease, drugs, etc.
Some physical factors are:
- Rate of flow of fluid. In the circulatory system, this rate is called heart rate, the rate at which blood (the fluid) is pumped by the heart. The higher the heart rate, the higher the blood pressure.
- Volume of fluid. In the case of the circulatory system, this is blood volume, the amount of blood present in the body. The more blood present in the body, the higher the blood pressure. Some research indicates there is a relationship between dietary salt intake and increased blood volume (resulting in higher blood pressure).
- In cardiac physiology, the rate and volume of flow are accounted for in a combined fashion by cardiac output. Cardiac output is the product of the heart rate, or the rate of contraction, mulitplied by the stroke volume, the amount of blood pumped out from the heart with each contraction. Basically, it represents the efficiency with which the heart circulates the blood throughout the body.
- Resistance. In the circulatory system, this is the resistance of the blood vessels. The higher the resistance, the higher the blood pressure. Resistance is related to size (The larger the blood vessel, the lower the resistance), as well as the smoothness of the blood vessel walls. Smoothness is reduced by the buildup of fatty deposits on the arterial walls. Substances called vasoconstrictors can reduce the size of blood vessels, thereby increasing blood pressure. Vasodilators (such as nitroglycerin) increase the size of blood vessels, thereby decreasing blood pressure.
- Viscosity, or thickness of the fluid. If the blood gets thicker, the result is an increase in blood pressure. Certain medical conditions can change the viscosity of the blood. Note that "blood thinners" such as aspirin affect the viscosity of the blood by reducing platelet clotting.
Other causes of low blood pressure
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:
- Sepsis
- Hemorrhage
- Toxins including toxic doses of blood pressure medicine
- Hormonal abnormalities, such as Addison's disease
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 aortic coarctation, aortic dissection, thrombosis or embolism) of an artery.
Venous pressure
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.
