Causes of blood pressure

The causes of high blood pressure vary. Causes may include narrowing of the arteries, a greater than normal volume of blood, or the heart beating faster or more forcefully than it should. Any of these conditions will cause increased pressure against the artery walls. High blood pressure might also be caused by another medical problem. Most of the time, the cause is not known. Although high blood pressure usually cannot be cured, in most cases it can be prevented and controlled.

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; gravity reduces the rate of blood return from the body veins below the heart back to the heart, thus reducing stroke volume and cardiac output.

When people are healthy, they quickly constrict the veins below the heart and increase their heart rate to minimize and compensate for the gravity effect. This is carried out involuntarily by the autonomic nervous system. The system usually requires a few seconds to fully adjust and if the compensations are too slow or inadequate, the individual will suffer reduced blood flow to the brain, dizziness and potential blackout. Increases in G-loading, such as routnely experienced by supersonic jet pilots "pulling Gs", greatly increases this effect. Repositioning the body perpendicular to gravity largely eliminates the problem.

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. The usual mechanisms are loss of blood volume, pooling of blood within the veins reducing adequate return to the heart and/or low effective heart pumping. Low blood pressure, especially low pulse pressure, is a sign of shock and contributes to/reflects decreased 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.

Source http://en.wikipedia.org/ & http://www.nhlbi.nih.gov