Blood Pressure Decrease (Hypotension)

Low blood pressure refers to inadequate intravascular pressure to maintain the body's oxygen requirements. Although commonly linked to shock, this sign may also result from cardiovascular, respiratory, neurological, or metabolic disorders. Hypoperfusion states especially affect the kidneys, brain, and heart, and may lead to renal failure, change in level of consciousness (LOC) or myocardial ischemia. Low blood pressure may also be caused by certain diagnostic tests- most commonly those using contrast media- and the use of certain drugs. It may stem from stress or a change of position - specifically, rising abruptly from a supine or sitting position to a standing position (orthostatic hypotension)
Normal blood pressure varies considerably; what qualifies as low blood pressure for one person may be perfectly normal for another. Consequently, ever blood pressure reading must be compared against the patent's baseline. Typically, a reading below 90/160 mm Hg or a drop of 30 mm Hg from the baseline, is considered low blood pressure.
Low blood pressure can reflect an expanded intravascular space (as in severe infections, allergic reactions, or adrenal insufficiency), reduced intravascular volume ( as in dehydration and hemorrhage), or decreased cardiac output (as in impaired cardiac muscle contractility). Because the body's pressure-regulating mechanisms are complex and interrelated, a combination of these factors usually contributes to low blood pressure.
(info- Professional Guide to Signs & Symptoms 5th Edition Lippincott Williams & Wilkins)