Blood pressure is a measure of the force exerted against the walls of the arteries as the heart pumps blood through them. A blood pressure reading is composed of two different pressures---diastolic and systolic. Systolic pressure, as described by the National Heart Blood and Lung Institute, is the pressure exerted when the heart beats and pumps blood. Diastolic pressure is the pressure while the heart is resting between beats. Low blood pressure occurs when either the diastolic or the systolic pressure is below normal.
A blood pressure reading states the systolic pressure first, followed by the diastolic pressure. Normal blood pressure, according to the American Heart Association, is pressure lower than 120/80. Low blood pressure occurs when the systolic pressure is below 90, or when the diastolic pressure is below 60.
Experts do not use a specific number to define dangerously low blood pressure. Blood pressure that is too low for one person may be normal for another. Low blood pressure can be dangerous, according to the Mayo Clinic, if it occurs suddenly or if it causes noticeable symptoms.
Three main types of low blood pressure cause noticeable symptoms that can be dangerous. Neutrally mediated hypotension (NMH) is low blood pressure that drops after standing for a long period or because of extreme emotional stress. Orthostatic hypotension is low blood pressure that occurs when rising from a sitting or lying down position. Severe hypotension is low blood pressure that can lead to shock, a potentially fatal condition in which blood pressure is so low that vital organs, such as the brain and kidneys, do not get sufficient blood to function properly.
Common symptoms of potentially dangerous low blood pressure are dizziness, fainting, lightheadedness, nausea, dehydration and blurred vision. Others include lack of concentration, skin that is cold, clammy or pale, fatigue, depression and shallow, rapid breathing.
Medical conditions that may cause dangerously low blood pressure include heart problems, an underactive or overactive thyroid, low blood sugar, dehydration and excessive blood loss. Others are severe infection, lack of nutrients in the diet and severe allergic reaction. Certain medications may also contribute to low blood pressure. Among the most common are diuretics, alpha blockers, beta blockers, drugs that treat Parkinson's disease and some types of antidepressants.